Amandine Merlin - Ecobio - Université de Rennes 1
Amandine Merlin - Ecobio - Université de Rennes 1
Amandine Merlin - Ecobio - Université de Rennes 1
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N° d’ordre : 4343 ANNÉE 2011<br />
THÈSE / UNIVERSITÉ DE RENNES 1<br />
sous le sceau <strong>de</strong> l’<strong>Université</strong> Européenne <strong>de</strong> Bretagne<br />
pour le gra<strong>de</strong> <strong>de</strong><br />
DOCTEUR DE L’UNIVERSITÉ DE RENNES 1<br />
Mention : Biologie<br />
Ecole doctorale Vie- Agro- Santé<br />
présentée par<br />
<strong>Amandine</strong> <strong>Merlin</strong><br />
préparée à l’unité <strong>de</strong> recherche 6553 <strong>Ecobio</strong><br />
Ecosystèmes, Biodiversité, Evolution<br />
UFR Sciences <strong>de</strong> la Vie et <strong>de</strong> l’Environnement<br />
Importance <strong>de</strong>s<br />
interactions biotiques<br />
et <strong>de</strong>s contraintes<br />
environnementales<br />
dans la structuration<br />
<strong>de</strong>s communautés<br />
végétales : le cas <strong>de</strong>s<br />
marais atlantiques et<br />
<strong>de</strong>s pelouses<br />
méditerranéennes.<br />
Soutenance prévue à <strong>Rennes</strong><br />
le 30 septembre<br />
<strong>de</strong>vant le jury composé <strong>de</strong> :<br />
Marie-Laure Navas<br />
PR Montpellier Sup Agro / rapporteur<br />
Eric Tabacchi<br />
CR <strong>Université</strong> Paul Sabatier / rapporteur<br />
Emmanuel Corcket<br />
MC <strong>Université</strong> Bor<strong>de</strong>aux 1 / examinateur<br />
Nicolas Gross<br />
CR Centre d’Etu<strong>de</strong>s Biologiques <strong>de</strong> Chizé/<br />
examinateur<br />
Jan-Bernard Bouzillé<br />
Pr <strong>Université</strong> <strong>Rennes</strong> 1 / directeur <strong>de</strong> thèse<br />
François Mesléard<br />
DR centre <strong>de</strong> recherches Tour du Valat, Pr associé<br />
IUT 2 d’Avignon / co-directeur <strong>de</strong> thèse
- Résumé -<br />
Le stress est connu pour être structurant <strong>de</strong>s communautés végétales. Cependant, son<br />
effet sur les mécanismes et facteurs régissant les patrons <strong>de</strong> distribution <strong>de</strong>s espèces et la<br />
structure <strong>de</strong>s communautés est encore mal compris.<br />
L’objectif <strong>de</strong> cette thèse est <strong>de</strong> comprendre les mécanismes structurant les patrons<br />
d’abondance <strong>de</strong>s espèces et les communautés végétales le long <strong>de</strong> gradients<br />
environnementaux, en quantifiant le stress à (i) l’échelle <strong>de</strong> l’individu et à (ii) l’échelle <strong>de</strong> la<br />
communauté. Deux modèles biologiques ont été considérés autour <strong>de</strong>squels s’articulent <strong>de</strong>ux<br />
problématiques <strong>de</strong> recherche. La première problématique vise à comprendre l’importance<br />
respective <strong>de</strong>s inondations et <strong>de</strong> la compétition dans les patrons d’abondance d’espèces dans<br />
les prairies humi<strong>de</strong>s du Marais poitevin, en considérant le concept <strong>de</strong> niche <strong>de</strong>s espèces. La<br />
secon<strong>de</strong> problématique est consacrée à l’étu<strong>de</strong> <strong>de</strong> l’effet <strong>de</strong>s conditions <strong>de</strong> salinité du sol sur<br />
les réponses <strong>de</strong>s pelouses xéro-halophiles <strong>de</strong> Camargue au pâturage et à la variabilité <strong>de</strong> la<br />
pluviosité. Des approches expérimentales, démographiques et fonctionnelles ont été utilisées<br />
pour traiter ces <strong>de</strong>ux problématiques <strong>de</strong> recherche.<br />
L’approche expérimentale a montré que la compétition conduit à une ségrégation<br />
spatiale <strong>de</strong>s niches <strong>de</strong>s espèces le long du gradient d’inondation, en fonction <strong>de</strong> leur <strong>de</strong>gré <strong>de</strong><br />
tolérance à l’inondation et leur capacité à répondre à la compétition. A l’échelle <strong>de</strong> l’espèce,<br />
l’importance <strong>de</strong> la compétition est, en géneral, prédite par l’intensité <strong>de</strong> la contrainte abiotique<br />
liée au régime hydrique. L’approche démographique a confirmé le rôle structurant <strong>de</strong>s<br />
inondations, en particulier par son impact sur les capacités <strong>de</strong> survie et <strong>de</strong> colonisation <strong>de</strong>s<br />
espèces. Cette approche a permis <strong>de</strong> quantifier la compétition in natura et a ainsi montré que<br />
l’importance <strong>de</strong> la compétition augmente lorsque le <strong>de</strong>gré d’inondation diminue. L’analyse<br />
<strong>de</strong>s patrons <strong>de</strong> distribution <strong>de</strong> traits aériens à l’échelle <strong>de</strong> la communauté (approche<br />
fonctionnelle) a permis <strong>de</strong> montrer que l’assemblage <strong>de</strong>s espèces est la résultante <strong>de</strong> processus<br />
déterministes. Les rôles structurants du patron d’inondation et <strong>de</strong> la compétition dans les<br />
patrons d’abondance <strong>de</strong>s espèces et la structure <strong>de</strong>s communautés le long du gradient<br />
d’inondation sont ainsi démontrés par l’ensemble <strong>de</strong>s approches conduites.<br />
Pour les communautés végétales méditerranéennes, <strong>de</strong>s relevés <strong>de</strong> terrain ont montré<br />
que sous <strong>de</strong>s conditions <strong>de</strong> salinité faible le pâturage était un facteur important contrôlant la<br />
structure <strong>de</strong>s communautés méditerranéennes tandis qu’une forte salinité du sol limitait les<br />
effets <strong>de</strong> la pluviosité et du pâturage sur la structure <strong>de</strong> la communauté. Ce travail a montré<br />
que les espèces dominantes jouent un rôle important dans la réponse <strong>de</strong> ces communautés au<br />
pâturage et à la variabilité <strong>de</strong>s patrons <strong>de</strong> pluviosité.<br />
D’une manière générale, le niveau <strong>de</strong> contrainte a un effet sur les patrons d’abondance<br />
<strong>de</strong>s espèces et sur la structure <strong>de</strong>s communautés et influence l’importance d’autres filtres<br />
écologiques.<br />
Mots-clés : Importance <strong>de</strong> la compétition, Intensité <strong>de</strong> la compétition, Intensité du stress,<br />
Niches écologiques, Inondations, Règles d’assemblage, Traits fonctionnels, Démographie,<br />
Pâturage
- Abstract –<br />
Stress is known to structure plant communities. However, its effect on the mechanisms<br />
and factors governing patterns of species distribution and community structure is poorly<br />
un<strong>de</strong>rstood.<br />
The objective of this thesis is to un<strong>de</strong>rstand the mechanisms structuring patterns of<br />
species abundance and plant communities along environmental gradients by quantifying the<br />
stress at (i) the individual level and (ii) the community level. Two biological mo<strong>de</strong>ls have<br />
been consi<strong>de</strong>red for which two research axes have been <strong>de</strong>veloped. The first axis aims at<br />
un<strong>de</strong>rstanding the relative importance of flooding and competition in the patterns of species<br />
abundance in wet grasslands of the Marais Poitevin, consi<strong>de</strong>ring the concept of niche of<br />
species. The second axis is <strong>de</strong>voted to the study of the effect of soil salinity conditions on the<br />
responses of xero-halophytic grasslands of the Camargue to grazing and rainfall variability.<br />
Experimental, <strong>de</strong>mographic and functional approaches were used to address these two axes.<br />
The experimental approach has shown that competition leads to a spatial segregation<br />
of species niches along the gradient of flooding, according both to their tolerance to flooding<br />
and to their ability to respond to competition. At the species level, the importance of<br />
competition is generally predicted by the intensity of abiotic stress related to water regime.<br />
The <strong>de</strong>mographic approach has confirmed the structuring role of flooding, especially through<br />
its impact on the species survival and colonization abilities. This approach allowed to quantify<br />
the competition in natura and has shown that the importance of competition increases as the<br />
<strong>de</strong>gree of flooding is reduced. The analysis of distribution patterns of functional traits at the<br />
community level (functional approach) has shown that the species assembly is the outcome of<br />
<strong>de</strong>terministic processes. The structuring role of flooding and competition in the patterns of<br />
species abundance and community structure throughout the flooding gradient is <strong>de</strong>monstrated<br />
by all the approaches initiated.<br />
For Mediterranean plant communities, field surveys have shown that un<strong>de</strong>r conditions<br />
of low salinity, grazing was an important factor controlling Mediterranean community<br />
structure whereas high soil salinity limited the effects of rainfall and grazing on community<br />
structure. This work has shown that the dominant species play an important role in the<br />
response of these communities to grazing and to the variability of rainfall patterns.<br />
In general, the stress level has an effect on patterns of species abundance and<br />
community structure and influences the importance of other ecological filters.<br />
Key-words: Competition Importance, Competition Intensity, Stress Intensity, Ecological<br />
Niches, Flooding, Assembly Rules, Functional Traits, Demography, Grazing
- Remerciements -<br />
Je tiens à commencer ces remerciements en exprimant ma plus profon<strong>de</strong> reconnaissance et ma<br />
gratitu<strong>de</strong> à l’ensemble <strong>de</strong>s coordinateurs <strong>de</strong> ce projet, Anne Bonis, Jan-Bernard Bouzillé et<br />
François Mesléard pour m’avoir accordée leur confiance et m’avoir guidée au cours <strong>de</strong> cette<br />
thèse.<br />
Merci à Jan-Bernard, qui a toujours réussi à trouver les bons mots pendant mes pério<strong>de</strong>s <strong>de</strong><br />
doute.<br />
Je tiens à remercier les membres du jury qui ont accepté <strong>de</strong> juger ce travail.<br />
Un grand merci aux membres (passés et présents) <strong>de</strong> l’équipe Forbio, que j’ai eue la joie <strong>de</strong><br />
côtoyer, et qui m’ont offert leur ai<strong>de</strong> à plusieurs reprises: Marie-Lise, Jean-Clau<strong>de</strong>, Ahmed,<br />
Benoît, pour ne citer qu’eux.<br />
Merci à Olivier et Guillaume pour leur ai<strong>de</strong> lors <strong>de</strong> la mise en place <strong>de</strong>s manips et la récolte<br />
<strong>de</strong>s données.<br />
Un merci tout particulier à Fabienne qui a toujours pris soin <strong>de</strong> moi.<br />
J’exprime toute ma reconnaissance et mes sincères remerciements à Nicole Yavercowski et<br />
Loïc Willm qui gèrent l’expérimentation et récoltent les données, ainsi que les membres <strong>de</strong> la<br />
Tour du Valat qui, à <strong>de</strong> nombreuses reprises, ont fournis une ai<strong>de</strong> terrain.<br />
Je remercie Le National Environmental Research Institute (NERI), Silkeborg, Danemark, et<br />
en particulier Christian Damgaard pour m’avoir accueillie chaleureusement à plusieurs<br />
reprises durant ma thèse.<br />
Merci au Collège Doctoral International <strong>de</strong> Bretagne pour m’avoir allouée une bourse <strong>de</strong><br />
mobilité.<br />
Merci aux personnes ayant participé à mes comités <strong>de</strong> thèse : Catherine Grimaldi, Arnaud<br />
Elger, Cyrille Violle, ainsi que mon tuteur <strong>de</strong> thèse, Jacques Baudry.<br />
Je remercie également à tous les stagiaires qui ont apporté leur ai<strong>de</strong> durant les pério<strong>de</strong>s <strong>de</strong><br />
terrain dans le Marais Poitevin ou en Camargue.<br />
Je remercie Marie-Lise, Véro, Cécile & Co., avec qui je ne partage pas seulement le goût <strong>de</strong> la<br />
recherche, mais aussi et surtout celui <strong>de</strong>s moments <strong>de</strong> détente autour d’un bon verre et surtout<br />
au son d’une bonne musique.<br />
Mention spéciale à Marie-Lise qui a toujours été présente pour moi : tout simplement, merci.<br />
Merci à Véro pour les corrections d’anglais !<br />
Merci à Helle, Kai et Line pour m’avoir accueillie chez eux comme un membre <strong>de</strong> leur propre<br />
famille.<br />
Je remercie ma famille et Alexis pour leur soutien sans faille, je leur dédie ce travail.
TABLE DES MATIERES<br />
INTRODUCTION GENERALE 1<br />
1. Les filtres écologiques 3<br />
1.1. Le concept <strong>de</strong> niche 4<br />
1.2. La variabilité <strong>de</strong>s conditions environnementales comme filtre écologique 6<br />
2. L’organisation <strong>de</strong> la végétation le long <strong>de</strong> gradients environnementaux 8<br />
2.1. Les différents types <strong>de</strong> gradient 8<br />
2.2. Les concepts <strong>de</strong> continuum et <strong>de</strong> communauté 9<br />
2.3. Le consensus actuel 10<br />
3. Les interactions compétitives le long <strong>de</strong> gradients environnementaux 11<br />
3.1. Les composantes <strong>de</strong> la compétition : intensité et importance 11<br />
3.2. Grime vs Tilman: un débat qui n’a donc plus lieu d’être 13<br />
3.3. La hiérarchie compétitive 14<br />
3.4. La nécessité <strong>de</strong> caractériser la contrainte 15<br />
3.5. Importance du niveau d’organisation 16<br />
3.6. Les méthodologies d’étu<strong>de</strong> <strong>de</strong> la compétition 16<br />
3.6.1. L’utilisation d’indices 16<br />
3.6.2. Les mesures <strong>de</strong> performance 17<br />
4. Les traits fonctionnels : outils d’étu<strong>de</strong> <strong>de</strong>s règles d’assemblage 18<br />
OBJECTIFS 23<br />
MODELES BIOLOGIQUES 29<br />
1. La microtopographie, génératrice d’une variété <strong>de</strong> communautés 29<br />
1.1. Les prairies humi<strong>de</strong>s du Marais Poitevin 29<br />
1.2. Les pelouses xéro-halophiles <strong>de</strong> Camargue 33<br />
2. La caractérisation <strong>de</strong> la variable environnementale d’intérêt 34<br />
2.1. Les patrons d’inondation dans les prairies humi<strong>de</strong>s du Marais Poitevin 34<br />
2.2. Les patrons <strong>de</strong> pluviosité en Camargue 39<br />
PARTIE 1: LA COMPETITION DANS LES MILIEUX INONDES 43<br />
Chapitre 1: Detecting changes in the intensity and importance of competition for grasslands<br />
species along a flooding gradient 45<br />
Chapitre 2: Fundamental niche versus realized niche: assessment of the importance of<br />
competition along a flooding gradient<br />
Part1: Fundamental niches of twelve wetland species in response to the variation of<br />
flooding conditions 71
Chapitre 3: Fundamental niche versus realized niche: assessment of the importance of<br />
competition along a flooding gradient<br />
Part 2: Tra<strong>de</strong>-off between competitive ability and tolerance of stress at the species level<br />
driving species distribution along the flooding gradient 93<br />
PARTIE 2: LA DEMOGRAPHIE COMME OUTIL D’ETUDE DES FILTRES ECOLOGIQUES 121<br />
Chapitre 4: Effect of environment on species competitive effect and on importance of<br />
competition along an elevation gradient 123<br />
Chapitre 5: The <strong>de</strong>mography of space occupancy: measuring plant colonization and survival<br />
probabilities using repeated pin-point measurements 145<br />
PARTIE 3: LA PART DES FILTRES ABIOTIQUES ET BIOTIQUES DANS LA STRUCTURE DES<br />
COMMUNAUTES VEGETALES 163<br />
Chapitre 6: Detecting community assembly patterns in floo<strong>de</strong>d and productive<br />
grasslands 165<br />
Chapitre 7: Stress level influences the effect of grazing regime and of variability of rainfall<br />
patterns on xero-halophytic communities 185<br />
DISCUSSION GENERALE 213<br />
1. La caractérisation <strong>de</strong>s contraintes 214<br />
1.1. Deux contraintes rencontrées le long du gradient d’inondation 215<br />
1.2. Les traits reliés à la tolérance aux inondations 216<br />
2. Variation <strong>de</strong> la compétition le long du gradient d’inondation 217<br />
2.1. Intensité <strong>de</strong> la compétition le long du gradient d’inondation 217<br />
2.1.1. Ségrégation spatiale <strong>de</strong>s niches le long du gradient d’inondation en réponse à<br />
la présence <strong>de</strong> compétiteurs 217<br />
2.1.2. Importance <strong>de</strong> la survie et <strong>de</strong> la colonisation pendant les inondations pour être<br />
compétitif 220<br />
2.2. Importance <strong>de</strong> la compétition le long du gradient d’inondation 221<br />
2.2.1. Importance <strong>de</strong> la compétition prédite par le « strain » 221<br />
2.2.2. Variation <strong>de</strong> l’importance <strong>de</strong> la compétition – apport <strong>de</strong> l’approche<br />
démographique 222<br />
2.3. Intensité et importance <strong>de</strong> la compétition le long du gradient d’inondation 223<br />
3. Les traits fonctionnels comme outils d’étu<strong>de</strong> <strong>de</strong>s règles d’assemblage 223<br />
4. Le stress à l’échelle <strong>de</strong> la communauté module les effets du pâturage dans les<br />
milieux méditerranéens 226<br />
5. Conclusion & Perspectives 228<br />
5.1. La mesure <strong>de</strong> la compétition en conditions naturelles 229<br />
5.2. Vers le développement <strong>de</strong>s tests <strong>de</strong>s règles d’assemblage 229
5.3. De la facilitation observée ? 230<br />
5.4. Une étu<strong>de</strong> plus précise <strong>de</strong> l’effet <strong>de</strong>s patrons <strong>de</strong> pluviosité dans les mileux<br />
méditerranéens 231<br />
BIBLIOGRAPHIE 233<br />
ANNEXE 1 : Liste <strong>de</strong>s abréviations <strong>de</strong>s espèces présentes dans les prairies humi<strong>de</strong>s du Marais<br />
Poitevin 245<br />
ANNEXE 2 : Liste <strong>de</strong>s abréviations <strong>de</strong>s espèces présentes dans pelouses xéro-halophiles <strong>de</strong><br />
Camargue 246<br />
ANNEXE 3 : Protocole <strong>de</strong> l’étu<strong>de</strong> <strong>de</strong> la compétition en conditions contrôlées 248<br />
ANNEXE 4 : Clonal growth strategies along flooding and grazing gradients in Atlantic coastal<br />
meadows 250
INTRODUCTION GÉNÉRALE<br />
A<br />
l’heure où les activités humaines induisent <strong>de</strong> nombreux changements<br />
globaux, la préservation <strong>de</strong> la biodiversité est au cœur <strong>de</strong>s préoccupations<br />
actuelles. Les activités humaines auraient déjà conduit à l’extinction entre 5 à<br />
20% <strong>de</strong>s espèces dans différents groupes d’organismes (Chapin et al. 2000).<br />
Les changements globaux sont <strong>de</strong> quatre types : les changements d’usage <strong>de</strong>s terres, les<br />
changements <strong>de</strong>s cycles biogéochimiques, les changements climatiques et l’introduction<br />
d’espèces. Ces changements affectent la distribution <strong>de</strong>s organismes altérant par conséquence<br />
les processus écosystémiques et les capacités <strong>de</strong> résilience <strong>de</strong>s écosystèmes aux changements<br />
environnementaux (Chapin et al. 2000).<br />
Comprendre les conséquences <strong>de</strong> la perte <strong>de</strong> diversité sur le fonctionnement <strong>de</strong>s<br />
écosystèmes et conserver la biodiversité nécessitent <strong>de</strong> caractériser les mécanismes régissant<br />
les patrons <strong>de</strong> diversité et donc <strong>de</strong> structure <strong>de</strong>s communautés végétales (Briske et al. 2005).<br />
La communauté végétale apparaît être un bon niveau d’organisation choisi pour étudier les<br />
patrons <strong>de</strong> distribution <strong>de</strong>s espèces car une communauté représente un ensemble d’espèces<br />
interagissant entre-elles mais aussi avec leur environnement. Etudier la structuration <strong>de</strong>s<br />
communautés le long <strong>de</strong> gradients environnementaux permet <strong>de</strong> caractériser les mécanismes<br />
régissant la structure <strong>de</strong>s communautés et in fine, <strong>de</strong> prédire les réponses <strong>de</strong>s espèces aux<br />
changements environnementaux en cours et/ou à venir.<br />
La compréhension <strong>de</strong>s patrons <strong>de</strong> diversité, d’abondance et <strong>de</strong> composition en espèces<br />
<strong>de</strong>s communautés et <strong>de</strong>s processus sous-jacents se situe au cœur <strong>de</strong> l’écologie <strong>de</strong>s<br />
communautés (Vellend 2010). De nombreuses théories écologiques ont été développées pour<br />
expliquer la coexistence <strong>de</strong>s espèces : les théories se distinguent par l’importance accordée<br />
aux quatre processus fondamentaux impliqués dans la coexistence <strong>de</strong>s espèces : les<br />
interactions interspécifiques, la dérive, la spéciation et la dispersion (Fig. 1).<br />
- Les interactions interspécifiques que les individus <strong>de</strong> différentes espèces peuvent<br />
expérimenter en se rencontrant dans un endroit et à un moment donnés. Le signe <strong>de</strong><br />
l’interaction (négatif, neutre ou positif) dépend alors <strong>de</strong> la valeur sélective <strong>de</strong>s individus en<br />
interaction.<br />
1
- La dérive écologique correspond aux changements <strong>de</strong>s abondances relatives <strong>de</strong>s<br />
espèces correspondant à la dynamique <strong>de</strong>s populations, <strong>de</strong>s fluctuations démographiques par<br />
une balance entre recrutement et mortalité inhérents à <strong>de</strong>s processus aléatoires.<br />
- La spéciation décrit la capacité <strong>de</strong>s espèces à s’adapter aux conditions abiotiques,<br />
pouvant amener à la création <strong>de</strong> nouvelles espèces.<br />
- La dispersion décrit le mouvement <strong>de</strong>s individus à travers l’espace entre<br />
communautés, et les échanges individus entre communautés.<br />
Fig. 1 : Lien entre les quatre processus fondamentaux : la sélection, la dérive, la spéciation et<br />
la dispersion et les patrons générés créés par d’innombrables manières à travers la boîte noire<br />
<strong>de</strong> l’écologie <strong>de</strong>s communautés. Extrait <strong>de</strong> Vellend (2010).<br />
Les règles d’assemblage ont été développées pour hiérarchiser l’importance <strong>de</strong> ces<br />
quatre processus fondamentaux. Ces règles d’assemblage peuvent être étudiées le long <strong>de</strong>s<br />
gradients environnementaux où différents facteurs ou filtres écologiques interviennent et<br />
pouvant affecter la performance <strong>de</strong>s espèces (Van Eck et al. 2004). En particulier, le long <strong>de</strong><br />
ces gradients peuvent varier <strong>de</strong> façon simultanée <strong>de</strong>s contraintes environnementales et les<br />
interactions compétitives. Comprendre l’importance <strong>de</strong>s contraintes abiotiques (facteurs<br />
<strong>de</strong> stress ou <strong>de</strong> perturbation) et celle <strong>de</strong>s interactions compétitives dans la distribution<br />
<strong>de</strong>s espèces, leur succès et in fine la structure <strong>de</strong>s communautés végétales le long <strong>de</strong><br />
gradients environnementaux sont l’objet <strong>de</strong> ce travail <strong>de</strong> recherche.<br />
2
1. Les filtres écologiques<br />
Les règles d’assemblage ont été développées pour comprendre l’assemblage <strong>de</strong>s<br />
espèces et <strong>de</strong>s traits au sein d’une communauté via la détermination et la quantification <strong>de</strong><br />
l’importance <strong>de</strong> filtres abiotiques et biotiques agissant à partir d’un pool d’espèces (Keddy<br />
1992 ; Diaz et al. 1998 ; Weiher & Keddy 1999 ; Lavorel & Garnier 2002). La communauté<br />
est composée d’espèces issues initialement d’un même pool régional, qui ont pu passer au<br />
travers <strong>de</strong> plusieurs types <strong>de</strong> filtres (Belyea & Lancaster 1999 ; Lortie et al. 2004) (Fig. 2).<br />
Tout d’abord le filtre biogéographique apparaît à une échelle large (continentale,<br />
biogéographique). Il dépend <strong>de</strong> processus stochastiques. Les espèces sont sélectionnées<br />
suivant leurs capacités à se disperser sur <strong>de</strong> longues distances.<br />
Le filtre abiotique regroupe l’ensemble <strong>de</strong>s facteurs physico-chimiques qui agissent<br />
<strong>de</strong> l’échelle régionale à locale. Les espèces sont sélectionnées suivant leur tolérance<br />
physiologique à ces facteurs. Ainsi seules les espèces capables <strong>de</strong> tolérer les conditions du<br />
milieu peuvent potentiellement être constitutives <strong>de</strong> la communauté.<br />
Le filtre biotique regroupe les interactions plante-plante dans la communauté<br />
végétale, et également les interactions plantes-organismes (prédation, parasitisme,<br />
mycorhizes…). Ce filtre agit par conséquent à une échelle très locale : ce filtre correspond<br />
aux interactions existantes au sein <strong>de</strong>s communautés végétales. Ces interactions peuvent<br />
modifier les conditions abiotiques d’un milieu, expliquant les interactions possibles entre les<br />
filtres biotique et abiotique.<br />
Ce modèle conceptuel hiérarchise ces différents filtres pour une meilleure<br />
compréhension <strong>de</strong> leurs effets sur les probabilités <strong>de</strong> présence <strong>de</strong>s espèces et sur les<br />
probabilités <strong>de</strong> <strong>de</strong>venir dominantes. Néanmoins, ces filtres agissent simultanément sur le pool<br />
d’espèces (Cingolani et al. 2007).<br />
3
Fig. 2 : Hiérarchisation <strong>de</strong>s filtres déterminant la composition et la structure <strong>de</strong>s communautés<br />
végétales (extrait <strong>de</strong> Lortie et al. 2004).<br />
De nombreuses théories ont été proposées pour expliquer l’assemblage <strong>de</strong>s espèces au<br />
sein <strong>de</strong>s communautés. L’objectif ici n’est pas d’en faire une liste exhaustive, mais plutôt <strong>de</strong><br />
se placer dans le cadre théorique le mieux adapté pour étudier l’importance <strong>de</strong> la compétition<br />
par rapport aux filtres abiotiques. En considérant la littérature, le concept <strong>de</strong> niche <strong>de</strong>s<br />
espèces apparaît être un cadre adéquat.<br />
1.1. Le concept <strong>de</strong> niche<br />
Le concept <strong>de</strong> niche est une théorie déterministe dans laquelle la structure <strong>de</strong>s<br />
communautés végétales reflète les exigences <strong>de</strong>s espèces au regard <strong>de</strong>s conditions<br />
environnementales et <strong>de</strong>s ressources, et également au regard <strong>de</strong>s limitations imposées par les<br />
interactions interspécifiques et intraspécifiques.<br />
La niche d’Hutchinson (Hutchinson 1957) est définie par un hyper-volume à n<br />
dimensions où chaque axe définit un type <strong>de</strong> conditions environnementales et/ou <strong>de</strong><br />
ressources. Au sein <strong>de</strong> cet hyper-volume, les populations d’une espèce peuvent maintenir un<br />
taux <strong>de</strong> croissance positif. Il a ainsi introduit la notion <strong>de</strong> niche fondamentale <strong>de</strong>s espèces,<br />
espace occupé par une espèce en l’absence <strong>de</strong> toute influence d’espèces; et la niche réalisée<br />
correspondant à l’espace occupé par l’espèce en présence d’autres espèces. La niche réalisée<br />
est moins large que la niche fondamentale car la compétition interspécifique réduit la fitness<br />
4
<strong>de</strong> l’espèce (taux <strong>de</strong> survie, fécondité, croissance…). Néanmoins, il ne faut pas oublier que les<br />
interactions interspécifiques peuvent être positives (facilitation) permettant aux espèces <strong>de</strong><br />
subsister dans <strong>de</strong>s conditions peu favorables, impliquant une niche réalisée plus large que la<br />
niche fondamentale.<br />
Le concept <strong>de</strong> niche développé d’Hutchinson décrit la niche comme étant un ensemble<br />
<strong>de</strong> conditions environnementales dans lesquelles un organisme peut subsister. Chase &<br />
Liebold (2003) propose <strong>de</strong> considérer en plus <strong>de</strong> la proposition d’Hutchinson, celle d’Elton<br />
(1927) considérant la niche d’un organisme comme étant son impact sur les autres organismes<br />
et l’environnement, <strong>de</strong>ux points <strong>de</strong> vue qui sont pour eux complémentaires.<br />
D’après le concept <strong>de</strong> niche, <strong>de</strong>ux espèces ne peuvent pas partager indéfiniment une<br />
même ressource limitante, elles ne peuvent donc pas occuper une même niche. Suivant le<br />
principe d’exclusion compétitive (Gause 1934), l’espèce la plus compétitive est amenée à<br />
exclure la moins compétitive. Ce concept est fortement dépendant d’un autre : le concept <strong>de</strong><br />
«limiting similarity» (MacArthur & Levins, 1967 ; Chesson 2000 ; Chase & Liebold 2003)<br />
disant que <strong>de</strong>ux espèces similairement proches, c’est-à-dire ayant <strong>de</strong>s stratégies d’acquisition<br />
et d’utilisation <strong>de</strong>s ressources similaires, entreront plus en compétition que <strong>de</strong>ux autres<br />
espèces présentant <strong>de</strong>s stratégies un peu plus contrastées.<br />
Tilman (1982; 1984) a développé le modèle du « Resource Ratio Hypothesis» pour<br />
expliquer pourquoi l’exclusion compétitive a lieu et dans quelles conditions les espèces<br />
coexistent. Dans ce modèle, les espèces sont caractérisées par <strong>de</strong>s exigences minimales pour<br />
<strong>de</strong>ux ressources, R1* et R2*, nécessaire à leur persistance dans un site. La différence entre ces<br />
exigences minimales mais également dans les taux <strong>de</strong> consommation <strong>de</strong> ces ressources sont<br />
les paramètres déterminant <strong>de</strong> leur coexistence. Une espèce tolérant le plus faible niveau <strong>de</strong><br />
ressource, et diminuant le niveau <strong>de</strong> cette ressource, c’est-à-dire présentant un R* faible, sera<br />
la plus compétitive. Pour ces <strong>de</strong>ux ressources, les espèces peuvent coexister quand une espèce<br />
est compétitive pour l’une <strong>de</strong> ces <strong>de</strong>ux ressources. Cette coexistence <strong>de</strong> <strong>de</strong>ux espèces est<br />
possible en raison d’un compromis entre les capacités compétitives <strong>de</strong>s espèces pour les<br />
ressources, car les espèces ne peuvent pas être compétitives pour toutes les ressources (Fig.<br />
3).<br />
5
Fig. 3 : Domaine <strong>de</strong> coexistence théorique <strong>de</strong> <strong>de</strong>ux espèces A et B qui interagissent pour <strong>de</strong>ux<br />
ressources. L’espèce A présente <strong>de</strong>s exigences minimales pour la ressource 2, l’espèce B<br />
présente <strong>de</strong>s exigences minimales pour la ressource 1. Des isoclines sont définies indiquant si<br />
une espèce exclut l’autre en raison d’une plus forte compétitivité en raison <strong>de</strong> besoins plus<br />
faibles pour la ressource, ou si elles coexistent grâce à un compromis entre les capacités<br />
compétitives <strong>de</strong>s espèces pour les <strong>de</strong>ux ressources.<br />
In natura, plus <strong>de</strong> <strong>de</strong>ux espèces peuvent coexister et interagir ensemble qui car ces<br />
espèces ont besoin <strong>de</strong> ressources similaires (Grubb 1977), il faut donc adapter le modèle <strong>de</strong><br />
Tilman en y intégrant un plus grand nombre d’espèces. Cependant même si le principe<br />
d’exclusion explique l’exclusion <strong>de</strong> certaines espèces, d’autres facteurs sont responsables <strong>de</strong><br />
la coexistence <strong>de</strong>s autres espèces, comme l’hétérogénéité spatiale et temporelle <strong>de</strong>s<br />
ressources.<br />
1.2. La variabilité <strong>de</strong>s conditions environnementales comme filtre écologique<br />
Les concepts précé<strong>de</strong>nts, que ce soit le concept <strong>de</strong> niche ou le modèle du « Resource<br />
Ration Hypothesis » ont été proposés suivant <strong>de</strong>s conditions environnementales stables.<br />
Néanmoins, les organismes sont soumis à une variabilité qui leur est intrinsèque et également<br />
à une variabilité extrinsèque produite par l’hétérogénéité spatiale et/ou temporelle <strong>de</strong>s<br />
ressources (Chase & Liebold 2003).<br />
La variabilité <strong>de</strong>s conditions environnementales peut influencer <strong>de</strong> manière importante<br />
les paramètres démographiques et les capacités compétitives <strong>de</strong>s espèces. Tenir compte <strong>de</strong>s<br />
paramètres qui témoignent <strong>de</strong> la dynamique <strong>de</strong>s populations au sein <strong>de</strong> leurs communautés,<br />
c’est-à-dire <strong>de</strong>s probabilités <strong>de</strong> recrutement et <strong>de</strong> survie <strong>de</strong>s espèces est par conséquent<br />
important (Harper 1977 ; Salguero-Gomez & <strong>de</strong> Kroon 2010). L’importance actuelle <strong>de</strong> ces<br />
6
étu<strong>de</strong>s <strong>de</strong> dynamique <strong>de</strong> populations a d’ailleurs fait l’objet d’un numéro spécial <strong>de</strong> Journal of<br />
Ecology (« Advances in plant <strong>de</strong>mography using matrix mo<strong>de</strong>ls », Mars 2010) permettant <strong>de</strong><br />
montrer les avancées dans l’étu<strong>de</strong> <strong>de</strong> la réponse <strong>de</strong>s populations <strong>de</strong> plantes où la variabilité <strong>de</strong><br />
l’environnement est maintenant mieux prise en compte.<br />
Les étu<strong>de</strong>s utilisant <strong>de</strong>s données démographiques pour comprendre l’importance <strong>de</strong>s<br />
filtres environnementaux sont nécessaires car elles tiennent compte du volet dynamique utile<br />
à toute compréhension du fonctionnement <strong>de</strong>s écosystèmes (Enright et al. 1995). De récents<br />
modèles permettent d’étudier l’effet <strong>de</strong> ces différents filtres environnementaux, notamment sur<br />
la variabilité <strong>de</strong>s conditions environnementales, sur différents sta<strong>de</strong>s du cycle <strong>de</strong> vie <strong>de</strong>s<br />
plantes. Ce type d’approche a été développé dans cette thèse proposant un modèle utilisant<br />
une métho<strong>de</strong> répétable au cours du temps et utilisant <strong>de</strong>s données récoltées in natura.<br />
L’importance <strong>de</strong>s variations du recrutement, <strong>de</strong> la mortalité et également <strong>de</strong> la<br />
dispersion sont les paramètres sur lesquels se fon<strong>de</strong> la théorie neutre (Hubbell 2001) où les<br />
processus déterministes comme par exemple les préférences d’habitats, la différenciation <strong>de</strong>s<br />
niches n’apparaissent pas pertinents pour expliquer la structure <strong>de</strong>s communautés. Longtemps<br />
perçues comme opposées, le concept <strong>de</strong> niche et la théorie neutre doivent être perçues comme<br />
complémentaires:<br />
“The next challenge ahead is to integrate niche and neutral theories<br />
that is to add more processes in neutral theories and more<br />
stochasticity in niche theories.”<br />
Chave (2004)<br />
Il s’agit donc <strong>de</strong> comprendre comment les processus déterministes et stochastiques<br />
peuvent interagir sur l’assemblage <strong>de</strong>s communautés (Heil 2004). Gewin (2006) synthétise les<br />
discussions actuelles sur le sujet via le recensement d’étu<strong>de</strong>s, notamment à partir du jeu <strong>de</strong><br />
données d’Hubbell (2001), montrant que les processus aléatoires ne peuvent seuls expliquer<br />
les diversités en espèces rencontrées. De récentes étu<strong>de</strong>s montrent que la considération <strong>de</strong><br />
processus stochastiques peut mieux expliquer la « limiting similarity » (Chesson 2000) et<br />
ainsi augmenter la différenciation <strong>de</strong>s niches <strong>de</strong>s espèces (Tilman 2004) ou <strong>de</strong> groupes<br />
d’espèces (Swilck & Ackerly 2005). Le succès <strong>de</strong>s processus déterministes par rapport aux<br />
processus stochastiques est fortement dépendant <strong>de</strong>s conditions environnementales (Lortie et<br />
7
al. 2004) et plus particulièrement <strong>de</strong> leur patron <strong>de</strong> distribution temporelle (Westoby et al.<br />
1989 ; Potts et al. 2005), ainsi que <strong>de</strong> l’échelle d’étu<strong>de</strong> considérée (Lortie et al. 2004). Heil<br />
(2004) et Cadotte (2007) proposent d’ailleurs <strong>de</strong> considérer différentes échelles spatiales où<br />
les mécanismes neutres et ceux fondés sur la théorie <strong>de</strong>s niches peuvent agir simultanément et<br />
ainsi expliquer la coexistence <strong>de</strong>s espèces.<br />
La part <strong>de</strong>s processus déterministes expliquant la structure <strong>de</strong>s communautés<br />
végétales peut être étudiée en testant ces processus déterministes face à un modèle neutre qui<br />
est une approche générant aléatoirement <strong>de</strong>s assemblages d’espèces (Gotelli & McCabe<br />
2002). Ce type d’approche a été utilisé dans cette thèse pour déterminer l’importance <strong>de</strong>s<br />
processus déterministes dans l’assemblage <strong>de</strong>s espèces le long d’un gradient d’inondation.<br />
2. L’organisation <strong>de</strong> la végétation le long <strong>de</strong> gradients environnementaux<br />
2.1. Les différents types <strong>de</strong> gradient<br />
Smith 1989) :<br />
Les gradients environnementaux peuvent être <strong>de</strong> trois types (Austin 1980 ; Austin &<br />
- les gradients <strong>de</strong> ressources, correspondant à la variation <strong>de</strong> ressources directement<br />
nécessaires à la croissance <strong>de</strong>s plantes.<br />
- les gradients directs, correspondant aux facteurs environnementaux qui vont impacter<br />
directement l’acquisition <strong>de</strong>s ressources et par conséquent la croissance <strong>de</strong>s plantes.<br />
- les gradients indirects, correspondant aux facteurs qui influencent <strong>de</strong> manière<br />
indirecte la croissance <strong>de</strong>s plantes car influencent eux-mêmes les facteurs directs.<br />
Pour la compréhension <strong>de</strong>s processus agissant sur l’assemblage <strong>de</strong>s espèces le long <strong>de</strong><br />
gradients environnementaux, il est nécessaire <strong>de</strong> se positionner par rapport à l’un <strong>de</strong> ces trois<br />
types <strong>de</strong> gradients. En effet, l’organisation <strong>de</strong> la végétation dépend du type <strong>de</strong> ressource, <strong>de</strong> la<br />
quantité <strong>de</strong> ressource disponible pour lesquelles les plantes interagissent, et également <strong>de</strong><br />
l’hétérogénéité <strong>de</strong> ces ressources (Chase & Liebold 2003). Les attendus en terme <strong>de</strong> résultante<br />
<strong>de</strong> la compétition peuvent par conséquent être différents suivant si on se trouve sous une<br />
contrainte abiotique <strong>de</strong> type ressource (disponibilité en nutriments) ou <strong>de</strong> type indirect.<br />
Néanmoins, il peut être parfois difficile <strong>de</strong> se positionner par rapport à ces propositions<br />
8
lorsque qu’un gradient <strong>de</strong> ressource coïnci<strong>de</strong> avec un gradient indirect (Kotowski et al. 2006).<br />
Les gradients d’inondations sont généralement décrits comme <strong>de</strong>s gradients indirects car ils<br />
correspon<strong>de</strong>nt en fait à un gradient altitudinal où la durée d’inondation varie. Néanmoins,<br />
certains auteurs remettent en cause ce classement car les inondations limitent la disponibilité<br />
en oxygène directement nécessaire les plantes (Blom & Voesenek 1996) et proposent <strong>de</strong><br />
considérer ce gradient comme ressource (Keddy et al. 1994).<br />
2.2. Les concepts <strong>de</strong> continuum et <strong>de</strong> communauté<br />
Deux concepts ont été développés pour expliquer l’organisation <strong>de</strong> la végétation le<br />
long <strong>de</strong> gradients environnementaux : le concept <strong>de</strong> la communauté développé par Clements<br />
(1936) et le concept du continuum développé par Whittaker (1951) et Curtis (1959). Le<br />
concept <strong>de</strong> la communauté propose que les communautés sont <strong>de</strong>s associations d’espèces<br />
fortement structurées et répétables avec <strong>de</strong>s limites finies. Une communauté peut être<br />
représentée par <strong>de</strong>s groupes d’espèces qui ne se chevauchent pas le long d’un gradient<br />
environnemental (Fig. 4A) (Collins et al. 1993 ; Austin 2005). Le concept du continuum<br />
propose quant à lui que les communautés végétales changent graduellement le long <strong>de</strong><br />
gradients environnementaux complexes au point où aucune association d’espèces ne peut être<br />
i<strong>de</strong>ntifiée, car chacune d’elles a une distribution le long du gradient qui lui est propre (Fig.<br />
4B) (Collins et al. 1993 ; Austin, 2005).<br />
Fig. 4 : Patrons <strong>de</strong> végétation hypothétiques le long d’un gradient environnemental<br />
représentant : A : le concept <strong>de</strong> communauté où les associations sont très structurées le long<br />
du gradient environnemental; B : le concept <strong>de</strong> continuum où chaque espèce a une distribution<br />
le long du gradient qui lui est propre (extrait <strong>de</strong> Austin 2005).<br />
9
2.3. Le consensus actuel<br />
La vision d’une organisation <strong>de</strong>s patrons <strong>de</strong> végétation sous forme <strong>de</strong> communautés et<br />
sous forme <strong>de</strong> continuum apparaissent maintenant moins incompatibles. En effet, les espèces<br />
composant les communautés végétales présentant <strong>de</strong>s réponses à l’environnement qui peuvent<br />
être différentes entre elles, argument en faveur du concept <strong>de</strong> continuum <strong>de</strong> l’organisation <strong>de</strong><br />
la végétation. Néanmoins, certaines espèces sont plus souvent observées en commun à<br />
certaines localisations du gradient permettant <strong>de</strong> construire <strong>de</strong>s typologies et donc <strong>de</strong> décrire<br />
<strong>de</strong>s communautés. Un patron intermédiaire à ces <strong>de</strong>ux points <strong>de</strong> vue est l’actuel consensus,<br />
avec d’espèces sténoèces à la base <strong>de</strong> chaque communauté et <strong>de</strong>s espèces euryèces qui leur<br />
sont communes (Austin 2005) (Fig. 5).<br />
Fig. 5 : Patron hypothétique d’organisation <strong>de</strong> la végétation le long d’un gradient<br />
environnemental caractérisé par <strong>de</strong>s espèces présentant <strong>de</strong>s gammes <strong>de</strong> distribution restreintes<br />
permettant <strong>de</strong> la distinction <strong>de</strong>s communautés, toutes trois ayant chacune <strong>de</strong>s espèces<br />
communes présentant une large distribution.<br />
Le choix du niveau d’organisation ainsi que les métho<strong>de</strong>s statistiques associées<br />
influencent la capacité à vali<strong>de</strong>r les hypothèses d’assemblage <strong>de</strong>s espèces le long <strong>de</strong> gradients<br />
environnementaux. D’une manière générale, le concept <strong>de</strong> communauté est démontré à l’ai<strong>de</strong><br />
d’analyses multivariées comme les métho<strong>de</strong>s d’ordination comme l’Analyse Canonique <strong>de</strong>s<br />
Correspondances (Ter Braak 1986) ou <strong>de</strong>s métho<strong>de</strong>s <strong>de</strong> classification comme TWINSPAN<br />
(Kent & Coker 1992) déterminant <strong>de</strong>s associations d’espèces. Les relevés floristiques groupés<br />
vont nous permettre <strong>de</strong> dégager les espèces caractérisant chaque communauté, les espèces<br />
dominantes et les espèces compagnes. L’organisation <strong>de</strong> la végétation sous forme <strong>de</strong><br />
continuum quant à lui est généralement mise en évi<strong>de</strong>nce à l’ai<strong>de</strong> <strong>de</strong> régressions <strong>de</strong> type<br />
modèles linéaires généralisés (GLM) ou modèles additifs généralisés (GAM), permettant <strong>de</strong><br />
10
déterminer les courbes <strong>de</strong> réponses <strong>de</strong>s espèces montrant ainsi <strong>de</strong>s changements continus le<br />
long <strong>de</strong> gradients environnementaux (Austin & Gaywood 1994). Ces métho<strong>de</strong>s constituent<br />
<strong>de</strong>s <strong>de</strong>scripteurs <strong>de</strong> la niche écologique <strong>de</strong>s espèces.<br />
3. Les interactions compétitives le long <strong>de</strong> gradients environnementaux<br />
La compétition a été largement étudiée via notamment <strong>de</strong>s approches expérimentales :<br />
Goldberg & Barton (1992) ont recensés <strong>de</strong> nombreuses expérimentations traitant<br />
majoritairement <strong>de</strong> l’effet <strong>de</strong> la compétition sur la fitness <strong>de</strong>s espèces cibles. Plus récemment,<br />
les étu<strong>de</strong>s se sont intéressées au rôle <strong>de</strong> la compétition sur les patrons d’abondance <strong>de</strong>s<br />
espèces et sur la structure <strong>de</strong>s communautés. Il reste néanmoins à l’heure actuelle <strong>de</strong>s<br />
imprécisions qui obscurcissent la compréhension du rôle <strong>de</strong> la compétition le long <strong>de</strong><br />
gradients environnementaux. De façon fondamentale, le concept d’importance <strong>de</strong> la<br />
compétition a été largement négligé dans la littérature (Craine 2007 ; Brooker & Kikvidze<br />
2008) et sa prise en compte permet <strong>de</strong> résoudre certains débats.<br />
3.1. Les composantes <strong>de</strong> la compétition : intensité et importance<br />
Distinguer le rôle <strong>de</strong> l’intensité <strong>de</strong> la compétition <strong>de</strong> celui <strong>de</strong> l’importance <strong>de</strong> la<br />
compétition est essentielle pour comprendre les patrons d’abondance <strong>de</strong>s espèces et <strong>de</strong><br />
structure <strong>de</strong>s communautés végétales le long <strong>de</strong> gradients environnementaux.<br />
L’intensité <strong>de</strong> la compétition correspond à la réduction <strong>de</strong> performances due à la<br />
présence <strong>de</strong> compétiteurs (Wel<strong>de</strong>n & Slauson 1986 ; Brooker et al. 2005 ; Brooker 2006). Des<br />
espèces fortement compétitives, que ce soit dans leur capacité à réduire la ressource<br />
disponible pour les voisins (effet compétitif) et/ou dans leur capacité à répondre à la réduction<br />
du niveau <strong>de</strong>s ressources par les voisins (réponse à la compétition) (Goldberg 1990 ; Goldberg<br />
& Landa 1991), induiront une compétition intense par rapport à <strong>de</strong> moins bons compétiteurs.<br />
Cette variation d’intensité <strong>de</strong> la compétition est schématisée sur la figure 6, où la compétition<br />
est plus intense pour les conditions environnementales A et B que pour les conditions C et D.<br />
L’importance <strong>de</strong> la compétition correspond à la réduction <strong>de</strong> performances due à la<br />
présence <strong>de</strong> compétiteurs, par rapport à la réduction <strong>de</strong> performances due à d’autres<br />
processus/conditions environnementales (Wel<strong>de</strong>n & Slauson 1986 ; Brooker et al. 2005 ;<br />
Brooker 2006). L’importance <strong>de</strong> la compétition est supposée forte sous <strong>de</strong>s conditions<br />
11
environnementales optimales pour l’espèce, mais faible pour <strong>de</strong>s conditions qui lui sont plus<br />
contraignantes. Sur la figure 6, l’importance <strong>de</strong> la compétition est plus forte pour les<br />
conditions environnementales B et C, car la réduction <strong>de</strong> performances <strong>de</strong> l’espèce s’explique<br />
majoritairement par ce processus.<br />
Même si Wel<strong>de</strong>n & Slauson (1986) affirment qu’une importance <strong>de</strong> la compétition et<br />
intensité <strong>de</strong> la compétition ne sont pas nécessairement reliés, une intensité <strong>de</strong> la compétition<br />
forte serait un pré-requis pour que la compétition soit importante (Bartelheimer et al. 2010).<br />
En effet, l’importance <strong>de</strong> la compétition sera élevée si la variation <strong>de</strong> l’intensité <strong>de</strong> la<br />
compétition le long <strong>de</strong> gradients est la cause <strong>de</strong> la variation <strong>de</strong> la structure <strong>de</strong>s communautés.<br />
Pour <strong>de</strong>s conditions environnementales égales, toute variation <strong>de</strong> l’intensité <strong>de</strong> la compétition<br />
fera varier proportionnellement l’importance <strong>de</strong> la compétition (Kikvidze et al. 2011). Mais<br />
les autres conditions environnementales n’étant pas constantes in natura, ces <strong>de</strong>ux<br />
composantes ne sont pas systématiquement corrélées (Wel<strong>de</strong>n & Slauson 1986 ; Kikvidze et<br />
al. 2011).<br />
Par exemple, une relation entre l’importance et l’intensité <strong>de</strong> la compétition n’est<br />
toujours mise en évi<strong>de</strong>nce : en effet, Lamb & Cahill (2008) montrent une intensité <strong>de</strong> la<br />
compétition racinaire très forte mais qui n’est pas importante, donc non structurante <strong>de</strong>s<br />
prairies à fétuque, milieux contraints par la disponibilité <strong>de</strong>s nutriments et <strong>de</strong> l’eau. La figure<br />
6 schématise cette idée autour <strong>de</strong> la relation possible entre intensité <strong>de</strong> la compétition et<br />
importance <strong>de</strong> la compétition. Pour une même intensité <strong>de</strong> compétition (par exemple sous les<br />
conditions environnementales A et B), l’importance <strong>de</strong> la compétition n’est pas la même : cet<br />
argument est en faveur d’une non-relation <strong>de</strong> ces <strong>de</strong>ux concepts. Néanmoins, pour une<br />
réduction similaire <strong>de</strong>s performances <strong>de</strong> l’espèce par les conditions environnementales<br />
(comme par exemple entre les conditions B et C), l’importance <strong>de</strong> la compétition apparaît être<br />
plus importante là où l’intensité <strong>de</strong> la compétition est la plus forte.<br />
Encore peu d’étu<strong>de</strong>s s’intéressent à la variation <strong>de</strong> l’importance <strong>de</strong> la compétition le<br />
long <strong>de</strong> gradients environnementaux ainsi que sa relation avec l’intensité <strong>de</strong> la compétition,<br />
limitant la compréhension du rôle <strong>de</strong> la compétition dans la structure <strong>de</strong>s communautés<br />
végétales (e.g. Greiner la Peyre et al. 2001 ; Gaucherand et al. 2006).<br />
12
Fig. 6: Représentation <strong>de</strong>s notions d’intensité et d’importance <strong>de</strong> la compétition. Pour <strong>de</strong>ux<br />
intensités <strong>de</strong> compétition différentes (forte dans la partie gauche et faible dans la partie droite)<br />
réduisant différemment les performances <strong>de</strong> l’espèce (différence entre niche fondamentale et<br />
niche réalisée : C), l’importance <strong>de</strong> la compétition correspond à la réduction <strong>de</strong> performances<br />
<strong>de</strong> l’espèce par la compétition et par tout autre processus suivant ce rapport (C/C+S). Lorsque<br />
S augmente, le résultat <strong>de</strong> ce rapport est faible : l’importance <strong>de</strong> la compétition est alors faible<br />
(conditions A et D). En revanche, quand S diminue, le rapport est élevé signifiant une<br />
compétition importante (conditions B et C) (adapté <strong>de</strong> Wel<strong>de</strong>n & Slauson 1986).<br />
3.2. Grime vs Tilman : un débat qui n’a donc plus lieu d’être<br />
Face à l’intérêt actuel <strong>de</strong> distinguer l’intensité <strong>de</strong> la compétition et <strong>de</strong> l’importance <strong>de</strong><br />
la compétition, il est maintenant reconnu que Grime (1977) a discuté non pas <strong>de</strong> l’intensité <strong>de</strong><br />
la compétition, mais <strong>de</strong> l’importance <strong>de</strong> la compétition, différence déjà mise en exergue par<br />
Wel<strong>de</strong>n & Slauson (1986) et Goldberg (1990).<br />
Cependant, <strong>de</strong> nombreuses étu<strong>de</strong>s ont entretenu cette confusion pendant <strong>de</strong><br />
nombreuses années concernant le point <strong>de</strong> vue <strong>de</strong> Grime : les étu<strong>de</strong>s se sont axées sur la<br />
variation <strong>de</strong> l’intensité <strong>de</strong> la compétition, soit augmentant avec la productivité suivant alors la<br />
thèse <strong>de</strong> Grime (1977), soit restant constante le long du gradient avec une bascule entre plus<br />
<strong>de</strong> compétition aérienne dans les milieux productifs et plus <strong>de</strong> compétition souterraine dans<br />
les milieux peu productifs (Tilman 1988). Certaines étu<strong>de</strong>s ont ainsi vérifié le modèle <strong>de</strong><br />
Grime (Pennings & Callaway 1992; Gau<strong>de</strong>t & Keddy 1995; Briones et al. 1998; Choler et al.<br />
2001) ; d’autres le modèle <strong>de</strong> Tilman (Berendse et al. 1992 ; Wilson 1993; Rea<strong>de</strong>r et al. 1994;<br />
Casper & Jackson 1997). Plusieurs citations en introduction d’articles traitant <strong>de</strong> la<br />
compétition témoignent et soutiennent cette confusion pour le débat Grime/Tilman:<br />
13
“Competition intensity may increase with productivity (Grime 1973,<br />
1974, 1979, […] Campbell and Grime 1992), remain constant as<br />
productivity changes (Newman 1973, Tilman 1982, 1988, Grubb<br />
1985) or be <strong>de</strong>pen<strong>de</strong>nt on the ratio of resource supply to <strong>de</strong>mand,<br />
which may be un-related to productivity (Taylor et al. 1990).”<br />
Twolan-Strutt & Keddy (1996)<br />
“A long-running <strong>de</strong>bate in plant ecology concerns whether the<br />
intensity of competitive interactions, or the <strong>de</strong>gree to which neighbors<br />
reduce plant growth, changes along a gradient of productivity,<br />
resource supply or non-resource stress (Grime 1973, 2001; Newman<br />
1973; Tilman 1988;[…]Rajaneimi 2003). In nutrient-poor or<br />
otherwise abiotically ‘stressed’ habitats, Grime (1973) asserted that<br />
the intensity of competition should be weakest; whereas Newman<br />
(1973) countered that competition should be intense (particularly<br />
belowground) in unproductive environments (Tilman 1988; Grace<br />
1991; Rajaneimi 2003; Craine 2005).”<br />
Fraser & Miletti (2008)<br />
Ce débat, lié à un problème <strong>de</strong> sémantique, doit maintenant être dépassé pour<br />
progresser dans la compréhension du rôle <strong>de</strong> la compétition dans la structuration <strong>de</strong>s<br />
communautés. Il est nécessaire d’étudier ces <strong>de</strong>ux composantes (intensité et importance) pour<br />
déterminer leur réelle opérationnalité (Grace 1995).<br />
3.3. La hiérarchie compétitive<br />
L’intensité <strong>de</strong> la compétition a été beaucoup plus étudiée que l’importance <strong>de</strong> la<br />
compétition (Brooker et al. 2005). Son étu<strong>de</strong> permet <strong>de</strong> classer les espèces en terme <strong>de</strong><br />
hiérarchie compétitive au sein d’une communauté (Keddy & Shipley 1989 ; Goldberg &<br />
Landa 1991). Le long <strong>de</strong> gradients environnementaux, la hiérarchie compétitive peut<br />
potentiellement varier et a le potentiel d’expliquer l’organisation <strong>de</strong>s communautés végétales<br />
car elle peut être un facteur important déterminant <strong>de</strong> l’abondance <strong>de</strong>s espèces (e.g. Gau<strong>de</strong>t &<br />
Keddy 1995 ; Fraser & Keddy 2005). L’effet <strong>de</strong>s changements <strong>de</strong> l’environnement sur la<br />
hiérarchie compétitive est largement étudié et les résultats apparaissent assez variables dans<br />
la littérature. Certaines étu<strong>de</strong>s mettent en évi<strong>de</strong>nce un maintien <strong>de</strong> la hiérarchie compétitive le<br />
long <strong>de</strong>s gradients, que ce soit le long <strong>de</strong> gradients d’inondations ou <strong>de</strong> disponibilité <strong>de</strong> la<br />
ressource en eau (e.g. Shipley et al. 1991 ; Keddy et al. 2002 ; Lenssen et al. 2004). Dans ce<br />
cas, la compétition n’est pas un processus majeur expliquant les patrons d’abondance<br />
d’espèces le long <strong>de</strong> ces gradients. En revanche d’autres étu<strong>de</strong>s mettent en évi<strong>de</strong>nce un<br />
14
changement dans la hiérarchie compétitive expliquant les patrons <strong>de</strong> structuration <strong>de</strong>s espèces<br />
(e.g. Keddy 1990 ; Wisheu & Keddy 1992). Les espèces moins compétitives sont alors<br />
observées dans les milieux les plus contraints. Cette variation <strong>de</strong> la hiérarchie compétitive<br />
permet <strong>de</strong> conclure à un rôle important <strong>de</strong> la compétition dans la structuration <strong>de</strong>s<br />
communautés. C’est donc indirectement que l’importance <strong>de</strong> la compétition dans la<br />
structuration <strong>de</strong>s communautés végétales a été étudiée.<br />
Une part <strong>de</strong> la diversité <strong>de</strong>s résultats entre les étu<strong>de</strong>s peut s’expliquer par le critère<br />
choisi pour quantifier l’aptitu<strong>de</strong> compétitive <strong>de</strong>s espèces : certaines considèrent l’effet<br />
compétitif d’autres la réponse à la compétition. Le long <strong>de</strong> gradients, l’effet compétitif tend à<br />
être constant tandis que la réponse à la compétition montre une plus gran<strong>de</strong> sensibilité au<br />
changement <strong>de</strong> l’environnement (Keddy et al. 1994 ; Wang et al. 2010). Les patrons <strong>de</strong> la<br />
réponse à la compétition expliqueraient, mieux que les patrons d’effet compétitif, les patrons<br />
d’abondance <strong>de</strong>s espèces le long <strong>de</strong> gradients (Howard & Goldberg 2001). Néanmoins, la<br />
variation <strong>de</strong> la réponse à la compétition le long <strong>de</strong> gradients environnementaux est encore peu<br />
comprise (Fraser & Miletti 2008). En effet, il existe différents types <strong>de</strong> réponses à la<br />
compétition en réponse à la présence <strong>de</strong> compétiteurs qui peuvent varier suivant les espèces et<br />
le milieu (Keddy et al. 1998) : certaines espèces présentent une stratégie <strong>de</strong> conservation <strong>de</strong>s<br />
ressources, d’autres au contraire, présentent la stratégie inverse <strong>de</strong> prélèvement rapi<strong>de</strong> <strong>de</strong>s<br />
ressources ; d’autres enfin décale le prélèvement <strong>de</strong>s ressources pour éviter cette compétition.<br />
3.4. La nécessité <strong>de</strong> caractériser la contrainte<br />
La caractérisation <strong>de</strong> la contrainte est une étape requise pour étudier la compétition,<br />
car le type <strong>de</strong> contrainte, c’est-à-dire (i) une ressource directement utilisable pour la<br />
croissance <strong>de</strong> la plante ou non, (ii) la distribution temporelle et spatiale <strong>de</strong> la contrainte qui<br />
peuvent influencer la compétition (Novoplanski & Goldberg 2001 ; Craine 2005). Les<br />
différents types <strong>de</strong> contraintes étudiées dans la littérature peuvent expliquer, au moins pour<br />
partie, la large palette <strong>de</strong> résultats obtenus concernant la variation <strong>de</strong> la compétition le long <strong>de</strong><br />
gradients environnementaux. En effet, Grime (2007) explique que les différentes<br />
interprétations du rôle <strong>de</strong> la ressource sujette à la compétition sont l’un <strong>de</strong>s éléments centraux<br />
<strong>de</strong> la controverse Grime-Tilman.<br />
Les résultats sont variables quant à la variation <strong>de</strong> l’intensité <strong>de</strong> la compétition le long<br />
<strong>de</strong> gradients. L’intensité peut être réduite avec l’augmentation <strong>de</strong> la contrainte, que ce soit le<br />
long <strong>de</strong> gradients <strong>de</strong> salinité et d’inondation (Pennings et al. 2005), ou le long d’un gradient<br />
15
<strong>de</strong> disponibilité en nutriments (Fraser & Keddy 2005). L’intensité <strong>de</strong> la compétition peut<br />
également être constante le long <strong>de</strong> gradients <strong>de</strong> disponibilité en nutriments et en eau (Corcket<br />
et al. 2003; Gaucherand et al. 2006; Carlyle et al. 2010) ou <strong>de</strong> gradients <strong>de</strong> salinité (Greiner la<br />
Peyre et al. 2001) ; la part <strong>de</strong> la compétition racinaire et <strong>de</strong> la compétition aérienne peut<br />
cependant varier en fonction <strong>de</strong> la disponibilité en ressources (Bartelheimer et al. 2010 ;<br />
Marion 2010). Comme les espèces peuvent présenter différentes réponses à la présence <strong>de</strong><br />
compétiteurs, il est important <strong>de</strong> savoir précisément quelle est la contrainte et quelle est la<br />
stratégie <strong>de</strong> l’espèce face à cette contrainte.<br />
3.5. Importance du niveau d’organisation<br />
Dans la littérature, on trouve fréquemment le terme « gradient <strong>de</strong> stress ». Or le stress<br />
est généralement défini à l’échelle <strong>de</strong> la communauté via <strong>de</strong>s mesures <strong>de</strong> productivité, suivant<br />
la définition du stress proposée par Grime (1979). Dans ce cadre, les différentes tolérances<br />
physiologiques <strong>de</strong>s espèces (niches fondamentales) le long d’un gradient environnemental, ne<br />
sont pas considérées. Les effets <strong>de</strong> la compétition à l’échelle <strong>de</strong> l’espèce ne sont cependant<br />
pas forcément transférables à l’échelle <strong>de</strong> la communauté dans la mesure où les différentes<br />
espèces <strong>de</strong> la communauté peuvent présenter <strong>de</strong>s niches fondamentales contrastées (Goldberg<br />
1994 ; Greiner la Peyre et al. 2001) : la contrainte doit être examinée à l’échelle <strong>de</strong> l’espèce.<br />
Cette nécessité <strong>de</strong> déterminer la contrainte à l’échelle <strong>de</strong> l’espèce a été introduite par Wel<strong>de</strong>n<br />
& Slauson (1986) : ils ont ainsi différencié le « strain » comme étant la réponse aux facteurs<br />
abiotiques au niveau spécifique et même au niveau individuel, tandis que le stress étant une<br />
mesure <strong>de</strong> la contrainte au niveau <strong>de</strong> la communauté et pouvait être approchée par le niveau<br />
<strong>de</strong> productivité à l’échelle <strong>de</strong> la communauté.<br />
3.6. Les méthodologies d’étu<strong>de</strong> <strong>de</strong> la compétition<br />
3.6.1. L’utilisation d’indices<br />
Les expérimentations in situ et en conditions contrôlées sont les approches<br />
majoritairement utilisées pour étudier la compétition (Damgaard 2004). Cela a permis, en<br />
parallèle, <strong>de</strong> développer <strong>de</strong> nombreux indices visant à quantifier la compétition. Weigelt &<br />
Jocliffe (2003) ont recensé <strong>de</strong> nombreux indices visant à quantifier notamment l’intensité <strong>de</strong><br />
la compétition, comme le Log Response Ratio (Hedges et al. 1999) ou encore le Relative<br />
16
Competition Intensity (Grace 1995). Plus récemment, <strong>de</strong>s indices pour quantifier l’importance<br />
<strong>de</strong> la compétition ont également été développés, comme Cimp (Brooker et al. 2005), Relative<br />
Neighbour Importance (Kikvidze & Armas 2010) ou encore Iimp (Seifan et al. 2010). Même<br />
s’il est indispensable <strong>de</strong> réaliser <strong>de</strong>s expérimentations pour comprendre le rôle <strong>de</strong> la<br />
compétition dans la structuration <strong>de</strong>s communautés grâce au contrôle d’un ou plusieurs<br />
facteurs (Kikvidze & Brooker 2010), le grand nombre d’indices utilisés dans la littérature<br />
limitent la généralisation <strong>de</strong>s résultats et la comparaison <strong>de</strong>s étu<strong>de</strong>s entre elles. De plus, le<br />
nombre limité <strong>de</strong> compétiteurs choisis dans les approches expérimentales limite<br />
l’extrapolation <strong>de</strong>s résultats à <strong>de</strong>s conditions réalistes <strong>de</strong> terrain (Goldberg 1996 ;<br />
Novoplanski & Goldberg 2001 ; Perkins et al. 2007 ; Engel & Weltzin 2008). En outre, dans<br />
les approches expérimentales en conditions contrôlées, les traitements expérimentaux choisis<br />
ne peuvent pas refléter tous les facteurs présents, leur potentielle interaction et leur variation<br />
au cours du temps affectant <strong>de</strong> surcroît les performances <strong>de</strong>s espèces (Wilson 2007).<br />
3.6.2. Les mesures <strong>de</strong> performance<br />
Les expérimentations in situ et en conditions contrôlées impliquent généralement <strong>de</strong>s<br />
mesures <strong>de</strong> biomasse et <strong>de</strong> survie <strong>de</strong>s espèces, récoltées à la fois en monocultures et en<br />
mixtures en fin d’expérimentations (Aarssen & Keogh 2002). Néanmoins, le résultat <strong>de</strong><br />
l’intensité <strong>de</strong> la compétition entre <strong>de</strong>ux espèces peut être différent suivant la mesure <strong>de</strong><br />
performance considérée, comme cela a été démontré entre l’analyse <strong>de</strong> données <strong>de</strong> biomasse<br />
et <strong>de</strong> survie (Goldberg & Novoplanski 1997 ; Goldberg 1999 ; Liancourt et al. 2005). De plus,<br />
les différents sta<strong>de</strong>s <strong>de</strong> développement <strong>de</strong>s plantes montrent différentes capacités compétitives<br />
et différentes capacités à tolérer la contrainte (Grubb 1977). En effet, il a été montré une<br />
variation <strong>de</strong> la hiérarchie compétitive d’espèces suivant le sta<strong>de</strong> mesuré : la germination, la<br />
croissance ou encore la survie (Howard & Goldberg 2001). Ces différents effets <strong>de</strong>s<br />
interactions compétitives sur les différents sta<strong>de</strong>s <strong>de</strong> développement auront <strong>de</strong>s conséquences<br />
sur les performances <strong>de</strong>s adultes (Fayolle et al. 2009). Cependant, peu d’étu<strong>de</strong>s ont testé les<br />
effets <strong>de</strong>s interactions sur différents sta<strong>de</strong>s <strong>de</strong> développement permettant ainsi <strong>de</strong> décrire le<br />
cycle <strong>de</strong> vie entier <strong>de</strong>s espèces ; alors que comprendre la relative importance <strong>de</strong> chaque<br />
composant <strong>de</strong> fitness est nécessaire pour comprendre la structuration <strong>de</strong>s communautés<br />
végétales (Howard & Goldberg 2001).<br />
17
Des approches utilisant <strong>de</strong>s données démographiques, via le développement <strong>de</strong><br />
modèles prenant en compte les différents composants du cycle <strong>de</strong> vie <strong>de</strong>s plantes comme les<br />
analyses LTRE (Life Table Response Experiments) sont utilisées pour évaluer l’influence <strong>de</strong><br />
la compétition sur les différents taux démographiques (e.g. Gustafsson & Ehrlen 2003;<br />
Fréville & Silvertown 2005). Dans cet objectif, <strong>de</strong>s modèles sont maintenant disponibles et<br />
paramétrisés pour estimer l’effet <strong>de</strong> la compétition entre espèces en conditions naturelles<br />
(Damgaard et al. 2009) représentant ainsi une voie à suivre pour évaluer la compétition in<br />
natura.<br />
Les modèles utilisant <strong>de</strong>s données démographiques sont actuellement développés pour<br />
évaluer la compétition. Ces approches sont prometteuses car celles-ci peuvent être conduites<br />
sous <strong>de</strong>s conditions naturelles et considèrent différentes mesures <strong>de</strong> performances <strong>de</strong>s<br />
plantes. Ce type d’approche a été choisi dans ce travail pour mesurer l’importance <strong>de</strong> la<br />
compétition le long d’un gradient environnemental d’inondation.<br />
4. Les traits fonctionnels : outils d’étu<strong>de</strong> <strong>de</strong>s règles d’assemblage<br />
Les traits fonctionnels représentent <strong>de</strong>s caractéristiques physiologiques,<br />
morphologiques ou phénologiques mesurés à l’échelle <strong>de</strong> l’individu reflétant l’adaptation <strong>de</strong>s<br />
plantes à leur milieu et leur réponse aux facteurs environnementaux (McGill et al. 2006 ;<br />
Violle et al. 2007). Depuis l’article séminal <strong>de</strong> McGill et al. (2006), les traits fonctionnels<br />
prennent une place importante dans l’étu<strong>de</strong> <strong>de</strong> la compréhension <strong>de</strong>s mécanismes <strong>de</strong><br />
coexistence <strong>de</strong>s espèces, <strong>de</strong> structure <strong>de</strong>s communautés et du fonctionnement <strong>de</strong>s écosystèmes<br />
(Lavorel & Garnier 2002). Ils permettent <strong>de</strong> s’affranchir <strong>de</strong> la taxonomie (Lavorel et al.<br />
1997). Ils permettent <strong>de</strong> prédire la réponse <strong>de</strong>s plantes à un filtre abiotique donné comme par<br />
exemple l’inondation (Blom & Voesenek 1996; Casanova & Brock 2000; Fraser & Karnesis<br />
2005), ou même la réponse <strong>de</strong>s plantes à la compétition pour la lumière par exemple (Keddy<br />
& Shipley 1989; Violle et al. 2006) et peuvent donc être considérés comme <strong>de</strong>s <strong>de</strong>scripteurs<br />
<strong>de</strong>s niches fondamentale et réalisée <strong>de</strong>s espèces (McGill et al. 2006 ; Violle & Jiang 2009)<br />
La considération du concept <strong>de</strong> niche dans l’approche fonctionnelle sous-entend que<br />
les différents filtres abiotiques et biotiques opèrent une sélection suivant les valeurs <strong>de</strong>s traits<br />
(Keddy 1992 ; McGill et al. 2006). L’étu<strong>de</strong> <strong>de</strong>s patrons <strong>de</strong> distribution <strong>de</strong>s traits <strong>de</strong>s espèces<br />
18
permettrait <strong>de</strong> déterminer l’importance <strong>de</strong>s différents processus responsables <strong>de</strong> ces patrons et<br />
donc d’étudier les règles d’assemblage <strong>de</strong>s espèces (Weiher & Keddy 1995). Dans ce sens,<br />
ont été introduites les notions <strong>de</strong> convergence et la divergence (MacArthur & Levins 1967).<br />
Face à un filtre abiotique, les espèces sont sélectionnées suivant leur tolérance physiologique<br />
à ces conditions environnementales : théoriquement, les espèces <strong>de</strong>vraient présenter <strong>de</strong>s<br />
valeurs <strong>de</strong> traits similaires en réponse à cette condition environnementale (Weiher & Keddy<br />
1995, 1998). On parle <strong>de</strong> convergence, <strong>de</strong> sous-dispersion ou encore d’« habitat filtering »<br />
(Fig. 7c). Par contraste, <strong>de</strong>ux espèces fonctionnellement proches qui interagissent ne peuvent<br />
pas coexister indéfiniment en raison du concept <strong>de</strong> la « limiting similarity ». Pour coexister et<br />
ainsi réduire l’intensité <strong>de</strong> leur interaction, les espèces doivent présenter une divergence<br />
compétitive <strong>de</strong>s traits d’exploitation <strong>de</strong>s ressources (Weiher & Keddy 1995, 1998), on<br />
s’attend à une sur-dispersion ou <strong>de</strong> « niche-differentiation » (Fig. 7b), permettant aux espèces<br />
coexistantes <strong>de</strong> diminuer l’intensité <strong>de</strong> leur interaction. Comme cela a été précisé dans la<br />
partie 1.2., les filtres agissent simultanément in natura : un patron intermédiaire à ceux <strong>de</strong> la<br />
divergence et convergence fonctionnelle est attendu (Fig. 7d). Pour tester l’effet <strong>de</strong> ces <strong>de</strong>ux<br />
processus déterministes dans la structuration <strong>de</strong>s communautés végétales, les données sont<br />
confrontées à un modèle nul qui considère une équivalence fonctionnelle entre les espèces<br />
(Fig. 7a).<br />
Fig. 7 : Représentation <strong>de</strong>s patrons hypothétiques <strong>de</strong>s traits (extrait <strong>de</strong> Weiher & Keddy<br />
1998). a : équivalence fonctionnelle entre toutes les espèces ; b : effet du filtre biotique<br />
sélectionnant <strong>de</strong>s espèces présentant <strong>de</strong>s valeurs <strong>de</strong> traits dispersés ; c : effet du filtre<br />
abiotique sélectionnant <strong>de</strong>s espèces présentant <strong>de</strong>s valeurs <strong>de</strong> traits similaires ; d : effet<br />
conjugué <strong>de</strong>s filtres abiotiques et biotiques résultant un patron <strong>de</strong> distribution <strong>de</strong>s traits<br />
intermédiaires aux <strong>de</strong>ux précé<strong>de</strong>nts.<br />
19
L’étu<strong>de</strong> <strong>de</strong> la diversité fonctionnelle est centrale dans la compréhension <strong>de</strong>s règles<br />
d’assemblage, car elle permet <strong>de</strong> relier la structure <strong>de</strong>s communautés au fonctionnement <strong>de</strong>s<br />
écosystèmes (Diaz & Cabido 2001 ; Diaz et al. 2007). Elle est définie comme étant l’étendue<br />
et les valeurs <strong>de</strong>s traits <strong>de</strong>s espèces influençant différents aspects du fonctionnement <strong>de</strong>s<br />
écosystèmes (Tilman 2001). La diversité fonctionnelle apparaît prometteuse pour étudier les<br />
règles d’assemblage dans les gradients car elle est influencée par les processus déterministes<br />
(Stubbs & Wilson 2004 ; Fukami et al. 2005 ; Grime 2006). La mesure <strong>de</strong> la diversité<br />
fonctionnelle est d’ailleurs le sujet <strong>de</strong> nombreux travaux récents proposant <strong>de</strong>s métho<strong>de</strong>s<br />
différentes pour la quantifier (e.g. Petchey &Gaston 2002 ; <strong>de</strong> Bello et al. 2009, 2010 ; Mason<br />
et al. 2003 ; Mouillot et al. 2005, 2007 ; Ricotta 2005), qui reste un champ <strong>de</strong> recherche<br />
ouvert.<br />
Les tests <strong>de</strong>s hypothèses <strong>de</strong> convergence et divergence en lien avec les processus<br />
déterministes le long <strong>de</strong> gradients environnementaux a fait l’objet d’étu<strong>de</strong>s récentes (e.g.<br />
Weiher et al. 1998 ; Ackerly & Cornwell 2007 ; Cornwell & Ackerly 2009 ; Jung et al. 2010)<br />
testant ces hypothèses <strong>de</strong> convergence et <strong>de</strong> divergence fonctionnelles face à un modèle nul.<br />
L’avantage <strong>de</strong> cette métho<strong>de</strong> est qu’elle est quantitative car les <strong>de</strong>grés <strong>de</strong> convergence et <strong>de</strong><br />
divergence fonctionnelle sont quantifiés. Globalement, cette métho<strong>de</strong> est intéressante pour<br />
étudier la distribution et l’abondance <strong>de</strong>s espèces car elle lie les stratégies écologiques, les<br />
théories d’assemblage <strong>de</strong>s espèces et la diversité fonctionnelle (McGill et al. 2006 ; Ackerly<br />
& Cornwell 2007 ; Violle & Jiang 2009). Les recherches sont encore en plein développement,<br />
elles nécessitent d’être approfondies car <strong>de</strong>s effets <strong>de</strong> l’échelle d’étu<strong>de</strong> (Weiher et al. 1998 ;<br />
Mokany & Roxburgh 2010), <strong>de</strong>s traits étudiés (Grime 2006 ; Schamp et al. 2008 ; Mokany &<br />
Roxburgh 2010) et <strong>de</strong> la mesure <strong>de</strong> diversité fonctionnelle choisie (Stubbs & Wilson 2004)<br />
sont mis en évi<strong>de</strong>nce, limitant la généralisation <strong>de</strong>s résultats.<br />
20
OBJECTIFS DE LA THESE<br />
L<br />
’approche déterministe visant à caractériser les effets <strong>de</strong>s filtres abiotiques et<br />
biotiques sur la structure et la dynamique <strong>de</strong>s communautés végétales a fait l’objet<br />
<strong>de</strong> nombreuses étu<strong>de</strong>s. Comme mentionné dans l’introduction, les interactions<br />
compétitives occupe une place majeure dans la compréhension <strong>de</strong> la structuration<br />
<strong>de</strong>s communautés végétales.<br />
Néanmoins, les différents points présentés précé<strong>de</strong>mment soulignent <strong>de</strong>s manques<br />
limitant la compréhension <strong>de</strong>s mécanismes agissant sur les patrons d’abondance d’espèces et<br />
<strong>de</strong> structure <strong>de</strong>s communautés le long <strong>de</strong> gradients environnementaux : i) le manque <strong>de</strong><br />
caractérisation fine <strong>de</strong> la variable environnementale majeure influençant les performances <strong>de</strong>s<br />
espèces, ii) le manque <strong>de</strong> distinction entre l’intensité <strong>de</strong> la compétition et l’importance <strong>de</strong> la<br />
compétition limitant la compréhension du rôle <strong>de</strong> la compétition dans la structuration <strong>de</strong>s<br />
communautés le long <strong>de</strong> gradients environnmentaux, iii) le manque <strong>de</strong> variabilité <strong>de</strong>s<br />
méthodologies utilisées pour caractériser l’importance <strong>de</strong> différents filtres environnementaux<br />
variables dans le temps et l’espace, couplant à la fois <strong>de</strong>s approches expérimentales et <strong>de</strong>s<br />
approches utilisant <strong>de</strong>s données récoltées in natura.<br />
suivantes :<br />
Face à ce constat, nous avons donc étudié les <strong>de</strong>ux problématiques <strong>de</strong> recherche<br />
- la quantification <strong>de</strong> la part <strong>de</strong>s interactions compétitives relativement aux<br />
facteurs environnementaux, c’est-à-dire l’importance <strong>de</strong> la compétition, pour<br />
comprendre la structuration <strong>de</strong>s communautés végétales.<br />
- l’évaluation <strong>de</strong> l’interaction <strong>de</strong> différents filtres environnementaux sur la structure<br />
<strong>de</strong>s communautés végétales, caractérisés par <strong>de</strong>s niveaux <strong>de</strong> productivité différentes et<br />
soumises à une variabilité <strong>de</strong>s patrons <strong>de</strong> pluviosité.<br />
Deux type <strong>de</strong> milieux correspondant à <strong>de</strong>ux types <strong>de</strong> climats contrastés ont été choisis: les<br />
prairies humi<strong>de</strong>s du Marais Poitevin et les pelouses xéro-halophiles <strong>de</strong> Camargue. La<br />
première problématique a été traitée en considérant le modèle <strong>de</strong> prairies humi<strong>de</strong>s du Marais<br />
Poitevin. Ces prairies sont productives et fertiles : dans ce contexte, il est attendu que les<br />
interactions entre plantes soient intenses. Les inondations, se produisant <strong>de</strong> manière régulière<br />
chaque année dans ce système (Amiaud et al. 1998). Cette occurrence régulière <strong>de</strong>s<br />
23
inondations permet l’étu<strong>de</strong> <strong>de</strong> l’interaction entre la contrainte abiotique et les interactions<br />
plantes-plantes. Les pelouses xéro-halophiles sont <strong>de</strong>s milieux peu fertiles et peu productifs,<br />
soumis à une forte variabilité <strong>de</strong>s conditions climatiques et en particulier <strong>de</strong> la ressources en<br />
eau : elles feront l’objet d’étu<strong>de</strong> <strong>de</strong> la secon<strong>de</strong> problématique. Cette variabilité <strong>de</strong>s patrons <strong>de</strong><br />
pluviosité a été appréciée via un jeu <strong>de</strong> données pluri-annuelles, en y analysant son poids<br />
respectif sur la structure <strong>de</strong>s communautés ainsi que sur la démographie <strong>de</strong>s espèces pérennes<br />
dominantes par rapport à différentes modalités <strong>de</strong> gestion pastorale.<br />
Ce travail est structuré en trois parties.<br />
La première partie est axée sur l’évaluation <strong>de</strong> la compétition le long d’un gradient<br />
environnemental d’inondation dans les prairies humi<strong>de</strong>s du Marais poitevin. L’étu<strong>de</strong> <strong>de</strong> la<br />
compétition est centrée espèce, et mobilise le concept <strong>de</strong> niche. Différentes méthodologies ont<br />
été appliquées pour étudier la compétition :<br />
- Le premier chapitre présente les patrons <strong>de</strong> variation <strong>de</strong>s <strong>de</strong>ux composantes <strong>de</strong> la<br />
compétition que sont l’intensité et l’importance, et ce en fonction <strong>de</strong> l’intensité <strong>de</strong> la<br />
contrainte à l’échelle <strong>de</strong> l’espèce. En se référant aux définitions <strong>de</strong> chaque composante<br />
<strong>de</strong> la compétition, nous avons testé les prédictions quant à leur patron <strong>de</strong> variation<br />
théorique le long d’un gradient <strong>de</strong> durée et <strong>de</strong> profon<strong>de</strong>urs d’inondation ont été testées.<br />
Cette étape requise pour comprendre la variation <strong>de</strong> chaque composante est pourtant<br />
souvent éludée (Brooker & Kikvidze 2008). Pour ce faire, nous avons fait l’hypothèse<br />
que l’importance <strong>de</strong> la compétition dépend du niveau <strong>de</strong> contraintes subies par les<br />
espèces cibles alors que l’intensité <strong>de</strong> la compétition ne dépend pas l’intensité <strong>de</strong> cette<br />
contrainte.<br />
- Les seconds et troisièmes chapitres concernent l’évaluation <strong>de</strong> l’importance <strong>de</strong> la<br />
compétition dans les patrons d’abondance d’espèces le long d’un gradient<br />
d’inondation, en testant l’hypothèse d’un tra<strong>de</strong>-off entre les capacités compétitives <strong>de</strong>s<br />
espèces et leur tolérance à la contrainte. La <strong>de</strong>scription <strong>de</strong>s niches fondamentales <strong>de</strong>s<br />
espèces est étudiée expérimentalement par <strong>de</strong>s transplantations d’individus à<br />
différentes positions le long du gradient in situ. La comparaison entre les niches<br />
fondamentales et les niches réalisées <strong>de</strong>s espèces permettra d’évaluer le rôle <strong>de</strong> la<br />
compétition dans les patrons d’abondance <strong>de</strong>s espèces (3 ème chapitre).<br />
24
Dans la secon<strong>de</strong> partie, l’importance <strong>de</strong>s différents filtres écologiques est évaluée à<br />
partir du développement <strong>de</strong> modèles utilisant <strong>de</strong>s données démographiques (survie,<br />
colonisation, croissance) reflétant la dynamique <strong>de</strong>s populations étudiées. Les données<br />
récoltées in natura expriment les conditions auxquelles les populations sont réellement<br />
soumises. Ces modèles sont développés dans les <strong>de</strong>ux sites d’étu<strong>de</strong>.<br />
- Le chapitre 4 concerne l’analyse <strong>de</strong> l’effet <strong>de</strong> l’environnement sur les effets<br />
compétitifs d’espèces présentant <strong>de</strong>s distributions différentes le long d’un gradient<br />
d’inondation, ainsi que l’analyse <strong>de</strong>s patrons <strong>de</strong> variation <strong>de</strong> l’importance <strong>de</strong> la<br />
compétition. Le modèle est développé à partir <strong>de</strong> relevés floristiques suivant la<br />
métho<strong>de</strong> <strong>de</strong>s points contact, permettant <strong>de</strong> tenir compte <strong>de</strong>s probabilités <strong>de</strong> survie, <strong>de</strong><br />
colonisation <strong>de</strong>s espèces et <strong>de</strong> leur croissance. Le développement <strong>de</strong> ce type<br />
d’approche permet <strong>de</strong> compléter la quantification <strong>de</strong> l’importance réalisée à partir<br />
d’indices (Kikvidze & Brooker 2010).<br />
- Le chapitre 5 concerne la proposition d’une métho<strong>de</strong> d’analyse du succès écologique<br />
<strong>de</strong>s espèces pérennes. Le modèle, testé sur les pelouses xero-halophyles<br />
méditerranéennes, prédit l’occupation <strong>de</strong> l’espace par ces espèces à partir <strong>de</strong> leurs<br />
probabilités <strong>de</strong> survie et <strong>de</strong> colonisation estimées à partir <strong>de</strong> points contacts. Le<br />
modèle permet d’évaluer l’élasticité <strong>de</strong> ces paramètres démographiques sous<br />
différentes conditions environnementales : la variabilité <strong>de</strong>s patrons <strong>de</strong> pluviosité, la<br />
gestion par le pâturage, pour <strong>de</strong>s communautés végétales soumis à différents niveau <strong>de</strong><br />
stress salins.<br />
La troisième partie vise à comprendre les rôles respectifs <strong>de</strong>s filtres biotiques et<br />
abiotiques dans la répartition <strong>de</strong>s espèces végétales et les patrons <strong>de</strong> végétation et <strong>de</strong><br />
structure <strong>de</strong>s communautés, à la fois dans les prairies humi<strong>de</strong>s du Marais Poitevin et dans<br />
les pelouses xéro-halophiles <strong>de</strong> Camargue.<br />
- Le chapitre 6 a pour problématique la compréhension <strong>de</strong>s règles d’assemblage<br />
responsables <strong>de</strong> la coexistence <strong>de</strong>s espèces le long du gradient d’inondation. Dans<br />
cette étu<strong>de</strong>, les hypothèses <strong>de</strong> d’habitat filtering et <strong>de</strong> niche differentiation sont testées<br />
face à un modèle nul. Ces hypothèses ont été testées en mesurant la distribution <strong>de</strong><br />
<strong>de</strong>ux traits fonctionnels. Ces traits fonctionnels, choisis suivant leur pertinence dans la<br />
25
éponse aux filtres écologiques étudiés, ont été mesurés in situ le long du gradient<br />
d’inondation.<br />
- Le chapitre 7 concerne la détermination <strong>de</strong>s rôles respectifs <strong>de</strong> différents facteurs<br />
environnementaux sur la structure et la composition <strong>de</strong>s communautés végétales<br />
annuelles méditerranéennes. Ces communautés sont soumises notamment à une forte<br />
variabilité inter et intra-annuelle <strong>de</strong>s patrons <strong>de</strong> pluviosité (hétérogénéité <strong>de</strong> la<br />
ressource en eau), à une gestion par pâturage et à une salinité variable du sol. L’effet<br />
du niveau <strong>de</strong> stress sur les réponses au pâturage et à la variabilité <strong>de</strong>s patrons <strong>de</strong><br />
pluviosité. Cette analyse a été réalisée à partir <strong>de</strong> huit années <strong>de</strong> relevés floristiques.<br />
26
MODELES BIOLOGIQUES<br />
D<br />
eux sites ont été étudiés : il s’agit <strong>de</strong>s <strong>de</strong>ux plus gran<strong>de</strong>s zones humi<strong>de</strong>s <strong>de</strong><br />
France : la Camargue et le Marais Poitevin. Ces <strong>de</strong>ux sites présentent <strong>de</strong>s<br />
intérêts écologique et patrimonial exceptionnels, avec notamment une richesse<br />
floristique remarquable s’organisant le long d’une microtopographie, résultant<br />
une variation <strong>de</strong>s conditions environnementales et <strong>de</strong> communautés végétales.<br />
1. La microtopographie, génératrice d’une variété <strong>de</strong> communautés<br />
1.1. Les prairies humi<strong>de</strong>s du Marais Poitevin<br />
Les prairies humi<strong>de</strong>s du Marais Poitevin sont caractérisées par un gradient micro-<br />
topographique d’amplitu<strong>de</strong> d’environ 40-45 cm décrivant un gradient <strong>de</strong> durée d’inondation<br />
(Fig. 8). Les inondations sont récurrentes chaque année, c’est-à-dire qu’elles apparaissent à la<br />
fin <strong>de</strong> l’automne jusqu’au printemps <strong>de</strong> l’année suivante. Malgré cette régularité, les dates <strong>de</strong><br />
mise en eau et <strong>de</strong> fin d’inondations sont néanmoins variables : elles dépen<strong>de</strong>nt <strong>de</strong>s patrons <strong>de</strong><br />
pluviosité. Les conditions d’inondations en hauteur d’eau et en durée le long <strong>de</strong> cette micro-<br />
topographie sont variables : elles peuvent influencer par conséquent l’expression <strong>de</strong>s espèces<br />
suivant leur tolérance physiologique à ces conditions environnementales.<br />
Fig. 8 : Profil micro-topographique durant la saison d’inondation avec la baisse inondée au<br />
centre, inondée jusqu’à six mois par an ; et à gauche les niveaux topographiques les plus hauts<br />
non inondés.<br />
29
Une Analyse Factorielle <strong>de</strong>s Correspondances (AFC) <strong>de</strong> relevés floristiques réalisés le<br />
long <strong>de</strong> cette microtopographie nous révèle l’existence <strong>de</strong> trois communautés végétales (Fig.<br />
9).<br />
Dans les niveaux topographiques les plus hauts est décrit la communauté mésophile<br />
composée majoritairement <strong>de</strong> Lolium perenne, Carex divisa, Bromus commutatus, Cynosurus<br />
cristatus et Hor<strong>de</strong>um secalinum. Cette communauté végétale est décrite dans les replats non<br />
inondés, sauf années exceptionnelles caractérisées par une forte pluviosité.<br />
La communauté méso-hygrophile est décrite au niveau intermédiaire (pentes). Elle<br />
est composée en gran<strong>de</strong> majorité <strong>de</strong> Juncus gerardii, espèce qui peut être associée à <strong>de</strong>s<br />
espèces comme Hor<strong>de</strong>um marinum, Plantago coronopus ou encore Leontodon taraxacoï<strong>de</strong>s.<br />
Cette communauté végétale est soumise à <strong>de</strong>s conditions d’inondation qui sont variables, <strong>de</strong><br />
l’ordre <strong>de</strong> quelques semaines dépendantes <strong>de</strong> la quantité <strong>de</strong>s pluies. Localement, le sol <strong>de</strong><br />
cette communauté végétale peut être salé, une caractéristique qui est en lien avec la nappe<br />
d’imbibition salée dans ce secteur et qui semble favorisée par le piétinement et le pâturage du<br />
bétail (Amiaud et al.1998; Bonis et al. 2005), en conséquence une végétation halotolérante<br />
peut être observée.<br />
La communauté hygrophile est décrite dans les niveaux topographiques les plus bas<br />
ou dépressions inondables. Cette communauté végétale est composée d’Eleocharis palustris,<br />
Juncus articulatus, Agrostis stolonifera, Oenanthe fistulosa ou encore Glyceria fluitans. Les<br />
inondations peuvent durer jusqu’à 6 mois dans ces dépressions, avec une hauteur d’eau au<br />
<strong>de</strong>ssus du sol pouvant atteindre jusqu’à 50 cm.<br />
Nous avons cherché à limiter l’influence <strong>de</strong> la salinité du sol afin d’éviter que ce<br />
facteur ne se surimpose au facteur inondation qui nous intéresse. Pour cela, nous avons<br />
travaillé le long <strong>de</strong> toposéquences où la salinité du sol était faible, salinité mesurée à l’ai<strong>de</strong><br />
d’un conductimètre.<br />
30
Fig. 9 : Analyse Factorielle <strong>de</strong>s Correspondances (AFC) <strong>de</strong> l’ensemble <strong>de</strong>s relevés floristiques<br />
(n=261 relevés). a) Carte factorielle <strong>de</strong>s espèces (le co<strong>de</strong> <strong>de</strong>s espèces est donné en Annexe 1)<br />
b) Carte factorielle <strong>de</strong>s relevés avec la distinction <strong>de</strong>s trois communautés végétales.<br />
La composition en espèces peut varier au sein <strong>de</strong> chacune <strong>de</strong>s trois communautés<br />
végétales (Marion 2010). En effet, certains relevés floristiques apparaissent être<br />
intermédiaires à <strong>de</strong>ux communautés. Une analyse <strong>de</strong> la distribution <strong>de</strong>s espèces le long du<br />
gradient micro-topographique indique que certaines espèces ont <strong>de</strong>s amplitu<strong>de</strong>s <strong>de</strong><br />
distributions larges, d’autres plus restreintes (Fig. 10). La distribution <strong>de</strong>s espèces a été<br />
analysée par l’Outlying Mean In<strong>de</strong>x (OMI) qui est un outil performant pour séparer les niches<br />
réalisées <strong>de</strong>s espèces (Dolé<strong>de</strong>c et al. 2000) et pour déterminer les variables environnementales<br />
déterminantes dans cette ségrégation <strong>de</strong>s niches (Thuillier et al. 2004). OMI permet une<br />
interprétation directe <strong>de</strong> la séparation <strong>de</strong>s niches <strong>de</strong>s espèces car la métho<strong>de</strong> calcule la<br />
déviation <strong>de</strong> la distribution d’une espèce par rapport aux conditions moyennes du milieu.<br />
Ainsi est représenté la position moyenne <strong>de</strong> l’espèce mais également la variabilité <strong>de</strong>s habitats<br />
occupés par chaque espèce, qui est considérée comme l’amplitu<strong>de</strong> <strong>de</strong> la niche réalisée <strong>de</strong>s<br />
espèces.<br />
31
Fig 10: Position <strong>de</strong> la niche réalisée <strong>de</strong>s espèces et <strong>de</strong> la largeur <strong>de</strong> leur niche sur l’axe <strong>de</strong><br />
l’OMI (gradient micro-topographique). Les bars horizontales correspon<strong>de</strong>nt à ± SD et sont<br />
interprétés comme les largeurs <strong>de</strong> niche. Les lignes verticales en bas du graphique<br />
correspon<strong>de</strong>nt à la position topographique <strong>de</strong>s relevés le long du gradient (le co<strong>de</strong> <strong>de</strong>s espèces<br />
est donné en Annexe 1).<br />
Une ségrégation <strong>de</strong>s niches <strong>de</strong>s espèces est décrite le long du gradient micro-<br />
topographique, avec <strong>de</strong>s espèces présentant <strong>de</strong>s amplitu<strong>de</strong>s <strong>de</strong> niche faibles comme Bal<strong>de</strong>llia<br />
ranunculoï<strong>de</strong>s (BALRAN) et Mentha pulegium (MENPUL) dans les zones inondées ou<br />
encore Juncus gerardii (JUNGER), Parapholis strigosa (PARSTR) dans les zones<br />
intermédiaires du gradient. La préférence <strong>de</strong> Bal<strong>de</strong>llia ranunculoï<strong>de</strong>s, Mentha pulegium ou<br />
encore Oenanthe fistulosa (OENFIS) pour les habitats longuement inondés a été démontré<br />
lors d’une précé<strong>de</strong>nte étu<strong>de</strong> visant à prédire les patrons <strong>de</strong> distribution théoriques à l’ai<strong>de</strong> <strong>de</strong><br />
régressions <strong>de</strong>s espèces (Violle et al. 2007). Des espèces ont, en revanche, <strong>de</strong> larges<br />
amplitu<strong>de</strong>s <strong>de</strong> niche réalisée comme par exemple Agrostis stolonifera (AGRSTO), Rumex<br />
conglomeratus (RUMCON) ou Galium <strong>de</strong>bile (GALDEB).<br />
Ces analyses multivariées montrent que l’organisation <strong>de</strong> la végétation se traduit par le<br />
rattachement <strong>de</strong>s espèces à <strong>de</strong>s communautés mais reflétant également un continuum <strong>de</strong> leur<br />
répartition le long du gradient <strong>de</strong> durée d’inondation. Nous sommes donc concrètement au<br />
cœur du consensus actuel <strong>de</strong> l’organisation <strong>de</strong> la végétation le long <strong>de</strong> gradients<br />
32
environnementaux à mi-chemin entre les concepts <strong>de</strong> communauté et <strong>de</strong> continuum (voir<br />
Introduction 2.2). En un endroit donné, il est ainsi possible <strong>de</strong> recenser <strong>de</strong>s espèces euryèces<br />
et sténoèces pour lesquelles la question <strong>de</strong> la variation <strong>de</strong>s capacités compétitives se pose.<br />
1.2. Les pelouses xéro-halophiles <strong>de</strong> Camargue<br />
Tout comme pour les prairies humi<strong>de</strong>s du Marais Poitevin, une micro-topographie est<br />
observée dans les systèmes méditerranéens d’une amplitu<strong>de</strong> maximale <strong>de</strong> 50cm. Le long <strong>de</strong><br />
cette micro-topographie, l’humidité du sol et la salinité varient en raison <strong>de</strong> la présence d’une<br />
nappe souterraine salée à faible profon<strong>de</strong>ur. Deux communautés végétales se développent le<br />
long <strong>de</strong> ce gradient (Fig. 11). Les pelouses strictes (Directive Habitats co<strong>de</strong> 6220-2) occupent<br />
les niveaux topographiques les plus élevés (Fig. 12). Elles sont composées d’une large<br />
proportion d’espèces annuelles : Brachypodium distachyon, Scorpiurus muricatus, Psilurus<br />
aristatus, Evax pygmaea, Filago vulgaris, Galium murale, Euphorbia exigua, Crepis sancta<br />
et quelques espèces pérennes comme Brachypodium phoenicoi<strong>de</strong>s et Crepis vesicaria. Une<br />
végétation plus halotolérante se développe dans les niveaux topographiques inférieurs. Cette<br />
végétation est dominée par les espèces suivantes : Halimione portulacoi<strong>de</strong>s, Bromus<br />
hor<strong>de</strong>aceus, Bupleurum semicompositum, Hymenolobus procumbens, Parapholis filiformis,<br />
Parapholis incurva, Plantago coronopus correspond aux prés salés méditerranéens (Directive<br />
Habitats co<strong>de</strong> 1310-4).<br />
Fig. 11 : Analyse Factorielle <strong>de</strong>s Correspondances (AFC) <strong>de</strong> l’ensemble <strong>de</strong>s relevés<br />
floristiques (n=63 relevés). a) Carte factorielle <strong>de</strong>s espèces (le co<strong>de</strong> <strong>de</strong>s espèces est donné en<br />
Annexe 2) b) Carte factorielle <strong>de</strong>s relevés et distinction <strong>de</strong>s <strong>de</strong>ux communautés végétales.<br />
33
Parmi les espèces sont communes aux <strong>de</strong>ux communautés végétales, certaines<br />
montrent <strong>de</strong>s abondances équivalentes entre communautés (Dactylis hispanica, Geranium<br />
molle, Medicago polymorpha ou Polypogon maritimus) ou <strong>de</strong> forts contrastes d’abondance<br />
entre communautés (Bromus madritensis, Bellis annua ou Plantago lagopus).<br />
Fig. 12 : Pelouses strictes au maximum <strong>de</strong> floraison <strong>de</strong> la pâquerette annuelle Bellis annua,<br />
fin mars-début avril.<br />
2. La caractérisation <strong>de</strong> la variable environnementale d’intérêt<br />
2.1. Les patrons d’inondation dans les prairies humi<strong>de</strong>s du Marais Poitevin<br />
Une caractérisation fine du gradient a été mise en place afin d’acquérir <strong>de</strong>s données<br />
précises sur la durée, l’intensité et la fréquence <strong>de</strong>s inondations. Des piézomètres ont été mis<br />
en place à différents points le long <strong>de</strong> différentes toposéquences (Fig. 13), équipés <strong>de</strong> son<strong>de</strong>s<br />
automatiques enregistrant les battements <strong>de</strong> la nappe d’eau du sol sur une année complète et<br />
sur un pas <strong>de</strong> temps horaire. Ce travail s’accompagne <strong>de</strong> mesures topographiques à l’ai<strong>de</strong> d’un<br />
théodolite mesurant l’altitu<strong>de</strong> en différents points du site (Fig. 14), complétant par la même<br />
occasion un même travail précé<strong>de</strong>mment effectué (Violle et al. 2007) dans le but d’extrapoler<br />
l’intensité <strong>de</strong>s inondations en différents points et <strong>de</strong> réaliser un modèle numérique <strong>de</strong> terrain.<br />
Enfin, nous avons fait appel à <strong>de</strong>s géomètres pour placer nos mesures topographiques dans un<br />
système géo-référencé.<br />
34
Fig. 13 : Mise en place <strong>de</strong>s piézomètres à l’automne 2008 dans une dépression inondable.<br />
Fig. 14 : Station fixe du théodolite servant aux mesures topographiques.<br />
Avec les enregistrements automatiques <strong>de</strong> cinq son<strong>de</strong>s placées le long d’une<br />
toposéquence, il est possible <strong>de</strong> suivre la dynamique <strong>de</strong> la nappe d’eau au cours du temps. Sur<br />
la figure 15, les mesures <strong>de</strong> niveau d’eau réalisées par ces 5 son<strong>de</strong>s sont représentées par <strong>de</strong>s<br />
couleurs différentes : <strong>de</strong> la courbe rouge reflétant les battements <strong>de</strong> la nappe d’eau dans les<br />
niveaux topographiques les plus hauts jusqu’à la courbe bleu foncé reflétant les battements <strong>de</strong><br />
la nappe d’eau dans le point le plus bas du gradient sur la pério<strong>de</strong> <strong>de</strong> janvier à juin 2009. Le<br />
long <strong>de</strong> la toposéquence, les battements <strong>de</strong> la nappe sont globalement synchronisés entre les<br />
cinq points topographiques, avec <strong>de</strong>s diminutions du niveau d’eau et <strong>de</strong>s remontées<br />
synchrones suite à <strong>de</strong>s pluies. Durant le printemps 2009, <strong>de</strong>ux dates <strong>de</strong> remise en eau ont été<br />
observées à quelques jours d’intervalle.<br />
35
Fig. 15 : Hydrographe <strong>de</strong> la saison <strong>de</strong> croissance 2009 montrant la dynamique temporelle <strong>de</strong><br />
la nappe d’eau dans le système mesurée en cinq points d’une toposéquence donnée (du rouge<br />
pour la son<strong>de</strong> placée au niveau du replat non inondé au bleu foncé pour la son<strong>de</strong> placée dans<br />
la dépression inondable). Les fluctuations <strong>de</strong> la nappe d’eau sont synchrones au cours du<br />
temps.<br />
Les inondations provoquent une anoxie au niveau racinaire en raison l’engorgement<br />
du sol. Après la fin <strong>de</strong>s inondations, un stress hydrique peut également être une contrainte<br />
forte pour les plantes. Gowing et al. (1998) ont proposé une métho<strong>de</strong> pour quantifier<br />
l’exposition <strong>de</strong>s plantes à ces <strong>de</strong>ux contraintes en chaque point du gradient : elle consiste en la<br />
détermination <strong>de</strong> <strong>de</strong>ux seuils, le premier correspond au seuil à partir duquel la zone racinaire<br />
commence à <strong>de</strong>venir engorgée (seuil d’engorgement), le second à un seuil à partir duquel les<br />
plantes ressentent le manque d’eau (seuil <strong>de</strong> déficit hydrique). Les <strong>de</strong>ux seuils sont déterminés<br />
suivant les caractéristiques du sol : la porosité, la conductivité hydraulique et le point <strong>de</strong><br />
flétrissement. A partir <strong>de</strong> chaque seuil est déterminé le SEV (Sum Exceedance Value)<br />
représentant le <strong>de</strong>gré à partir duquel la nappe d’eau dans le sol se trouve au <strong>de</strong>ssus du seuil<br />
d’engorgement (SEV for aeration) et en <strong>de</strong>ssous du seuil où le stress hydrique apparaît (SEV<br />
for soil drying). Le « SEV for aeration » mesure donc l’exposition <strong>de</strong>s plantes à la contrainte<br />
anoxie et le « SEV for soil drying » mesure la contrainte <strong>de</strong>s plantes au stress hydrique.<br />
Il a été déterminé que le seuil pour le calcul du « SEV for aeration » se trouve à<br />
19.1cm au <strong>de</strong>ssous <strong>de</strong> la surface du sol et celui pour le calcul du « SEV for soil drying » à 42.2<br />
cm (Fig. 16). Ces <strong>de</strong>ux indices sont calculés durant la saison <strong>de</strong> croissance et sont exprimés en<br />
cm.jour -1 ou en m.semaine -1 . De récentes étu<strong>de</strong>s ont démontré l’utilité <strong>de</strong> ces indices pour<br />
expliquer la ségrégation <strong>de</strong>s niches d’espèces le long d’un gradient hydrologique (Silvertown<br />
36
et al. 1999; Silvertown 2004) car ces mesures sont réalistes <strong>de</strong>s conditions auxquelles les<br />
plantes sont soumises.<br />
Fig. 16 : Hydrographe <strong>de</strong> la saison <strong>de</strong> croissance 2009 montrant la dynamique temporelle <strong>de</strong><br />
la nappe d’eau dans le système et <strong>de</strong> visualiser la pério<strong>de</strong> où les plantes sont exposée à une<br />
anoxie du sol est observée (zone bleue au <strong>de</strong>ssus du seuil référence d’engorgement à -19.1cm)<br />
et la pério<strong>de</strong> où les plantes sont exposées à la sécheresse (zone beige en <strong>de</strong>ssous du seuil<br />
référence <strong>de</strong> déficit hydrique à -42.2 cm).<br />
Grâce à ces mesures topographiques géoréférencées pour lesquelles les SEV ont été<br />
déterminés, il est possible <strong>de</strong> réaliser <strong>de</strong>s cartes prédictives <strong>de</strong>s conditions d’anoxie et <strong>de</strong><br />
déficit en eau, à l’ai<strong>de</strong> <strong>de</strong> métho<strong>de</strong>s d’interpolation spatiale (Fig. 17). Ainsi il est possible par<br />
exemple <strong>de</strong> visualiser les zones où l’intensité <strong>de</strong> l’anoxie est forte <strong>de</strong> celles où elle est faible,<br />
reflétant la micro-topographie.<br />
37
Fig. 17 : Carte prédictive du « SEV for aeration » au cours <strong>de</strong> la saison <strong>de</strong> croissance 2009<br />
réalisée à partir d’une interpolation spatiale. La métho<strong>de</strong> a discriminé 9 intensités <strong>de</strong><br />
contraintes allant <strong>de</strong> 0 cm.jour -1 , anoxie inexistante, à 7.08 cm.jour -1 , anoxie la plus forte dans<br />
cette zone cartographiée.<br />
38
Les gradients d’inondations peuvent se surimposer à d’autres variables<br />
environnementales, elles aussi variant le long <strong>de</strong> gradients environnementaux comme par<br />
exemple la disponibilité en nutriments. Dans notre site d’étu<strong>de</strong>, l’analyse du taux <strong>de</strong><br />
minéralisation <strong>de</strong> l’azote, qui est un proxy <strong>de</strong> la quantité d’azote disponible pour les plantes<br />
(Rossignol 2006), a montré que la disponibilité en azote ne varie pas le long du gradient<br />
d’inondation (Rossignol, unpubl.). Nous n’avons donc pas <strong>de</strong> gradient <strong>de</strong> productivité en plus<br />
du gradient d’inondation.<br />
2.2. Les patrons <strong>de</strong> pluviosité en Camargue<br />
La pluviosité annuelle moyenne est <strong>de</strong> l’ordre <strong>de</strong> 600 mm. Les occurrences <strong>de</strong>s pluies<br />
sont surtout automnales et printanières. Pour caractériser le niveau <strong>de</strong> contraintes subi par les<br />
plantes, la pluviosité - évapotranspiration (P-ETP) est quantifiée : il s’agit d’une mesure qui<br />
reflète la disponibilité <strong>de</strong> la ressource en eau pour les plantes (Scarnati et al. 2009). Le bilan<br />
hydrique est généralement déficitaire dès le début du mois <strong>de</strong> février et ce déficit se poursuit<br />
jusqu’en septembre : par conséquent le bilan hydrique annuel est par lui-même déficitaire. Ce<br />
déficit hydrique entraine <strong>de</strong>s remontées capillaires <strong>de</strong> la nappe salée et ainsi <strong>de</strong>s salinisations<br />
<strong>de</strong> sol en surface.<br />
Les patrons <strong>de</strong> P-ETP montrent une forte variabilité inter-annuelle, avec par exemple<br />
un printemps très sec en 1997 succédant à un printemps très humi<strong>de</strong> en 1996 (Fig. 18). La<br />
variabilité est également intra-annuelle : par exemple le mois <strong>de</strong> mars 2001 fut plus sec que le<br />
mois d’avril 2001.<br />
Fig. 18: Pluviosité - Evapotranspiration mensuelle sur une pério<strong>de</strong> <strong>de</strong> trois mois montrant à la<br />
fois la variabilité inter et intra-annuelle sur une pério<strong>de</strong> <strong>de</strong> 20 ans.<br />
39
Afin d’évaluer le rôle potentiel <strong>de</strong>s conditions hydriques (patrons <strong>de</strong> pluviosité) sur la<br />
structure et la composition <strong>de</strong>s communautés végétales, une expérimentation in situ a été mise<br />
en place à l’hiver 2009 visant à simuler <strong>de</strong>s précipitations printanières et/ou automnales,<br />
possible grâce à l’ajout d’eau sur le terrain.<br />
Des quadrats permanents ont été soumis à quatre traitements : (i) un traitement témoin<br />
non arrosé reflétant les conditions <strong>de</strong> pluviosité naturelles, (ii) un traitement arrosage au cours<br />
du printemps, (iii) au cours <strong>de</strong> l’automne et (iiii) au printemps et en automne. Les résultats ne<br />
seront pas présentés dans ce manuscrit, l’expérimentation est toujours en cours.<br />
40
Partie 1 :<br />
La compétition dans les milieux inondés<br />
43
- Chapitre 1 -<br />
Detecting changes in the intensity and importance of competition<br />
for grasslands species along a flooding gradient.<br />
Soumis à Journal of Vegetation Science<br />
45
Chapitre I<br />
Detecting changes in the intensity and importance of competition for<br />
grasslands species along a flooding gradient<br />
<strong>Amandine</strong> <strong>Merlin</strong> 1, 2 , Jan-Bernard Bouzillé 1 , François Mesléard 2, 3 , Anne Bonis 1<br />
1 UMR CNRS 6553 EcoBio, <strong>Université</strong> <strong>de</strong> <strong>Rennes</strong> 1, Campus Beaulieu, F-35042 <strong>Rennes</strong><br />
Ce<strong>de</strong>x, France<br />
2 Centre <strong>de</strong> recherche La Tour du Valat, Le sambuc, F-13200, France<br />
3 UMR CNRS-IRD 6116 Institut Méditerranéen d’Ecologie et <strong>de</strong> Paléoécologie, <strong>Université</strong><br />
d’Avignon IUT site Agroparc BP 1207, F-84911 Avignon Ce<strong>de</strong>x 09, France<br />
Abstract:<br />
Questions: Do competition importance and competition intensity <strong>de</strong>crease with the intensity<br />
of the species' strain? Do competitor performance and similarity with the competitor predict<br />
competition intensity? Does competition importance increase with competition intensity?<br />
Location: Greenhouse, <strong>Rennes</strong> University, Brittany, France<br />
Methods: Twelve wetland species were grown in monocultures and mixtures with the<br />
presence of one competitor in five experimental flooding treatments that controlled the<br />
duration of flooding and water <strong>de</strong>pth. The intensity of the strain, i.e. the intensity of the<br />
constraint at the species level, was quantified for each species by the log response ratio<br />
(lnRR(strain)). The intensity and importance of competition were quantified for each species<br />
using the LnRR(interaction) and Cimp indices. The functional similarity between the target and<br />
competitor was measured by the Gower similarity in<strong>de</strong>x for Specific Leaf Area (SLA) and<br />
plant height.<br />
Results: The twelve species experienced different strain types and intensities: long and <strong>de</strong>ep<br />
flooding conditions and the absence of flooding. For all species, competition importance was<br />
significantly related to strain intensity. By contrast, no general relationship was found<br />
between competition intensity and strain intensity, except for two species. The functional<br />
similarity between the target species and competitor did not predict competitive interactions.<br />
Together, competition intensity and importance vary wi<strong>de</strong>ly and in<strong>de</strong>pen<strong>de</strong>ntly along the<br />
flooding gradient.<br />
Conclusion: This study emphasizes the need to distinguish competition intensity and<br />
importance because they represent two facets of competition.<br />
Key-words: lnRR(strain), Cimp, lnRR(interaction), functional similarity, wetland species,<br />
strain<br />
Nomenclature: Tutin, T.G. et al. 1964-1980, Flora Europea, Cambridge University Press,<br />
Cambridge.<br />
47
Chapitre I<br />
48
Introduction<br />
Chapitre I<br />
The effect of competition within plant communities is generally measured without<br />
consi<strong>de</strong>ring the effect of the environment on plant success. This has led to a poor<br />
un<strong>de</strong>rstanding of the role of competition along environmental gradients, as both the abiotic<br />
environment and competition are important factors with respect to the structuring of plant<br />
communities (Keddy 1990; Greiner la Peyre et al. 2001; Brooker et al. 2005). The importance<br />
of competition, i.e. the role of competition in plant success relative to the role of the abiotic<br />
factor, is still rarely measured. However, together with the measurement of the intensity of<br />
competition (effect of competition per se), it is required for a better un<strong>de</strong>rstanding of species<br />
abundance patterns and the variation in the plant community structure along environmental<br />
gradients (Violle et al. 2010; Brooker & Kikvidze 2008).<br />
Competition intensity is <strong>de</strong>fined as the reduction of the fitness of a species due to the<br />
presence of neighbors (Wel<strong>de</strong>n & Slauson 1986). The intensity of competition experienced by<br />
the target species is positively related to the performance of its competitor (e.g. Gerry &<br />
Wilson 1995; Corcket et al. 2003; Liancourt et al. 2005; Jung et al. 2009). Along an<br />
environmental gradient, it can therefore be expected that any stress that locally reduces the<br />
performance of the competitor will thus reduce the intensity of the competition with other<br />
species. The pattern of variation in competition intensity along an environmental gradient<br />
may thus be related to the performance of the competitor. The functional approach of<br />
vegetation patterns (Mc Gill et al. 2006; Ackerly & Cornwell 2007) suggests that, in relation<br />
to the principle of limiting similarity (MacArthur & Levins 1967), two species with close<br />
values in some of their key functional traits are expected to experience intense competition<br />
(Johansson & Keddy 1991; Wilson 2007; Zhang et al. 2008). This prediction remains poorly<br />
studied experimentally (Elmendorf & Moore 2007) with some results fitting the expectation<br />
(e.g. Elmendorf & Moore 2007), whereas other results do not (e.g. Resetarits 1995).<br />
Competition importance is <strong>de</strong>fined as the reduction of fitness due to competition<br />
relative to the reduction of fitness by any other process or condition (Wel<strong>de</strong>n & Slauson<br />
1986). The importance of competition will thus correspond to the relative contribution of<br />
competition to the total reduction in the fitness of an individual relative to its physiological<br />
optimum (Wel<strong>de</strong>n & Slauson 1986). We can therefore expect that the <strong>de</strong>viation from the<br />
physiological optimum of a species in response to competition may correctly assess the<br />
importance of competition (Brooker et al. 2005; Jung et al. 2009; Carlyle et al. 2010;<br />
Kikvidze & Brooker 2010; Violle et al. 2010). Accordingly, when the environmental<br />
49
Chapitre I<br />
conditions are very close to the physiological optimum of a species, the proportion of the<br />
fitness reduced by competition is expected to be high. Symmetrically, the reduction of fitness<br />
will be mainly due to environmental conditions in a situation where they are further away<br />
from the physiological optimum of the studied species (Carlyle et al. 2010). It is therefore<br />
expected that there will be a negative relationship at the species level between competition<br />
importance and the intensity of the constraint (e.g. Greiner la Peyre et al. 2001; Gaucherand et<br />
al. 2006).<br />
The aim of this study was to experimentally <strong>de</strong>termine the patterns of variation in<br />
competition intensity and importance along an experimental flooding gradient which ranged<br />
from long floo<strong>de</strong>d conditions up to a never floo<strong>de</strong>d condition. The 12 species studied are<br />
typical of natural wet grasslands, representative of the three plant communities <strong>de</strong>scribed in<br />
natura along a flooding gradient that varies in flooding duration and water <strong>de</strong>pth. In this study<br />
we will: (i) quantify the intensity of stress at the individual level along a flooding gradient, i.e.<br />
the intensity of the strain (Suding et al. 2003; Gross et al. 2010), comparing species growth<br />
un<strong>de</strong>r optimal and non-optimal environmental conditions to assess the variation of the<br />
competition along environmental gradients as advocated by Elmendorf & Moore (2007); (ii)<br />
measure the competition intensity along the experimental flooding gradient for 12 species in<br />
mixture with one generalist competitor using the in<strong>de</strong>x <strong>de</strong>veloped by Hedges et al. (1999);<br />
(iii) measure the importance of competition in the success of the 12 studied species using the<br />
in<strong>de</strong>x <strong>de</strong>veloped by Brooker et al. (2005) by measuring the variation in plant success due to<br />
the presence of competitors relative to the variation in plant success due to the change in the<br />
flooding regime; and (iv) investigate if the functional similarity among the species in mixture<br />
with respect to the Specific Leaf Area (SLA) and plant height predict the variation in the<br />
intensity of their interaction along the flooding gradient, as both traits were found to vary in<br />
response to the flooding conditions (Blom & Voesenek 1996; Mommer et al. 2006; Colmer &<br />
Voesenek 2009) and the biotic environment (Violle et al. 2011).<br />
Four hypotheses were tested: (i) competition intensity varies in<strong>de</strong>pen<strong>de</strong>ntly from the<br />
intensity of strain experienced by the target species; (ii) competition intensity is higher when<br />
the growth performance of the competitor is high, and when the traits of both the target and<br />
competitor species show high similarity; (iii) competition importance increases with the<br />
<strong>de</strong>crease in strain intensity experienced by the target species; and (iv) competition importance<br />
varies in<strong>de</strong>pen<strong>de</strong>ntly with competition intensity.<br />
50
Methods<br />
Plant material<br />
Chapitre I<br />
Wet grasslands of the Marais Poitevin located on the French Atlantic coast (46°28’N,<br />
1°13’W) are characterized by an elevation gradient of approximately 40 to 45 cm, along<br />
which the flooding duration and water <strong>de</strong>pth vary (Violle et al. 2007). Twelve wetland<br />
species, presenting different life forms, were selected according to their field distribution<br />
along the flooding gradient (Violl et al. 2007). Four mesophilous species are typical of upper<br />
locations that are never floo<strong>de</strong>d: Carex divisa, Cynosurus cristatus, Hor<strong>de</strong>um secalinum and<br />
Lolium perenne (one sedge and three grasses). Three meso-hygrophilous species, Juncus<br />
gerardii, Bellis perennis and Leontodon autumnalis (one rush and two forbs), are found on the<br />
intermediate slopes enduring a variable duration of flooding during some weeks per year.<br />
Four hygrophilous species are found in the lower parts of the elevation gradient where<br />
flooding occurs for several months per year: Glyceria fluitans, Juncus articulatus, Mentha<br />
pulegium and Trifolium fragiferum (one grass, one rush and two forbs). In natura, four<br />
species only occur over a very restricted portion of the gradient (C. cristatus, G. fluitans, J.<br />
articulatus and M. pulegium). The seven other species are wi<strong>de</strong>spread over a large portion of<br />
the gradient (personal observation).<br />
Experimental <strong>de</strong>sign<br />
The effect of competition on the studied species was assessed by comparing their<br />
performances in monocultures and in mixtures with A. stolonifera, in five experimental<br />
hydrological treatments. Six tillers of each of the 12 target species were transplanted from the<br />
field to 3-L pots in the beginning of March 2009, with a potting soil corresponding to a<br />
mixture of gar<strong>de</strong>n soil and compost. Tillers of similar size and stage were transplanted.<br />
Monocultures and mixtures were submitted to five experimental treatments representing the<br />
range of flooding duration and water <strong>de</strong>pth observed in natura: (i) a non-floo<strong>de</strong>d treatment<br />
(NF); (ii) a floo<strong>de</strong>d treatment characterized by a water-<strong>de</strong>pth of 10 cm above the soil surface<br />
for a short duration (10-S); (iii) 10 cm above the soil surface for a long duration (10-L); (iv)<br />
20 cm above the soil surface for a short duration (20-S); and (v) 20 cm above the soil surface<br />
for a long duration (20-L). The duration of flooding lasted nine weeks for the short duration<br />
treatment and 13 weeks for the long duration treatment.<br />
51
Chapitre I<br />
Agrostis stolonifera was used as a standard neighbor as it occurs in situ all along the<br />
studied gradient. This species was also consi<strong>de</strong>red as a target species.<br />
The experiment was conducted at the experimental gar<strong>de</strong>n of the University of <strong>Rennes</strong><br />
(France) in outdoor tanks, with four replicate tanks for each flooding treatment. For each<br />
treatment, six replicates per species were randomly assigned to the tanks. Before applying the<br />
experimental treatments, the species were submitted to an acclimatization phase in<br />
waterlogged conditions for one week (from 9-15 March 2009). The water level was then<br />
progressively adjusted to 10 cm and 20 cm according to the treatment and was kept stable<br />
from mid-March up until 4May 2009 for the short duration treatments (10-S and 20-S) and 3<br />
June 2009 for the long duration treatments (10-L and 20-L). At the end of the experiment, the<br />
aboveground biomass was harvested, cleaned, and oven-dried at 70°C for 72 h, and then<br />
weighed.<br />
Plant traits<br />
Specific Leaf Area (SLA), i.e. the ratio of fresh leaf area to leaf dry mass, and plant<br />
height were measured for each individual in monocultures in every experimental treatment.<br />
The functional traits are related to species flooding responses allowing the improvement of<br />
gas diffusion (Blom & Voesenek 1996; Lenssen et al. 1998; Mommer et al. 2004). They are<br />
also related to the species' competitive ability (Weiher et al. 1999), indicating the resource<br />
acquisition strategy of the species. Following Weiher et al. (1999) and Cornelissen et al.<br />
(2001), the SLA was measured on the youngest fully grown leaf in the light using a leaf area<br />
meter (Li-Cor, Lincoln, NE, USA). Plant height was measured as the difference between the<br />
height of the highest photosynthetic leaf and the base of the plant. Height was not measured<br />
for species with roset life forms (e.g. Bellis perennis and Leontodon autumalis).<br />
Calculation of indices approaching strain intensity, competition intensity and competition<br />
importance<br />
The measure of individual stress: strain<br />
Strain was estimated by comparing the biomass of the species in monoculture and in<br />
optimal conditions with the biomass in non-optimal conditions (Suding et al. 2003, Gross et<br />
al. 2010). This was measured by the log response ratio (lnRR), where:<br />
lnRR (strain) = ln (individual biomass production in non-optimal conditions without<br />
neighbors/mean biomass production in optimal conditions)<br />
52
Chapitre I<br />
Negative values for lnRR (strain) indicate that the growth performance of the target species is<br />
<strong>de</strong>pressed by the environmental conditions. For each species, the optimal treatment<br />
correspon<strong>de</strong>d to the experimental condition in which survival and high biomass production<br />
were the highest.<br />
Intensity of competition<br />
The intensity of competition was characterized by the lnRR (interaction) in or<strong>de</strong>r to<br />
quantify the reduction of individual performances due to the presence of neighbors (Hedges et<br />
al. 1999), where:<br />
lnRR (interaction) = ln (individual biomass production with neighbors/mean biomass<br />
production without neighbors)<br />
A negative value for the log response ratio indicates competition and a positive value<br />
indicates facilitation.<br />
Importance of competition<br />
The in<strong>de</strong>x of competition importance Cimp (Brooker et al. 2005) was used to quantify<br />
the importance of competition relative to the impact of all other environmental factors, where:<br />
Cimp= (Biomasscomp-Biomasstarget)/(MaxBiomasstarget-y)<br />
Biomasscomp is the probability of survival of the target species in the presence of the<br />
competitor A. stolonifera, Biomasstarget is the probability of survival without a competitor,<br />
MaxBiomass is the maximum value of the probability of survival without a competitor among<br />
the hydrological treatments and y is the smallest value of either Biomasscomp or Biomasstarget.<br />
This in<strong>de</strong>x varies between -1 and +1. The contribution of competition relative to the<br />
hydrological conditions increases when Cimp tends toward -1. The contribution of the abiotic<br />
factor relative to competition increases when Cimp tends toward +1.<br />
Functional similarity between the target species and neighbor<br />
The functional similarity was measured by the Gower similarity in<strong>de</strong>x (Gower 1971),<br />
which is calculated as follows:<br />
with<br />
53
Chapitre I<br />
where n= number of functional traits and x = the functional trait. sijk is a measure of<br />
disagreement between species j and k, the competitor A. stolonifera, for variable i. This in<strong>de</strong>x<br />
was calculated for the SLA and height for each pair of the 12 species used as target species<br />
and A. stolonifera, in each replicate and in each experimental treatment. This in<strong>de</strong>x varies<br />
between 0 and 1. A value of 0 indicates a strong similarity between the target species and the<br />
competitor, whereas a value of 1 indicates no similarity in the trait value among the two<br />
species (Podani & Schmera 2006). To avoid an effect of the similarity in<strong>de</strong>x chosen, the<br />
functional similarity was also calculated with the Eucli<strong>de</strong>an distance which is more<br />
commonly used in the literature.<br />
Data analyses<br />
For all species and all water treatments, the tank effect (n=4 per treatment) was not<br />
significant (two-way ANOVA, P>0.05): accordingly, all repetitions were thereafter combined<br />
for analysis.<br />
Effect of the hydrological treatments<br />
Strain – A one-way ANOVA was run for each species to test whether the strain intensity<br />
was different between the experimental treatments, followed by post-hoc Tukey HSD tests<br />
after verifying normality. The effect of strain on the aboveground biomass production of the<br />
plant-target was investigated using linear regression, taking all the experimental treatments<br />
into account.<br />
Competition intensity – A one-way ANOVA was run for each species to test whether the<br />
competition intensity varied between the experimental treatments, followed by post-hoc<br />
Tukey HSD tests after verifying normality. Moreover, two-way ANOVAs were performed to<br />
test the effect of the experimental treatments and the effect of the species on competition<br />
intensity and competition importance. The two-way ANOVA showed a significant interaction<br />
between the species x hydrological treatment term, indicating a shift in the competitive<br />
hierarchy among the species with hydrological treatments (Lenssen et al. 2004).<br />
Effect of strain intensity on competition intensity and importance<br />
The relationships between, on one hand, competition intensity and importance and, on<br />
the other hand, strain intensity were investigated for each of the 12 species using linear<br />
54
Chapitre I<br />
regressions. Because some experimental treatments impact species survival to various extents,<br />
the number of replicates varied between species, and was low for B. perennis, C. divisa and C.<br />
cristatus: it was not possible to use this method for these three species.<br />
Effect of neighbor performances on competition<br />
The relationships between the aboveground biomass of the neighbor A. stolonifera and<br />
both competition intensity and importance were investigated using linear regressions. These<br />
relationships were first tested in<strong>de</strong>pen<strong>de</strong>ntly for each species and then a second time for all<br />
species combined.<br />
Relationship between competition intensity and importance<br />
The relationship between competition importance and intensity was investigated using<br />
linear regression. This relationship was first tested in<strong>de</strong>pen<strong>de</strong>ntly for each species, with the<br />
dataset including all experimental treatments together and then a second time combining all<br />
species and all experimental treatments together.<br />
Relationship between competition intensity and the <strong>de</strong>gree of similarity with the<br />
competitor<br />
The relationship between the <strong>de</strong>gree of similarity between the neighbor and target<br />
species (measured for SLA and plant height) and competition intensity was investigated for<br />
each species separately using linear regressions.<br />
Statistical tests were calculated with the software R (R version 2.7.2., 2008, R<br />
Foundation for Statistical Computing, Vienna, Austria) and the VEGAN package (Oksanen et<br />
al. 2008) was used to calculate Gower’s in<strong>de</strong>x.<br />
Results<br />
Differential patterns of strain<br />
The 12 species showed contrasted strain intensity <strong>de</strong>pending on the hydrological<br />
conditions (Fig. 1). The range of tolerance to the flooding treatments varied strongly between<br />
the species and <strong>de</strong>pen<strong>de</strong>d on the plant community they are related to in situ. Three of the four<br />
55
Chapitre I<br />
mesophilous species did not survive the long flooding duration treatment (C. cristatus, L.<br />
perenne un<strong>de</strong>r the 10-L and 20-L treatments and C. divisa un<strong>de</strong>r the 20-L treatment;<br />
Appendix S1) and H. secalinum was highly constrained by long flooding. As for the meso-<br />
hygrophilous species, various levels of tolerance to flooding were observed: from no survival<br />
(B. perennis, Appendix S1), growth performance very strongly diminished (L. autumnalis) or<br />
only slightly impacted (J. gerardii). Among the four hygrophilous species, the growth of three<br />
species (G. fluitans, J. articulatus and M. pulegium) was constrained by the absence of<br />
flooding whereas their survival was only slightly impacted by all the experimental treatments<br />
(Appendix S1). One hygrophilous species (T. fragiferum) was found to be highly constrained<br />
by a long duration of flooding.<br />
Fig. 1: Strain intensity, evaluated by biomass measurements, for the species in each floo<strong>de</strong>d<br />
treatment. Similar letters indicate no significant differences in the mean between the floo<strong>de</strong>d<br />
treatments from the one-way ANOVA analysis followed by Tukey HSD tests. The optimal<br />
treatment chosen for the calculation of strain was different according to the species: for most<br />
species, this was the non-floo<strong>de</strong>d treatment (NF); for A. stolonifera and M. pulegium: 10-S;<br />
for J. aticulatus: 10-L; for G. fluitans: 20-L.<br />
56
Intensity of competition along the experimental gradient<br />
Chapitre I<br />
Interaction with A. stolonifera negatively impacted the survival rate for all species, but<br />
to varying extents, <strong>de</strong>pending on the species and the experimental treatment (Appendix S2).<br />
For all species combined together, competition intensity was positively and linearly related to<br />
the biomass of the neighbor A. stolonifera (F=20.1; P
Chapitre I<br />
Competition intensity was not significantly related to the strain intensity experienced<br />
by the target species for ten of the 12 species studied. By contrast, for A. stolonifera,<br />
competition intensity increased with the strain intensity whereas for T. fragiferum, the reverse<br />
pattern was found (Fig 3).<br />
Fig. 3: Negative relationships between strain intensity for the target species and competition<br />
intensity for T. fragiferum and positive relationship for A. stolonifera- no significant<br />
relationships were found for all nine other species.<br />
Functional similarity did not predict the intensity of competition<br />
Competition intensity was not significantly related to the similarity between the target<br />
species and the competitor, except for C. cristatus (F=8.93; adjusted r²= 0.665; P=0.05) and<br />
T. fragiferum (F=9.78; adjusted r²= 0.37; P=0.007) which were similar to A. stolonifera in<br />
terms of plant height (data not shown). Only these two significant regressions were<br />
highlighted with the measure of similarity using the Eucli<strong>de</strong>an distance (data not shown).<br />
Competition importance along the experimental gradient<br />
The importance of competition varied between the treatments and species (two-way<br />
ANOVA; F=3.638; P
Chapitre I<br />
Fig. 4: Contribution of the competition compared to hydrological conditions (importance of<br />
competition): increased with the reduction of the strain intensity. The regression was<br />
significant at 5%. Not studied for B. perennis, C. cristatus and C. divisa due to not enough<br />
replicates (low survival rate).<br />
No relationship between competition intensity and importance<br />
For all species together, competition intensity was not related to competition<br />
importance (F=0.733; P>0.05; data not shown).<br />
Discussion<br />
The main purpose of this study was to investigate the variation in the intensity and<br />
importance of competition in relation to the intensity of strain that each species sustained. As<br />
expected, competition intensity was not found to vary with the strain intensity for ten of the<br />
12 studied species. Competition intensity appears to be strongly related to the biomass of the<br />
competitor, with no effect of the trait similarity among the target and competitor, with only<br />
one exception (plant height similarity between A. stolonifera and T. fragiferum). By contrast<br />
and as expected, competition importance increases when the strain intensity of strain<br />
diminishes. It could be conclu<strong>de</strong>d that competition intensity and importance vary<br />
in<strong>de</strong>pen<strong>de</strong>ntly along the flooding gradient.<br />
59
Species specific strain<br />
Chapitre I<br />
Fundamental niches regarding flooding conditions were found to be contrasted among<br />
the 12 species studied, as <strong>de</strong>monstrated for other wetland flora by Shipley et al. (1991),<br />
Baruch (1994) and Jung et al. (2009). The success of species related to mesophilous and<br />
meso-hygrophilous communities appears to be negatively impacted by long duration flooding<br />
and water <strong>de</strong>pth. As expected, hygrophilous species are rather constrained by the absence of<br />
flooding, except T. fragiferum.<br />
While the main contrasts with respect to flooding tolerance occurred among species<br />
connected to different plant communities, our results <strong>de</strong>monstrate that, within a given<br />
community type (i.e. the mesophilous, meso-hygrophilous or hygrophilous community)<br />
species could however show contrasted <strong>de</strong>grees of tolerance to hydrological treatments and<br />
strain appears to be highly species-specific (Gross et al. 2010). The greater tolerance of C.<br />
divisa to experimental flooding compared to the other mesophilous species and of J. gerardii<br />
compared to the other meso-hygrophilous species fits well with their occasional occurrence in<br />
situ in floo<strong>de</strong>d environments. Similarly, the low tolerance to long duration flooding of C.<br />
cristatus fits well with their distributions, which are restricted to non-floo<strong>de</strong>d locations<br />
(Violle et al. 2007).<br />
Competition importance related to the strain intensity of the target species<br />
The importance of competition <strong>de</strong>creases with the increase in strain intensity that each<br />
species sustained, i.e. either the absence of flooding or exposure to a long duration of<br />
flooding; this was observed for all the species studied regardless of their life forms (grasses,<br />
sedges or forbs). This supports the findings of other studies focused on other types of<br />
constraints: the level of productivity (Brooker et al. 2005; Gaucherand et al. 2006) with either<br />
disturbance (Carlyle et al. 2010) or soil salinity (Greiner la Peyre et al. 2001). The<br />
relationship between competition importance and strain intensity for a species thus appears to<br />
be a general one among the species and types of constraints. This was actually expected as, by<br />
<strong>de</strong>finition, the importance of competition represents the balance between the effect of<br />
competition and the effect of the abiotic factor (strain) on species performance. Our findings<br />
<strong>de</strong>monstrate that when species are close to their optimal conditions (the absence of flooding<br />
60
Chapitre I<br />
or a long duration of flooding), competition has a great role in explaining their ecological<br />
success.<br />
Intensity of competition along experimental gradient<br />
As we hypothesized, competition intensity appears unrelated to the strain intensity<br />
experienced by the species, for all studied species except T. fragiferum. The intensity of<br />
competition measured the effect of the presence of competitors per se. The performance of the<br />
generalist competitor, A. stolonifera, should then predict the intensity of competition, and this<br />
was in fact what was found for all the species studied. The performance of the competitor A.<br />
stolonifera may directly impact the plant target performance by shading effect or indirectly,<br />
by reducing the nutrient resource availability. Accordingly, the similarity among plants in<br />
mixture with respect to the SLA and plant height similarity was expected to predict the<br />
competition intensity, as these two functional traits constitute a good proxy of the resource<br />
uptake and light capture ability of the plants (Weiher et al. 1999; Lavorel & Garnier 2002;<br />
Garnier et al. 2004).<br />
However, for ten of the 12 studied species, there does not appear to be any significant<br />
relationship between functional similarity in those two traits and the intensity of their<br />
interaction with A. stolonifera. Only C. cristatus and T. fragiferum showed a <strong>de</strong>crease in their<br />
competitive response with increasing trait similarity with A. stolonifera with respect to plant<br />
height. This does not invalidate the limiting similarity hypothesis as only two traits were<br />
consi<strong>de</strong>red in this study given that the species niche is multidimensional, with a set of traits<br />
associated to each dimension of the niche. Moreover, the duration of the experiment may have<br />
also been too short for such a pattern to emerge as reported in other experiments (Resetarits<br />
1995; Perkins et al. 2007; Wang et al. 2010).<br />
The pattern in competition intensity across the hydrological treatments varied between<br />
the species. The intensity of competition was constant among the treatments for a first group<br />
of species (i.e. L. perenne, C. divisa, B. perennis, G. fluitans and J. articulatus). This kind of<br />
consistency in the intensity of competition for species has already been reported along<br />
productivity gradients (Brooker et al. 2005; Gaucherand et al. 2006; Carlyle et al. 2010). This<br />
constant pattern in competition intensity across a gradient could hi<strong>de</strong> a switch in the balance<br />
between shoot and root competition (Tilman 1988), indicating that more work is required to<br />
distinguish above-ground and below-ground competition intensity in our system. For another<br />
group of species (i.e. H. secalinum, J. gerardii and M. pulegium), the intensity of competition<br />
61
Chapitre I<br />
varied across the hydrological treatments, with different patterns specific to each species.<br />
Such variety in a specific response pattern was already reported by Wilson & Tilman (1995)<br />
and Jung et al. (2009). In<strong>de</strong>ed, the competitive response, which was consi<strong>de</strong>red here along the<br />
experimental treatments, was already reported to vary wi<strong>de</strong>ly according to the environment<br />
and species i<strong>de</strong>ntity (Greiner Peyre et al. 2001; Brooker et al. 2005; Jung et al. 2009; Carlyle<br />
et al. 2010).<br />
Importance and intensity: two facets of competition<br />
Numerous studies have focuses on how competition varies along gradients, which has<br />
also led to much confusion with regards to its measurement (Brooker et al. 2005; Brooker &<br />
Kikividze 2008). Our study helps clarify the expectations about the variation of competition<br />
along environmental gradients by showing that, following Greiner la Peyre et al. (2001),<br />
Gaucherand et al. (2006), Lamb & Cahill (2008), Zhang et al. (2008) and Lamb et al. (2009),<br />
competition intensity and importance varied in<strong>de</strong>pen<strong>de</strong>ntly from each other along the<br />
experimental gradient. Our study, like the re-analysis of Rea<strong>de</strong>r et al.'s data (1994) by<br />
Brooker et al. (2005), also shows that competition intensity and competition importance<br />
correspond to two different facets of competition.<br />
On one hand, the importance of competition measures how much competition shapes<br />
the structure of plant communities. On the other hand, the intensity of competition reflects the<br />
success of species' strategies to coexist with competitors in a given environment and then<br />
measure the species' competitive ability. Lamb et al.'s study (2009) provi<strong>de</strong>s clarification on<br />
the distinction between the intensity and importance of competition. Based on a mesocosm<br />
study controlling the level of fertility, they showed that, even if root competition is intense<br />
between species in mixture, such negative interactions do not directly shape the plant<br />
community, i.e. the do not directly explain the structure of their communities.<br />
Conclusion<br />
Until recently, the shaping role of competition along gradients was generally assessed<br />
through the measurement of competitive hierarchy changes along environmental gradients<br />
(e.g. Bertness 1991; Gau<strong>de</strong>t & Keddy 1995; Costa et al. 2003; Lenssen et al. 2004;<br />
Bartelheimer et al. 2010). This approach was also used in our study. A recent and<br />
62
Chapitre I<br />
complementary approach relies on the recent <strong>de</strong>velopment of indices for measuring the<br />
importance of competition (Brooker et al. 2005; Kikvidze & Armas 2010; Seifan et al. 2010).<br />
Moreover, competition mo<strong>de</strong>ls have been <strong>de</strong>veloped that take variation among the different<br />
stages of the plant life-cycle into account (Frekleton et al. 2009; Damgaard & Fayolle 2010),<br />
which may be differently affected by competition (Howard & Goldberg 2001). This<br />
information is nee<strong>de</strong>d in or<strong>de</strong>r to predict the importance of competition on population<br />
dynamics during a period of time that is as close to reality as possible rather than in a short-<br />
term experiment.<br />
It has become clear that community ecologists must now focus on both components of<br />
competition, i.e. its intensity and importance, especially now that the novel methodological<br />
tools are available for this kind of work.<br />
Acknowledgements<br />
The authors would like to thank the persons who provi<strong>de</strong>d technical support for this study, in<br />
particular Alwin Bleomelen for his contribution toward data collection. This work is a<br />
contribution to GDR 2574 ‘TRAITS’. This study was supported by a doctoral research grant<br />
from the Ministère <strong>de</strong> l’Enseignement Supérieur et <strong>de</strong> la Recherche.<br />
63
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Jung, V., Mony, C., Hoffmann, L. & Muller, S. 2009. Impact of competition on plant<br />
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competition – a reply to Freckleton et al. (2009). Journal of Ecology, 98: 719–724.<br />
Lamb, E.G. & Cahill, F.J. 2008. When Competition Does Not Matter: Grassland Diversity<br />
and Community Composition. The American Naturalist, 171(6): 777-787.<br />
Lamb, E.G., Kembel, S.W. & Cahill, J.F. 2009. Shoot, but not root, competition reduces<br />
community diversity in experimental mesocosms. Journal of Ecology, 97: 155–163.<br />
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functioning from plant traits: revisiting the Holy Grail. Functional Ecology, 16: 545- 556<br />
Lenssen, J.P.M., Ten Dolle, G.E. & Blom, C. 1998. The effect of flooding on the recruitment<br />
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Lenssen, J.P., van <strong>de</strong> Steeg, H.M. & <strong>de</strong> Kroon, H. 2004. Does disturbance favour weak<br />
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Mommer, L., Lenssen, J.P., Huber, H., Visser, E.J. & D.E. Kroon, H. 2006. Ecophysiological<br />
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Oksanen, J., Kindt, R., Legendre, P., O'Hara, B., Simpson, GL., Solymos, P., Stevens, MHH.<br />
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Podani, J. & Schmera, D. 2006. On <strong>de</strong>ndrogram-based measures of functional diversity. Oikos<br />
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Zhang, J., Cheng, G., Yu, F., Kräuchi, N. & Li, M.H. 2008. Intensity and importance of<br />
competition for a grass (Festuca rubra) and a legume (Trifolium pratense) vary with<br />
environmental changes. Journal of Integrative Plant Biology, 50: 1570–1579.<br />
Supporting information<br />
Appendix S1. Species survival along the flooding gradient without competitors<br />
Appendix S2. Species survival along the flooding gradient with competitors<br />
Appendix S3. Competitive hierarchy along the flooding gradient<br />
Supporting information<br />
Appendix S1<br />
Patterns of species survival without a competitor indicated that survival <strong>de</strong>creased with an<br />
increase in water <strong>de</strong>pth and duration for ten species. The survival rate was constant along the<br />
experimental treatments for M. pulegium and G. fluitans.<br />
67
Appendix S2<br />
Chapitre I<br />
Species survival was reduced in the presence of a competitor (A. stolonifera), especially for L.<br />
perenne, C. divisa, H. secalinum and T. fragiferum.<br />
Appendix S3<br />
The competitive hierarchy changed across the hydrological treatments: the species position<br />
varied between the treatments. In the non-floo<strong>de</strong>d treatments, the hierarchy of the species<br />
position was as follows: T. fragiferum, C. divisa, C. cristatus, J. gerardii, M. pulegium, L.<br />
perenne, G. fluitans, L. autumnalis, H. secalinum, B. perennis, A. stolonifera and J.<br />
articulatus.<br />
In the 10-S treatment, the hierarchy was: C. divisa, G. fluitans, B. perennis, J. articulatus, T.<br />
fragiferum, L. autumnalis, M. pulegium, A. stolonifera, H. secalinum, J. gerardii, L. perenne<br />
In the 10-L treatment: T. fragiferum, L. autumnalis, M. pulegium, G. fluitans, J. articulatus, J.<br />
gerardii, A. stolonifera.<br />
In the 20-S treatment, the hierarchy was: M. pulegium, G. fluitans, L. autumnalis, C. divisa,<br />
H. secalinum, J. gerardii, A. stolonifera, B. perennis, J.articulatus, T. fragiferum, L. perenne<br />
In the 20-L treatment: L. autumnalis, G. fluitans, T. fragiferum, M. pulegium, J. gerardii, J.<br />
articulatus, H. secalinum, A. stolonifera.<br />
68
- Chapitre 2 -<br />
Fundamental niche versus Realized niche: Assessment of the<br />
importance of competition along a flooding gradient.<br />
Part 1: Fundamental niches of twelve wetland species in response to the<br />
variation of flooding conditions.<br />
En préparation<br />
71
Chapitre II<br />
Fundamental niche versus Realized niche: Assessment of the importance of<br />
competition along a flooding gradient.<br />
Part 1: Fundamental niches of twelve wetland species in response to the<br />
variation of flooding conditions.<br />
<strong>Amandine</strong> <strong>Merlin</strong> 1,2 , Jan-Bernard Bouzillé 1 , François Mesléard 2,3 & Anne Bonis 1<br />
1 UMR CNRS 6553 EcoBio, <strong>Université</strong> <strong>de</strong> <strong>Rennes</strong> 1, Campus Beaulieu, F-35042 <strong>Rennes</strong><br />
Ce<strong>de</strong>x, France<br />
2 Centre <strong>de</strong> recherche La Tour du Valat, Le sambuc, F-13200, France<br />
3 UMR CNRS-IRD 6116 Institut Méditerranéen d’Ecologie et <strong>de</strong> Paléoécologie, <strong>Université</strong><br />
d’Avignon IUT site Agroparc BP 1207, F-84911 Avignon Ce<strong>de</strong>x 09, France<br />
Abstract:<br />
Stress is a fuzzy notion in literature concerning its characterization and its<br />
quantification: biomass measurements are often used to <strong>de</strong>fine the stress but they still appear<br />
insufficient. In this study, survival and biomass data are paired to quantify the intensity of the<br />
stress at individual level, i.e. strain, along a flooding gradient and then <strong>de</strong>termine whether<br />
fundamental niches of species, presenting different distribution patterns along the gradient,<br />
are contrasted.<br />
An in situ removal experiment was run along a flooding gradient: twelve wetland<br />
species were grown at 6 locations along the gradient and in monocultures. Analyses of<br />
survival and biomass patterns using the log response ratio (strain), GLM and Kaplan-Meier<br />
analysis, allow a precise <strong>de</strong>scription of fundamental niches. Shoot elongation and Specific<br />
Leaf Area (SLA) are used to appreciate species responses to the range of conditions.<br />
Fundamental niches differed significantly amongst species along the flooding gradient.<br />
Two kinds of constraints were <strong>de</strong>scribed: the increase in duration of flooding (for 9 species)<br />
and the absence of flooding (for 2 species). Only one species was not affected by the range of<br />
environmental conditions. Species success was generally more related to the length of shoot<br />
than to SLA.<br />
Our methods pairing survival and biomass data, few used in literature, quantified<br />
precisely the strain and the fundamental niche of species along the flooding gradient, useful to<br />
un<strong>de</strong>rstand species abundance patterns in natura.<br />
Key-words: strain, survival, biomass, SLA, shoot elongation<br />
73
Chapitre II<br />
74
Introduction<br />
Chapitre II<br />
The concept of stress is not precise and leads to some <strong>de</strong>bates and controversies<br />
between authors (Elmendorf & Moore 2007), reflecting the difficulty to comprehend it: the<br />
discussion between Körner (2003, 2004) and Lortie (2004) is certainly the most well known.<br />
For the former, the term “stress” is excessively used. He disagrees with the i<strong>de</strong>a that life<br />
conditions are often stressful for organisms, expressed as the “imperfect world” by Lortie et<br />
al. (2004). The lack of precision in literature on the study scale (species versus community)<br />
and on the kind of stress (resource versus non-resource) may be responsible for this<br />
confusion.<br />
In spite of the proposition of Wel<strong>de</strong>n & Slauson (1986) to differentiate the “stress” at the<br />
community level and the “strain” at the species level, this distinction was used seldom since<br />
Suding et al. (2003) followed by Gross et al. (2010). Their proposition to quantify the strain<br />
using controlled experiments un<strong>de</strong>r highly favorable conditions represents an original way to<br />
<strong>de</strong>termine the constraint at the species level and then characterize more precisely fundamental<br />
niches of species, quantitatively comparable between each others. Moreover, the concept of<br />
niche was poorly used these last years, although so essential to un<strong>de</strong>rstand processes acting on<br />
species success (Silvertown 2004). Körner’s and Lortie’s points of view may thus be tested<br />
through the quantification of strain.<br />
Along a flooding gradient, two hydrological stresses may be observed at the species<br />
level. In lower parts of the gradient, waterlogging which is a non-resource stress, leads to a<br />
reduction of oxygen diffusion in the root zone and may be paired with submergence affecting<br />
light quality and quantity available for species; whereas drought, a resource stress, occurs in<br />
higher parts of the gradient (Blom & Voesenek 1996; Braendle & Crawford 1999; Casanova<br />
& Brock 2000; Barber et al. 2004; Bartelheimer et al. 2010).<br />
Species responses to the variations of hydrological conditions are studied a lot. The<br />
production of aerenchyma and of adventitious roots was largely <strong>de</strong>scribed: these<br />
morphological changes facilitate gas exchanges among individuals un<strong>de</strong>r oxygen-<strong>de</strong>ficient<br />
conditions (Wample & Reid 1975; Laan et al. 1989; Baruch 1994; Blom & Voesenek 1996;<br />
Grimoldi 1999; Bouma et al. 2000; Visser et al. 2000; Insausti et al. 2001). Besi<strong>de</strong>s the<br />
difficulty to measure these morphological changes, these responses are observed along a wi<strong>de</strong><br />
range of hydrological conditions, i.e. from short durations of waterlogged conditions to <strong>de</strong>ep<br />
and prolonged submerged conditions (Colmer & Voesenek 2009): they are not sufficient to<br />
distinguish and explain the effects of waterlogging and submergence on species<br />
75
Chapitre II<br />
performances. In addition, other plant traits are well known to express species adaptations to<br />
variations of hydrological conditions: the shoot elongation and the Specific Leaf Area (SLA)<br />
(Blom & Voesenek 1996; Loreti & Oesterheld 1996; Clevering 1998; Lenssen et al. 1998;<br />
Insausti et al. 2001; Mommer et al. 2004; Voesenek et al. 2004; Violle et al. 2011), which<br />
respond strongly to submergence and allow the distinction between the effects of waterlogged<br />
and floo<strong>de</strong>d conditions (Colmer & Voesenek 2009).<br />
Functional traits can be used to predict species performance, measured either by<br />
biomass production, survival or reproduction. However, these plant traits were frequently<br />
related to vegetative biomass (Wright & Westoby 2001; Huber et al. 2009).When SLA and<br />
plant height are related to vegetative biomass, it is not easy to disentangle a response of a<br />
variation of hydrological conditions to an allometric relationship among individuals. On the<br />
other hand, patterns of survival, representing the first measure of species tolerance to a<br />
constraint (Lenssen et al. 1998; Lenssen et al. 2004; Mommer et al. 2006), are poorly used<br />
because of the difficulty to follow plant across time (Violle & Jiang, 2009). Nevertheless, the<br />
relationship between plant survival and plant traits is essential to un<strong>de</strong>rstand how variations of<br />
traits act on species success and thus consi<strong>de</strong>r plant traits as surrogate of species performances<br />
(Violle & Jiang 2009).<br />
The objective of this study is to <strong>de</strong>termine precisely the fundamental niches of twelve<br />
wetland species characterized by in situ different abundance patterns along a flooding<br />
gradient. Few studies have sought to characterize finely fundamental niches of species by<br />
quantifying the environmental gradient and using both different measures of performance that<br />
are the analyses of survival and strain patterns though biomass measurements. SLA and shoot<br />
elongation will be measured to test whether they can be substituted to species performances.<br />
We aimed particularly at answering the following questions:<br />
1) Do species, characterized by contrasted in situ patterns, present contrasted fundamental<br />
niches along the flooding gradient?<br />
2) Are SLA and shoot elongation linked to species success appreciated by plant survival<br />
and plant biomass?<br />
76
Material and methods<br />
Plant material<br />
Chapitre II<br />
Twelve wetland species, collected from the floo<strong>de</strong>d meadow of the Marais Poitevin on<br />
the French Atlantic coast (46°28’N; 1°13’W), were selected according to their distribution<br />
patterns along an elevation gradient <strong>de</strong>scribing a flooding gradient, along which duration of<br />
flooding and water <strong>de</strong>pth vary (Violle et al. 2007) (Table 1). Three compartments were<br />
<strong>de</strong>scribed along the elevation gradient: the higher parts which were never floo<strong>de</strong>d and<br />
characterized by the presence of mesophilous species Carex divisa, Cynosurus cristatus,<br />
Hor<strong>de</strong>um secalinum and Lolium perenne; the intermediate slopes enduring a variable duration<br />
of flooding during some weeks per year <strong>de</strong>pending on climate and characterized by meso-<br />
hygrophilous species: Juncus gerardii, Bellis perennis and Leontodon autumnalis; and the<br />
lower parts of the gradient where flooding occurred up to 6 months per year and characterized<br />
by hygrophilous species: Glyceria fluitans, Juncus articulatus, Mentha pulegium and<br />
Trifolium fragiferum. One species presented a large distribution along the overall gradient<br />
(Agrostis stolonifera).<br />
Table 1: Position and maximal abundance of the twelve studied species along the flooding<br />
gradient and their family.<br />
Species Family Position along Maximal<br />
the gradient abundance (%)<br />
Agrostis stolonifera Poaceae All elevations 100<br />
Bellis perennis Asteraceae Slopes 15<br />
Carex divisa Cyperaceae Higher elevations 50<br />
Cynosurus cristatus Poaceae Higher elevations 22<br />
Glyceria fluitans Poaceae Lower elevations 91<br />
Hor<strong>de</strong>um secalinum Poaceae Higher elevations 22<br />
Juncus articulatus Juncaceae Lower elevations 56<br />
Juncus gerardii Juncaceae Slopes 67<br />
Leontodon autumnalis Asteraceae Slopes 5<br />
Lolium perenne Poaceae Higher elevations 65<br />
Mentha pulegium Lamiaceae Lower elevations 52<br />
Trifolium fragiferum Fabaceae Lower elevations 41<br />
A precise characterization of the waterlogging constraint experienced by plants was<br />
neglected before the work of Gowing et al. (1998) <strong>de</strong>veloping the Sum Exceedance Value for<br />
aeration (SEV for aeration): this quantification was realized in this study (See appendix). The<br />
77
Chapitre II<br />
range of SEV for aeration observed along the studied topographical sequence was from 0 to<br />
8.27cm.day -1 .<br />
Assessment of species’ tolerance to flooding<br />
Field removal and transplant experiment<br />
To <strong>de</strong>termine the range of tolerance to hydrological conditions, a removal experiment<br />
was established in September 2008 before the beginning of flooding season and was run until<br />
June 2009, in the floo<strong>de</strong>d grasslands located in the Marais Poitevin. At 6 locations along a<br />
topographical sequence, three plots were created by a removal of vegetation and by a cutting<br />
of vegetation and roots around the edge of plots, repeated regularly to avoid re-growth of<br />
vegetation. Within each plot, 2 individuals of each species were transplanted randomly and<br />
spaced by 20 cm to avoid any interaction. Each transplant was transplanted at similar size and<br />
life stage. A total of 432 individuals were transplanted into the field experiment.<br />
Measurements of the elevation of plots using a Trimble (Trimble M3,Ohio, USA) were<br />
realized in or<strong>de</strong>r to <strong>de</strong>termine the intensity of the constraint experienced by transplants at each<br />
location related to flooding duration and water <strong>de</strong>pth (Violle et al. 2007). From the location at<br />
the highest part of the gradient to the location at the lowest part of the gradient, we quantified<br />
the following range of SEV for aeration: 0, 0.44, 0.77, 1.42, 4.86 and 8.27cm.day -1<br />
(corresponding to 0, 0.031, 0.054, 0.1, 0.341 and 0.579m.week -1 ). Moreover for the two<br />
lowest locations, submergence occurred with a <strong>de</strong>pth of 5cm and 18cm respectively.<br />
Highly favorable controlled conditions<br />
To measure the stress at the individual level, i.e. the strain sensu Wel<strong>de</strong>n & Slauson<br />
(1986), target of the 12 species were grown un<strong>de</strong>r highly favorable controlled conditions<br />
(experimental gar<strong>de</strong>n, <strong>Rennes</strong>, France). For 10 species, highly favorable conditions<br />
correspond to non limiting water conditions without the presence of interspecific competition.<br />
In contrast, G. fluitans and J. articulatus are hygrophilous species that need long period of<br />
flooding during their growth cycle. However, G. fluitans tolerates high water level compared<br />
to J. articulatus (<strong>Merlin</strong> et al. in prep). Thus, even for a long period of flooding, treatment of<br />
20cm of water was chosen as a reference for G. fluitans and treatment of 10 cm of water to J.<br />
articulatus. One tiller of each species per pot (3L), with a potting soil with a mixture of<br />
78
Chapitre II<br />
gar<strong>de</strong>n soil and compost, was grown early March 2009 and run until July 2009 with 6<br />
replicates per species. Each transplant was transplanted at similar size and life stage.<br />
Plant performance survey<br />
For the field experiment, individuals’ survival was surveyed all along the growing<br />
season, i.e. from mid-February to the end of the experiment at the end of May:<br />
presence/absence of each individual was noted at 0, 20, 29, 36, 42, 50, 57, 71 and 98 days.<br />
Survival was noted, and individual aboveground biomass was harvested after 9 months of<br />
experiment for the field experiment and after 4 months of experiment for the controlled<br />
experiment. Aboveground biomass was dried for 72 h at 60°C before weighing.<br />
Measurements of plant functional traits<br />
Two functional traits were measured to study species response to hydrological<br />
conditions: Specific Leaf Area (SLA) and shoot length. They were measured at the end of the<br />
experiment for each living individual at each location of the field experiment. Following the<br />
standardized protocols (Weiher et al. 1999; Cornelissen et al. 2001), SLA was measured on<br />
the youngest fully grown leaf in the light using a leaf area meter (Li-Cor, Lincoln, Nebraska,<br />
USA). Shoot length was measured as the difference between the elevation of the highest<br />
photosynthetic leaf and the base of the plant, but was not measured for Bellis perennis and<br />
Leontodon autumalis as they present a roset life-form.<br />
Data analysis<br />
The “plot effect” in the in situ experiment was tested for all species and for the 6<br />
locations: the non significant results indicated that the 6 transplants of a species transplanted<br />
at each location can be consi<strong>de</strong>red as 6 replicates (two-way ANOVA, data not shown).<br />
Survival pattern<br />
The <strong>de</strong>termination of the fundamental niches of each species along the flooding<br />
gradient was realized using GLM regression for binary data at the end of the experiment. Chi-<br />
square tests from analysis of variance were realized to test significance of regressions.<br />
Moreover, to assess flooding tolerance of each species, we estimated the median lethal time<br />
79
Chapitre II<br />
LT50, i.e. the number of days after which 50% of individuals had died (Lenssen et al. 2004;<br />
Van Eck et al. 2004; Mommer et al. 2006). The Kaplan-Meier survival analysis followed by<br />
log-rank test was realized to test the patterns of survival of individual species between the 6<br />
locations along the 10 weeks of survey during the flooding period. Such a survival dynamics<br />
analysis across time is poorly used in community ecology (Ozinga et al. 2007), but is essential<br />
to characterize species response patterns. In this analysis, un<strong>de</strong>r the null hypothesis, i<strong>de</strong>ntical<br />
survival patterns along time were expected across the different hydrological conditions, i.e.<br />
between locations.<br />
Measure of strain using biomass data<br />
Following the proposition of Gross et al. (2010), strain intensity was approached by<br />
the comparison of species growth un<strong>de</strong>r non-limiting conditions without competitors in the<br />
controlled experiment, with growth without competitors in the in situ experiment. The log<br />
response ratio was calculated following this formula:<br />
lnRR (strain) = ln (individual biomass production in the field without competitors/mean<br />
biomass production un<strong>de</strong>r non-limiting conditions)<br />
A negative value indicates that target species performance is <strong>de</strong>pressed compared with non-<br />
limiting conditions whereas a positive value indicates increased performances of species<br />
un<strong>de</strong>r the studied conditions.<br />
One-way ANOVA was realized to test the effect of the hydrological conditions on the<br />
species aboveground biomass production, followed by post-hoc Tukey HSD tests after a log-<br />
transformation to verify the hypotheses of normality and homogeneity of variance. A two-<br />
way ANOVA was performed to test the effect of the different topographical locations and the<br />
effect of species on strain intensity after the verification of normality and homogeneity of<br />
variance.<br />
Linear regressions were realized between each measure of performance and intensity<br />
of strain for each species to investigate whether strain affects species performances (survival<br />
and biomass).<br />
80
Relationship between plant traits and species success<br />
Chapitre II<br />
To <strong>de</strong>termine whether plant traits were related to species performances, linear<br />
regressions were performed between SLA, shoot length and survival in a first time, and<br />
between SLA, shoot length and aboveground biomass in a second time. For these analyses,<br />
functional traits data were transformed to fit the hypotheses of normality and homogeneity of<br />
variance.<br />
Statistical tests were calculated with the software R (R version 2.7.2., 2008, R<br />
Foundation for Statistical Computing, Vienna, Austria).<br />
Results<br />
Different ranges of tolerance to hydrological conditions<br />
Effect of hydrological conditions on survival<br />
Four responses groups were <strong>de</strong>scribed concerning the patterns of survival along the<br />
gradient estimated by the GLM (Fig. 1). The first group correspon<strong>de</strong>d to a complete mortality<br />
of individuals at the end of the experiment at one or two floo<strong>de</strong>d locations of the experiment:<br />
two species were inclu<strong>de</strong>d in this group, Cynosurus cristatus and Bellis perennis. In<strong>de</strong>ed,<br />
Kaplan-Meier analysis <strong>de</strong>monstrated that 0 and 20 days after the beginning of the growing<br />
season, half of the transplanted individuals were already <strong>de</strong>ad for these floo<strong>de</strong>d locations<br />
(Table 2). The second group concerned species survival patterns characterized by a reduction<br />
of survival probability in response to the increase of hydrological conditions tested by Chi-<br />
square tests, but without a complete mortality at the end of the experiment: Carex divisa,<br />
Lolium perenne, Hor<strong>de</strong>um secalinum, Juncus gerardii, Leontodon autumnalis and Trifolium<br />
fragiferum. Their survival patterns across the flooding period were significantly different for<br />
the last location characterized by <strong>de</strong>ep flooding for these species, while Leontodon autumnalis<br />
and Trifolium fragiferum showed early median lethal time intervene for the two floo<strong>de</strong>d<br />
locations (Table 2). The third group was only composed of one species, Mentha pulegium,<br />
characterized by an equivalent probability of survival all along the gradient. The last group<br />
was composed of 3 species presenting a reduction of survival probability in the highest parts<br />
of the gradient: Agrostis stolonifera, Glyceria fluitans and Juncus articulatus, confirmed by<br />
81
Chapitre II<br />
differential survival patterns among locations <strong>de</strong>monstrated for Juncus articulatus and<br />
Glyceria fluitans (Table 2).<br />
Fig 1: Fundamental niches of species predicted by GLM in response to the variation of<br />
hydrological conditions, quantified through the measure of the SEV for aeration (from 0<br />
cm.day -1 to 8.27cm.day -1 ), with the results of the chi-square tests indicating the <strong>de</strong>gree of<br />
significance of regressions.<br />
82
Chapitre II<br />
Table 2: Number of days after which 50% of individuals had died, the LT50, at the 6<br />
topographical locations (i<strong>de</strong>ntified from the driest (1) to the wettest (6)) from mid-February,<br />
<strong>de</strong>termined by Kaplan-Meier analysis followed by log-rank tests. In bold: significance at<br />
P
Chapitre II<br />
Table 3: Results of two-way ANOVA testing the effect of species, the topographical locations<br />
and their interaction on the values of strain.<br />
Df Mean Square F value P-value<br />
Species 11 24.71 28.06 < 2.2e-16<br />
Topographical locations 5 6.62 7.52 1.34e-6<br />
Species*Topographical locations 52 5.01 5.69 < 2.2e-16<br />
Residuals 254 0.88<br />
Fig 2: Comparisons of strain intensity (ln RR(strain)) between topographical locations for<br />
each species. Statistical analyses for each location (i<strong>de</strong>ntified from the driest (1) to the wettest<br />
(6)) were performed in<strong>de</strong>pen<strong>de</strong>ntly. Results of ANOVA, P-values and Tukey HSD post-hoc<br />
tests are indicated (different letters indicated a significant difference).<br />
The intensity of strain was significantly related to the survival for the majority of<br />
species (P
Relationship between plant traits and plant success<br />
Chapitre II<br />
Returning the response groups inferred from patterns of strain intensity along the<br />
gradient of flooding, we noted that for species of group 4 (G. fluitans and J. articulatus), the<br />
length of the stem was positively related to their survival pattern (G. fluitans: r²=0.838,<br />
P=0.007; J. articulatus: r²=0.956, P=0.001) and the amount of biomass (G. fluitans: r²=0.297,<br />
P=0.001; J. articulatus: r²=0.575, P=6.82e-6). Survival of species from the second group (L.<br />
perenne, M. pulegium and T. fragiferum) was positively related to the length of stem<br />
(respectively r²=0.907, P=0.002; r²=0.771, P=0.014; r²=0.983, P=7.04e-5); biomass<br />
production of M. pulegium was related to SLA (r²= 0.1956, P=0.012). Success of species from<br />
the first group (survival and biomass production) (B. perennis, H. secalinum and L.<br />
autumnalis) was not related to SLA and stem length except for the survival pattern of H.<br />
secalinum, related to SLA (r²=0.613, P=0.040) and stem length (r²=0.862, P=0.005). For the<br />
group 3 consisting A. stolonifera, J. gerardii, C. divisa and C. cristatus, it was more difficult<br />
to generalize the results. Success, especially survival of A. stolonifera and C. divisa were<br />
related to SLA (r²=0.73, P=0.019; r²=0.627, P=0.036) and length stem (respectively r²=0.975,<br />
P=0.0001; r²=0.771, P=0.013). Survival of J. gerardii was positively related to SLA<br />
(r²=0.860, P=0.005); biomass of C. cristatus related to length of stem (r²=0.369, P=0.002).<br />
Discussion<br />
Few studies have attempted to <strong>de</strong>fine precisely the fundamental niches of species by<br />
quantifying the environmental gradient studied and combining survival and biomass data.<br />
Biomass is the measure of performance preferentially used to <strong>de</strong>fine the <strong>de</strong>gree of the<br />
tolerance to a constraint. Even if the propositions from Suding et al. (2003) and Gross et al.<br />
(2010) represent an interesting approach to quantifying the strain from biomass data allowing<br />
comparisons between species (see the part: Contrasted fundamental niches along the flooding<br />
gradient: habitats’ preferences), it is necessary to combine this information with species<br />
survival patterns, the first measure of tolerance to a constraint (Lenssen et al. 1998; Lenssen et<br />
al. 2004; Mommer et al. 2006). In this way, species growth strategies expressed to tolerate the<br />
constraint can be approximated.<br />
85
Contrasted fundamental niches along the flooding gradient: habitat preferences<br />
Chapitre II<br />
Two kinds of strain were emphasized along the flooding gradient. Then species<br />
presented different fundamental niches along the flooding gradient. Moreover, each species<br />
experiences and tolerates the hydrological conditions differently resulting in habitat<br />
preferences (Shipley 1991; Baruch 1994; Jung et al. 2009; Bartelheimer et al. 2010). The<br />
strain is thus species-specific (Körner 2003; Liancourt et al. 2005; Gross et al. 2010).<br />
Interestingly, our results support Lortie et al. (2004)’s point of view: individuals growing in<br />
their natural habitat are limited by abiotic environmental conditions compared to the optimal<br />
conditions, a result also found by Gross et al. (2010). This indicates that in natura, individual<br />
growth is not only limited by the flooding conditions, but perhaps also by the effect of<br />
flooding on resources.<br />
The increase of flooding conditions represents the constraint for nine species, for<br />
which a reduction in survival and/or in aboveground biomass is observed (Baruch 1994;<br />
Grimoldi 1999; Fraser & Karnesis 2005). Among these species, different <strong>de</strong>grees of tolerance<br />
are observed. Two species, Bellis perennis and Cynosurus cristatus, are intolerant to<br />
submergence with a complete mortality of individuals whatever the <strong>de</strong>pth of flooding. For the<br />
other seven species (Carex divisa, Hor<strong>de</strong>um secalinum, Lolium perenne, Juncus gerardii,<br />
Leontodon autumnalis, Mentha pulegium and Trifolium fragiferum), performances are<br />
reduced with the increase of flooding conditions: the range of tolerance is wi<strong>de</strong> for these<br />
species compared to the two previous species, as mortality was not total in the floo<strong>de</strong>d parts<br />
of the gradient. But these seven species characterized by similar responses curves along the<br />
gradient were constrained differently by flooding. In<strong>de</strong>ed, the analysis of the median lethal<br />
time informs precisely on the pattern of survival across time. Among these species,<br />
Leontodon autumnalis and Trifolium fragiferum tolerate less the submergence than the other<br />
species as their mortality occurred more quickly and for the two floo<strong>de</strong>d locations whereas<br />
mortality occurred only for the last location for the other species. In addition, Carex divisa,<br />
Hor<strong>de</strong>um secalinum, Juncus gerardii, Lolium perenne and Mentha pulegium tolerate longer<br />
flooding with 10 cm high water around than flooding with higher water level. As low water<br />
<strong>de</strong>pth implies shorter duration of flooding compared to high water <strong>de</strong>pth (Casanova & Brock<br />
2000), Leontodon autumnalis and Trifolium fragiferum tolerate (less exten<strong>de</strong>d) flooding, an<br />
effect of duration of flooding <strong>de</strong>monstrated on other species (Fraser & Karnesis 2005).<br />
The second strain is represented by the absence of flooding and is found to reduce<br />
survival and aboveground biomass of Glyceria fluitans and Juncus articulatus. These species<br />
86
Chapitre II<br />
clearly show a preference for floo<strong>de</strong>d habitats. In<strong>de</strong>ed, un<strong>de</strong>r hydrological conditions lower<br />
than the 4cm.day -1 threshold from which submergence occurred, survival of both species<br />
<strong>de</strong>creases, <strong>de</strong>monstrating the low tolerance of the absence of submergence.<br />
Only one species, Agrostis stolonifera, shows a large tolerance to the hydrological<br />
conditions (large fundamental niche) without negative effects on survival and biomass<br />
production. The pattern of strain reveals that the range of hydrological conditions, i.e. from<br />
low anoxia to <strong>de</strong>ep flooding has an equal intensity on its performances.<br />
Species strategy along the gradient<br />
Pairing biomass and survival analyses allow the <strong>de</strong>termination of species strategy, and<br />
especially stress-tolerance strategy (Grime, 1977), <strong>de</strong>fined as a constant survival probability<br />
all along the gradient and a reduction of biomass production in stressful parts of the gradient<br />
resulting in a strategy of resources conservation. Moreover, the species strategy appears<br />
essential in the outcome of competition (Choler et al. 2001; Corcket et al. 2003; Liancourt et<br />
al. 2005; Maestre et al. 2009; Bowker et al. 2010). In<strong>de</strong>ed these performances may behave<br />
differently along the gradient (Van Eck et al. 2004; Fraser & Karnesis 2005; Mommer et al.<br />
2006). It is the case for Mentha pulegium which presents contrasted patterns of survival and<br />
biomass along the range of hydrological conditions <strong>de</strong>scribing a stress-tolerance strategy.<br />
Shoot length more related to species success than SLA<br />
The success of the species measured by their survival and their biomass indicates that<br />
the tolerance of species to the range of hydrological conditions along the gradient may be<br />
largely explained by variations of shoot length rather than by variations of SLA.<br />
For species having a preference for floo<strong>de</strong>d habitats, i.e. Glyceria fluitans and Juncus<br />
articulatus, the positive relationship between species success and shoot length was expected<br />
and <strong>de</strong>monstrated. In<strong>de</strong>ed the shoot elongation is a response to flooding in or<strong>de</strong>r to improve<br />
gas diffusion among individuals (Blom & Voesenek 1996; Loreti & Oesterheld 1996;<br />
Clevering 1998; Lenssen et al. 1998; Insausti et al. 2001; Mommer et al. 2004; Voesenek et<br />
al. 2004; Mommer et al. 2006). This increase in shoot elongation is not paired with an<br />
increase in SLA, a response also known to improve gas exchanges (Blom & Voesenek 1996;<br />
Mommer et al. 2004; Mommer et al. 2006). This may be un<strong>de</strong>rstood by two in<strong>de</strong>pen<strong>de</strong>nt<br />
clusters in response to flooding expressed by species <strong>de</strong>monstrated by Mommer et al. (2006):<br />
species can either respond to flooding by an increase in traits in relation to stem such as the<br />
87
Chapitre II<br />
increase in stem elongation and in aerenchyma proportion, or by leaf traits such as the<br />
variation in SLA and in chlorophyll content.<br />
The positive relationship between shoot length and survival is also observed for<br />
species having preferences for dry habitats, i.e. where the intensity of the anoxia is low, as<br />
Lolium perenne or Mentha pulegium. In<strong>de</strong>ed in this part of this gradient, water resource is<br />
available during the beginning of the growing season, this high availability of resources allow<br />
a good growth of species. For similar reasons, the successes of Carex divisa, Cynosurus<br />
cristatus, Hor<strong>de</strong>um secalinum and Juncus gerardii in dry habitats are related to SLA and to<br />
shoot length: the simultaneous variations of SLA and shoot length reflect an efficient strategy<br />
of resource use (Weiher et al. 1999; Jung et al. 2010), with a high ability of capture of<br />
resources, especially light and space, for high competitive species (Gau<strong>de</strong>t & Keddy 1995).<br />
Interestingly, species responses to floo<strong>de</strong>d and dry habitats are similar i.e. the<br />
elongation of the stem, even if the causalities of these responses are different. The increase<br />
may be due on the one hand to the availability of resource for species preferring dry habitats,<br />
and on the other hand to the increase of flooding conditions for species preferring floo<strong>de</strong>d<br />
habitats.<br />
Conclusion<br />
Our study helps precisely characterizing the fundamental niches of twelve species<br />
using the quantification of strain and the analysis of survival and biomass patterns according<br />
to a range of hydrological conditions, compared to other studies <strong>de</strong>monstrating fundamental<br />
niches of wetlands species (Fraser & Karnesis 2005). Two constraints are <strong>de</strong>monstrated along<br />
the gradient: the increase of the duration of flooding and the absence of flooding, which<br />
differently affect species. Moreover, the species success related to the tolerance to the<br />
constraint may be expressed by the plant height, thus consi<strong>de</strong>red as a surrogate to species<br />
performances along the flooding gradient.<br />
The knowledge of fundamental niches is essential for the un<strong>de</strong>rstanding of processes<br />
acting on species distribution along the gradient and especially the importance of competition,<br />
which will be studied in the second part of this paper.<br />
88
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91
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92
- Chapitre 3 -<br />
Fundamental niche versus Realized niche: Assessment of the<br />
importance of competition along a flooding gradient.<br />
Part 2: Tra<strong>de</strong>-off between competitive ability and tolerance to stress at the species<br />
level driving species distribution along a flooding gradient<br />
En préparation<br />
93
Chapitre III<br />
Fundamental niche versus Realized niche: Assessment of the importance of<br />
competition along a flooding gradient.<br />
Part 2: Tra<strong>de</strong>-off between competitive ability and tolerance to stress at the<br />
species level driving species distribution along a flooding gradient<br />
<strong>Amandine</strong> <strong>Merlin</strong> 1,2 , Jan-Bernard Bouzillé 1 , François Mesléard 2,3 & Anne Bonis 1<br />
1 UMR CNRS 6553 EcoBio, <strong>Université</strong> <strong>de</strong> <strong>Rennes</strong> 1, Campus Beaulieu, F-35042 <strong>Rennes</strong><br />
Ce<strong>de</strong>x, France<br />
2 Centre <strong>de</strong> recherche La Tour du Valat, Le sambuc, F-13200, France<br />
3 UMR CNRS-IRD 6116 Institut Méditerranéen d’Ecologie et <strong>de</strong> Paléoécologie, <strong>Université</strong><br />
d’Avignon IUT site Agroparc BP 1207, F-84911 Avignon Ce<strong>de</strong>x 09, France<br />
Abstract<br />
The variation in the importance of competition is still poorly un<strong>de</strong>rstood along<br />
environmental gradients. In this study, we examined the role of competition across a flooding<br />
gradient using the niche concept, with the hypothesis of a tra<strong>de</strong>-off between competitiveability<br />
of species and the tolerance to stress responsible for species distribution along the<br />
gradient.<br />
This work involved twelve wetland species presenting different levels of flooding<br />
tolerance. Comparisons between fundamental and realized niches of species, predicted by<br />
generalized mo<strong>de</strong>ls, were realized to assess the competitive ability of species and <strong>de</strong>termine<br />
which species are submitted to the competitive exclusion. The importance of competition was<br />
assessed using the in<strong>de</strong>x of competition importance Cimp.<br />
Five species were exclu<strong>de</strong>d in the stressed parts of the gradient with a displacement of<br />
their niche’s optima observed, whereas no optima displacement was observed for six other<br />
species suggesting a high competitive ability. Only one species presented a large niche width<br />
in presence of neighbors all along the flooding gradient. At individual level, the importance of<br />
competition was related to the intensity of the strain experienced for six species.<br />
The tra<strong>de</strong>-off between competitive ability of species and the tolerance to stress at<br />
species level is responsible for species distribution and their abundance patterns along the<br />
flooding gradient, with the competition importance predicted by the intensity of the<br />
constraint. This experiment shows that niche concept is useful for un<strong>de</strong>rstanding the assembly<br />
rules of communities along a constraint gradient and provi<strong>de</strong>d testable hypothesis.<br />
Key-words: response curves, niche shift, optima displacement, Cimp<br />
95
Chapitre III<br />
96
Introduction<br />
Chapitre III<br />
The study of the competition importance is crucial to <strong>de</strong>termine the roles of the abiotic<br />
and biotic filters in structuring plant communities. However its assessment along<br />
environmental gradients is seldom studied and still poorly un<strong>de</strong>rstood (Goldberg 1994;<br />
Sammul et al. 2000; Greiner La Peyre et al. 2001; Corcket et al. 2003; Gaucherand et al.<br />
2006; Zhang et al. 2008; Jung et al. 2009; Damgaard & Fayolle 2010).<br />
Many studies have been conducted to <strong>de</strong>termine whether the distribution of plant<br />
species along environmental gradients resulted from a tra<strong>de</strong>-off between the competitive<br />
abilities of species and their tolerance to stress (Keddy 1989; Wisheu & Keddy 1992; Lenssen<br />
et al. 1999; Brose & Tielbörger 2005; Jung et al. 2009), with species highly competitive<br />
excluding less competitive from more stressful areas and then increasing their tolerance to<br />
stress. In contrast, very few studies have inclu<strong>de</strong>d the concepts of intensity and importance of<br />
competition relative to this tra<strong>de</strong>-off, thus limiting the un<strong>de</strong>rstanding of their patterns of<br />
variation along gradients. By <strong>de</strong>fining the intensity of competition as a process and the<br />
importance of the competition as a product (Wel<strong>de</strong>n & Slauson 1986), we un<strong>de</strong>rstand that<br />
competitive exclusion of species, allowing the <strong>de</strong>scription of the tra<strong>de</strong>off, represents the<br />
product of the interaction between species, corresponding to the process.<br />
Few studies reported the competitive exclusion of species, viewable by a niche shift.<br />
The <strong>de</strong>monstration of this tra<strong>de</strong>-off requires the knowledge of width of fundamental niches<br />
(Sommer & Worm 2002), however less <strong>de</strong>termined (Aerts 1999; Silvertown 2004). In<strong>de</strong>ed,<br />
the use of niche concept represents an interesting approach to study the importance of<br />
competition along environmental gradients. The <strong>de</strong>viation of species distribution from their<br />
physiological range of tolerance in response to interspecific competition can assess<br />
competition importance at species distribution limits (Wel<strong>de</strong>n & Slauson 1986; Jung et al.<br />
2009). Thus, only a small effect of competition on niche positions is expected among high<br />
competitive species (Parrish & Bazzaz 1982). Moreover, a reduction of competition<br />
importance with the increase of stress is expected, i.e. at each si<strong>de</strong> of the physiological<br />
optimum (Jung et al. 2009), but competition importance may be different between each si<strong>de</strong><br />
inducing an asymmetric species’ distribution along the gradient (Austin 1990).<br />
Different fundamental niches of species may be <strong>de</strong>scribed along a flooding gradient<br />
(see part 1): as a consequence, the reduction of performances by competition <strong>de</strong>pends on<br />
species and on locations along the gradient (Jung et al. 2009). In addition, the knowledge of<br />
the fundamental niches is <strong>de</strong>cisive in the outcome of interspecific competition because this<br />
97
Chapitre III<br />
outcome may <strong>de</strong>pend of individual responses (Greiner la Peyre et al. 2001; Choler et al. 2001;<br />
Bruno et al. 2003; Corcket et al. 2003; Liancourt et al. 2005; Gaucherand et al. 2006): this<br />
information is essential to extrapolate the result of tra<strong>de</strong>-off from the species level to the<br />
community level aggregating responses of all species having various ranges of tolerance<br />
(Greiner la Peyre et al. 2001). In<strong>de</strong>ed, this tra<strong>de</strong>-off at the species level may not be observed<br />
at the community level (Violle et al. 2010). Consequently, these different ranges of tolerance<br />
imply different levels of competition importance which may explain the difficulty to evaluate<br />
the variation of competition importance along environmental gradients.<br />
The use of niche concept has to be paired with a precise characterization of the<br />
environmental factor acting strongly on species distribution, which is a complex logistic,<br />
especially when a resource gradient coinci<strong>de</strong>s with a stress gradient (Kotowski et al. 2006):<br />
disentangling the effects of competition from physical factors is essential to un<strong>de</strong>rstand<br />
communities’ distribution. In<strong>de</strong>ed, communities’ distribution may come from species<br />
segregation according to their competitive ability, from distinct physiological preference for<br />
habitats, or from an interaction between species responses to both physical and resource<br />
gradients (Emery et al. 2001; Kotowski et al. 2006).<br />
The aim of this present study is to <strong>de</strong>termine the relative importance of flooding<br />
conditions and competition in the distribution of species along a flooding gradient, through<br />
the measure of the in<strong>de</strong>x of competition importance (Brooker et al. 2005). We hypothesize,<br />
first, the existence of a tra<strong>de</strong>-off between competitive abilities and tolerance to stress, driving<br />
species ecological success along the flooding gradient. Second, there should be an exclusion<br />
of less competitive species from the higher stressed parts of the flooding gradient, as the<br />
product of strong competitive interactions in the lesser stressed parts of the gradient.<br />
To respond to this objective, the concept of niche will be used through the comparison<br />
of fundamental and realized niches of species, informing then on the competitive ability of<br />
species measured as the <strong>de</strong>gree of niche <strong>de</strong>viation. Moreover, the <strong>de</strong>termination of the<br />
individual strain (Suding et al. 2003; Gross et al. 2010) representing an interesting way to<br />
quantify stress at the species level (see part 1), will allow comparing the importance of<br />
competition between species according to the intensity of strain experimented (Brooker et al.<br />
2005; Gaucherand et al. 2006).<br />
98
Material and methods<br />
Study site<br />
Chapitre III<br />
The study site is a flood meadow located in the Marais Poitevin on the French Atlantic<br />
coast (46°28’N; 1°13’W) characterized by a mild Atlantic climate. The study area is<br />
characterized by a micro-topographical gradient, with an elevation range of approximately 40<br />
cm, with floo<strong>de</strong>d <strong>de</strong>pressions, intermediate slopes and higher level flats resulting in a range of<br />
flooding intensity, frequency and duration (Violle et al. 2011). Three plant communities have<br />
been distinguished on these three topographical compartments (Tourna<strong>de</strong> & Bouzillé 1995).<br />
The higher level flats are never floo<strong>de</strong>d: the mesophilous community is dominated by Lolium<br />
perenne, Agrostis stolonifera, Elymus repens, Cynosurus cristatus and Hor<strong>de</strong>um secalinum.<br />
The hygrophilous community dominated by Agrostis stolonifera, Glyceria fluitans, Oenanthe<br />
fistulosa and Eleocharis palustris in the floo<strong>de</strong>d <strong>de</strong>pressions are submitted to 6 months long<br />
flooding. The intermediate slopes experience a variable duration of flooding <strong>de</strong>pending on<br />
climate during some weeks per year, dominated by Juncus gerardii, Hor<strong>de</strong>um marinum,<br />
Alopecurus bulbosus.<br />
The productivity of the study site is stable along the gradient: flooding intensity did<br />
not affect biomass production, so nutrients availability and flooding factors are not<br />
confoun<strong>de</strong>d (Violle et al. 2011).<br />
Floristic samplings<br />
To <strong>de</strong>termine species distribution according to hydrological conditions in the presence<br />
of interspecific competition, 266 floristic samplings were realized randomly in paddocks<br />
managed by mo<strong>de</strong>rate grazing. Within each quadrate (25 cm*25 cm), the presence of species<br />
was noted. As for the part 1, measurements of the elevation of plots with a Trimble (Trimble<br />
M3, Ohio, USA) were realized in or<strong>de</strong>r to <strong>de</strong>termine the intensity of the constraint<br />
experienced by species following the proposition of Gowing et al. (1998) (See appendix).<br />
99
Data analysis<br />
Assessment of species competitive ability<br />
Chapitre III<br />
The <strong>de</strong>gree of displacement of the ecological (realized) optima from the fundamental<br />
optima sensu Austin (1990) was assessed to <strong>de</strong>termine the existence of a tra<strong>de</strong>-off between<br />
competitive ability and tolerance to stress at the species level. Following this hypothesis of<br />
competitive hierarchy, a coinci<strong>de</strong>nce of fundamental and realized optima is expected for high<br />
competitive species, whereas a displacement of optima is expected for low competitive<br />
species (competitive exclusion) (Fig. 1).<br />
Fig 1: A. The response of a high competitive species to the presence of neighbors, i.e. a<br />
reduction of performance without <strong>de</strong>viation of optima; B. The response of a less competitive<br />
species to the presence of neighbors, i.e. a competitive exclusion observable through the<br />
displacement of the ecological optima (adapted from Austin 1990).<br />
Measure of competition importance<br />
Following the prediction of fundamental niches realized with GLM in the part 1 of the<br />
paper, the probability of occurrence of each species in presence of competitors along the<br />
gradient was predicted from binary data. The Akaike Information Criterion (AIC) was used to<br />
select the better fitting mo<strong>de</strong>l between quadratic GLM and GAM (Thuiller et al. 2003; Austin<br />
et al. 2009). In<strong>de</strong>ed a <strong>de</strong>bate exists concerning the prediction of response curves with GLM<br />
100
Chapitre III<br />
and with GAM: better fitting of asymmetric response curves were expected with GAM<br />
(Guisan et al. 1999; Austin 2009; Heikkinen & Makipaa 2010). Bootstrap procedure though a<br />
re-sampling of residuals was realized for each regression to <strong>de</strong>termine whether parameters of<br />
mo<strong>de</strong>ls were well predicted because the number of observations per species was low. Chi-<br />
square tests from analysis of variance were realized to test significance of regressions.<br />
The in<strong>de</strong>x of competition importance Cimp (Brooker et al. 2005) was used to quantify<br />
the importance of competition relative to the impact of all others factors in the environment.<br />
This in<strong>de</strong>x was calculated with the probability of occurence data for each value of SEV for<br />
aeration <strong>de</strong>scribed at the 6 locations along the gradient, following this formula:<br />
Cimp= (Occcomp-Occtarget)/(MaxOcc-y)<br />
Occcomp is the probability of occurence in presence of competitors, Occtarget is the probability<br />
of occurence without competitors, MaxOcc is the maximum value of the probability of<br />
occurence without competitors along the gradient and y is the smaller of either Occcomp or<br />
Occtarget. The more Cimp tends to -1, the more important is the contribution of competition<br />
against the hydrological conditions; whereas the more Cimp tends to +1, the more important is<br />
the contribution of abiotic factor relative to competition.<br />
Linear regressions were performed to test whether the importance of competition was<br />
related to the intensity of strain.<br />
Characterization of niche shape<br />
To <strong>de</strong>termine whether an interaction between hydrological conditions and competition<br />
occurs and was more important for one part of the gradient, the skewness ( ) was calculated<br />
to characterize niche shape of realized niches (Heikkinen & Makipaa, 2010). It was calculated<br />
for a threshold equal to e -1/2 of the ecological optimum of niche, threshold <strong>de</strong>termined by<br />
previous mo<strong>de</strong>ling works (Heegaard 2002; Heikkinen & Makipaa 2010). However, this<br />
calculation can be applied only for non-truncated responses curves. Skewness gives a value<br />
between +1 and -1: a negative value indicates that niche is larger for low values of the<br />
hydrological conditions whereas a positive value indicates that niche is larger for high values<br />
of the hydrological conditions.<br />
Statistical tests were computed with R software (R Development Core Team 2008),<br />
with the boot and GAM packages.<br />
101
Results<br />
Niche shifts and <strong>de</strong>viation of optima in response to diffuse competition<br />
Chapitre III<br />
The range of hydrological conditions experienced by transplants was only 1/4 long<br />
(from 0 to 8.27 cm.day -1 ) (see part 1) compared with the range along the various sites where<br />
the floristic samplings were realized (from 0 to 36.92cm.day -1 ). The quantification of the<br />
importance of competition was possible only on this 1/4 range of hydrological conditions.<br />
Ecological responses curves were better predicted by GAM than by quadratic GLM<br />
because of low AIC values (see Appendix): the bootstrap procedure indicated reliable<br />
predictions because estimators were reasonably unbiased and the bootstrap standard errors<br />
smaller (Table 2).<br />
Table 2: Results of bootstrapping realized on residuals of simple GLM for fundamental niches<br />
and on residuals of GAM for realized niches, indicating unbiased estimators of regressions<br />
paired with small standard errors.<br />
Fundamental niches Realized niches<br />
Parameters Intercept ~SEVa Intercept ~SEVa<br />
Bias Std. error Bias Std. error Bias Std. error Bias Std. error<br />
A. stolonifera -0.063 0.084 0.024 0.032 -0.004 0.105 0.0002 0.006<br />
B.perennis -0.010 0.068 0.004 0.026 0.149 0.071 -0.009 0.004<br />
C. cristatus -0.078 0.102 0.030 0.039 0.059 0.083 -0.003 0.005<br />
C. divisa -0.090 0.162 0.034 0.062 -0.263 0.118 0.015 0.007<br />
G. fluitans -0.017 0.134 0.007 0.051 0.022 0.108 -0.001 0.006<br />
H. secalinum -0.046 0.144 0.018 0.055 -0.011 0.108 0.0006 0.006<br />
J. articulatus -0.039 0.125 0.015 0.048 0.219 0.140 -0.013 0.008<br />
J. gerardii -0.073 0.150 0.028 0.057 -0.068 0.127 0.004 0.007<br />
L. autumnalis -0.109 0.111 0.041 0.042 0.120 0.055 -0.007 0.003<br />
L. perenne -0.067 0.151 0.025 0.058 -0.201 0.112 0.011 0.006<br />
M. pulegium 0.013 0.156 -0.005 0.059 -0.063 0.108 0.004 0.006<br />
T. fragiferum -0.071 0.160 0.027 0.061 -0.070 0.126 0.004 0.007<br />
For overall species, interspecific competition induced niche shifts with some<br />
displacements of niche optima. A succession of ecological optima was observed along the<br />
overall range of hydrological conditions, and four response groups were <strong>de</strong>scribed (Fig 3).<br />
The first group was characterized by species presenting a probability of occurrence<br />
strongly reduced for Juncus gerardii, Bellis perennis, Leontodon autumnalis, Mentha<br />
pulegium and Trifolium fragiferum. On the range of 0 to 8.27cm.day -1 , the ecological optima<br />
of these species were displaced to lower elevations of the gradient for the hydrological<br />
102
Chapitre III<br />
conditions superior to 8.27cm.day -1 : ecological optima were equal to 17, 13, 19, 24 and<br />
20cm.day -1 respectively. Among these species, the characterization of the skewness was<br />
possible only for the non-truncated ecological niches of Bellis perennis, Leontodon<br />
autumnalis, Mentha pulegium and Trifolium fragiferum. For the two first species, the<br />
ecological niche was larger in the higher elevations of the gradient ( =-0.11 and =-0.45<br />
respectively) whereas the ecological niche of the two last species was larger for the lower<br />
elevations of the gradient ( =0.14 and =0.27 respectively).<br />
Fig. 3: Realized niches of species predicted through the probability of occurrence using GAM<br />
in response to the variation of hydrological conditions and to the presence of interspecific<br />
competition with the result of the chi-square test.<br />
The second group was composed of Carex divisa, Lolium perenne, Hor<strong>de</strong>um<br />
secalinum and Cynosurus cristatus where the probability of occurrence was reduced in<br />
response to interspecific competition except for Carex divisa on the range of SEV aeration<br />
from 0 to 8.27cm.day -1 . Moreover, along the range of hydrological conditions, the success of<br />
species was greater for a SEV for aeration superior to 8.27cm.day -1 in presence of<br />
interspecific competition.<br />
103
Chapitre III<br />
The third group was composed of Glyceria fluitans and Juncus articulatus for which<br />
the ecological optima were <strong>de</strong>scribed for high values of SEV for aeration. The ecological<br />
success of these species was reduced for low values of SEV for aeration. Agrostis stolonifera<br />
composed the last group: this ecological success was high along the flooding gradient and<br />
appeared bimodal.<br />
Contribution of competition against hydrological conditions: the quantification on a short<br />
range of hydrological conditions.<br />
Through the use of the probability of occurrence from fundamental and realized niches<br />
for the 6 values of SEV for aeration corresponding to the 6 locations of the field experiment,<br />
the contribution of competition against hydrological conditions seemed to be higher (Cimp<br />
closed to -1) for Mentha pulegium, Trifolium fragiferum, Leontodon autumnalis, Bellis<br />
perennis and Juncus articulatus for which the contribution increased slowly with SEV for<br />
aeration. Cimp was closed to -1 for Bellis perennis, Leontodon autumnalis and Juncus gerardii<br />
for higher locations of the gradient and increased with SEV for aeration, especially strongly<br />
for Bellis perennis. The contribution of competition against hydrological conditions was the<br />
lowest (Cimp closed to 1) for Carex divisa.<br />
Figure 4: Pattern of competition importance, Cimp, for the twelve species in each location (1<br />
for the driest to 6 for the wettest) of the in situ experiment, calculated from the probability of<br />
occurrence of species in presence and in absence of interspecific competition. We observed<br />
that the importance of competition is generally higher for the hygrophilous and mesohygrophilous<br />
species (Cimp closed to -1).<br />
104
Chapitre III<br />
Significant regressions were <strong>de</strong>scribed between the intensity of strain and the<br />
importance of competition, except for Cynosurus cristatus, Carex divisa, Hor<strong>de</strong>um secalinum,<br />
Leontodon autumnalis and Juncus articulatus (Table 5). For Agrostis stolonifera, the<br />
competition importance was high where the intensity of strain was high. On the other hand,<br />
for Bellis perennis, Glyceria fluitans, Lolium perenne, Mentha pulegium, Trifolium<br />
fragiferum, the importance of competition was high where the strain intensity was low; a<br />
ten<strong>de</strong>ncy was <strong>de</strong>scribed for Juncus gerardii.<br />
Table 5: Significant regressions between competition importance Cimp and strain intensity for<br />
A. stolonifera, B. perennis, G. fluitans, L. perenne, M. pulegium and T. fragiferum.<br />
A. stolonifera B. perennis C. cristatus C. divisa<br />
t 3.74 -8.21 -1.44 -0.74<br />
df 4 3 2 4<br />
P-value 0.02 0.0038 0.2869 0.498<br />
cor 0.882 -0.978 -0.713 -0.348<br />
G. fluitans H. secalinum J. articulatus J. gerardii<br />
t -12.69 0.79 0.72 -2.34<br />
df 4 4 4 4<br />
P-value 0.0002 0.4759 0.512 0.079<br />
cor -0.988 0.366 0.338 -0.761<br />
L. autumnalis L. perenne M. pulegium T. fragiferum<br />
t -0.56 -3.23 -3.46 -32.07<br />
df 4 4 4 4<br />
P-value 0.607 0.032 0.026 5.63e-6<br />
cor -0.268 -0.850 -0.875 -0.998<br />
Discussion<br />
Competitive ability-strain tolerance tra<strong>de</strong>-off driving species success along the flooding<br />
gradient<br />
Interspecific competition induces a spatial segregation of species niches along the<br />
flooding gradient, particularly according to the <strong>de</strong>gree of anoxia which is the variable strongly<br />
explaining niches segregation (Silvertown et al. 1999; Silvertown 2004; Araya et al. 2010).<br />
This segregation confirms our hypothesis of a tra<strong>de</strong>-off between species competitive ability<br />
and their tolerance to strain driving species distribution along the flooding gradient with a<br />
displacement of species (Lenssen et al. 1999; Brose & Tielbörger 2005; Pennings et al. 2005;<br />
Jung et al. 2009). However all species are not driven by this tra<strong>de</strong>-off.<br />
105
Chapitre III<br />
Even if ranges of hydrological conditions were not equivalent between the<br />
topographical sequence where the field experiment was implanted and the various sites where<br />
floristic samplings were realized, it is possible to formulate hypothesis and extrapolate results<br />
on the importance of competition quantified over the short range of the hydrological<br />
conditions to the overall gradient.<br />
Importance of competition <strong>de</strong>pending on the strain intensity<br />
A niche shift is <strong>de</strong>fined as any change in the position, in the mo<strong>de</strong> and/or a reduction<br />
in the variance of the distribution of a species along a niche axis (Silvertown 2004). In our<br />
study, interspecific competition is responsible for the niche shifts and for the optima<br />
displacements for Juncus gerardii, Bellis perennis, Leontodon autumnalis, Mentha pulegium<br />
and Trifolium fragiferum. For these species, competition importance <strong>de</strong>creases with strain<br />
intensity which is in accordance with others studies (Sammul et al. 2000; Greiner La Peyre et<br />
al. 2001; Gaucherand et al. 2006; Jung et al. 2009). In other words, competition is important<br />
when environmental conditions are close to the optimal hydrological conditions of the<br />
species. Thus, these species are exclu<strong>de</strong>d from floo<strong>de</strong>d elevations of the gradient due to a low<br />
competitive ability. Moreover, at the limits of species’ distribution, i.e. at each si<strong>de</strong> of the<br />
ecological niche, the contribution of competition is not equal (Austin 1990). In<strong>de</strong>ed,<br />
interspecific competition strongly limits the species’ upper distribution of the ecological<br />
niches except for Juncus gerardii, Bellis perennis and Leontodon autumnalis whereas<br />
flooding strongly limits their distribution in the lower elevations. The distribution of previous<br />
species is thus driven by the tra<strong>de</strong>-off between competitive abilities and tolerance to stress at<br />
species level, resulting in the prediction of competition importance, <strong>de</strong>pending on the<br />
individual tolerance to the absence of flooding or to the presence of long durations of<br />
flooding.<br />
Our study only allows hypotheses to be formulated for Glyceria fluitans and Juncus<br />
articulatus. In<strong>de</strong>ed, no available data on species performances for high <strong>de</strong>gree of anoxia<br />
exists. However the absence of flooding represents the constraint (part 1). This may<br />
confirmed the relationship between competition importance and the strain intensity: in<strong>de</strong>ed,<br />
the contribution of competition in species success is high in the higher parts of the gradient<br />
for Glyceria fluitans. This relationship is not significant for Juncus articulatus: the low<br />
number of replicates may influence the result of this relationship. In opposition to previous<br />
species, no displacement of niche optima could occur in response to interspecific competition<br />
106
Chapitre III<br />
because of their preference for floo<strong>de</strong>d habitats. However, the range of environmental<br />
conditions is strongly reduced by the presence of neighbors, otably in low floo<strong>de</strong>d parts of the<br />
gradient.<br />
Enlargement of mesophilous species’ realized niches in presence of competitors<br />
No displacement of niche optima are observed for Cynosurus cristatus, Carex divisa,<br />
Hor<strong>de</strong>um secalinum and Lolium perenne in response to interspecific competition but only a<br />
reduction of the ecological success. This indicates a high competitive ability for these species<br />
(Parrish & Bazzaz 1982) which are certainly responsible for the exclusion of the previous<br />
meso-hygrophilous species and hygrophilous species (Mentha pulegium and Trifolium<br />
fragiferum).<br />
Moreover, an expected result is observed: the unrelated variations of competition<br />
importance and strain intensity, except for Lolium perenne. This may be explained by the<br />
increase realized niche width of mesophilous species compared to fundamental niches. This<br />
may suggest a facilitative effect of neighbors then increasing their range of tolerance to<br />
flooding. In<strong>de</strong>ed, following Bruno et al. (2003), in the context of niche theory, facilitation<br />
may extent the realized niche of species until this extent can be larger than the fundamental<br />
niche. Recent studies <strong>de</strong>monstrated that facilitation did not only occur in harsh environments<br />
but <strong>de</strong>pends on target species and on the kind of resources (Choler et al. 2001; Liancourt et al.<br />
2005; Maestre et al. 2009; Bowker et al. 2010). In<strong>de</strong>ed facilitation <strong>de</strong>pends on the low<br />
tolerance of species to the constraint and at the same time on their high competitive ability<br />
(Liancourt et al. 2005). In stressful environments, an amelioration of habitats by neighbors<br />
may be observed, thus facilitating less adapted species (Choler et al. 2001) but this was<br />
<strong>de</strong>monstrated for resource gradient where the availability of water was low. Nevertheless,<br />
neighbors may improve the availability of soil oxygen (Keddy 1994): plants may compete for<br />
this soil oxygen, and this proposition to consi<strong>de</strong>r the aeration as a resource stress seems to be<br />
a<strong>de</strong>quate (Bartelheimer et al. 2010), contrary to the proposition of Austin (1990, 2002)<br />
consi<strong>de</strong>ring this abiotic variable as a non-resource gradient. However, we cannot exclu<strong>de</strong> the<br />
effect of the mo<strong>de</strong>rate grazing present in this study, although grazing effect was <strong>de</strong>monstrated<br />
to restrict realized niches (Lau et al. 2008), not enlarge them. Moreover, an experimental bias<br />
cannot be exclu<strong>de</strong>d because the removal of vegetation may locally change the environmental<br />
conditions.<br />
107
The particular case of Agrostis stolonifera<br />
Chapitre III<br />
One species presents a high success along the gradient with the presence of<br />
competitors, Agrostis stolonifera: its ecological success does not seem to be driven by the<br />
competitive ability-stress tolerance tra<strong>de</strong>-off. This may be explained by a high plasticity of<br />
this species (Callaway et al. 2003), not <strong>de</strong>monstrated in this study. In<strong>de</strong>ed, plastic<br />
morphological responses of this species may reduce the effect of interspecific competition<br />
(Silvertown & Gordon, 1989). Moreover, this species <strong>de</strong>monstrated a high competitive ability<br />
after flooding events thanks to an efficient clonal growth (Lenssen et al. 2004).<br />
A possible extrapolation at the community scale<br />
To extrapolate results of the tra<strong>de</strong>-off from the species level to the community level,<br />
our results support the i<strong>de</strong>a suggesting an aggregation of species responses, which could have<br />
various tolerances to the abiotic variables (Greiner la Peyre et al. 2001). The exclusion of<br />
Juncus gerardii, Bellis perennis and Leontodon autumnalis, <strong>de</strong>scribing the meso-<br />
hygrophilous community, from stressful habitats (slopes) by high competitive species<br />
characterizing the mesophilous community (Carex divisa, Lolium perenne, Hor<strong>de</strong>um<br />
secalinum and Cynosurus cristatus) allows the extrapolation of this tra<strong>de</strong>-off at these two<br />
communities’ scale. In<strong>de</strong>ed the meso-hygrophilous community is certainly composed of a<br />
large number of subordinate species, resulting in a <strong>de</strong>crease of competition importance with<br />
the increase of the constraint. Moreover, the predicted ecological success seems to be<br />
concordant with the competitive hierarchy with subordinate presenting a weak relative<br />
abundance, especially for Leontodon autumnalis and Bellis perennis. However relative<br />
abundances of Cynosurus cristatus and Hor<strong>de</strong>um secalinum were low compared to the<br />
predicted ecological success. This may be explained by the fact that these species are more<br />
sensitive to interspecific competition than Carex divisa and Lolium perenne. In<strong>de</strong>ed, the study<br />
of competitive hierarchy among mesophilous community <strong>de</strong>monstrated that the low<br />
competitive abilities of Cynosurus cristatus and Hor<strong>de</strong>um secalinum and the high competitive<br />
abilities of other species especially Lolium perenne (Marion et al. in prep.).<br />
However, the extrapolation at the community level of the reduction of the contribution<br />
of competition with the increase of the constraint is not possible because of floo<strong>de</strong>d habitats’<br />
preference of two species: Juncus articulatus and Glyceria fluitans, explaining then their<br />
abundance patterns. Then, because of these habitats preferences, the assessment of<br />
competition importance along environmental gradients is not easy: in our study, at the<br />
108
Chapitre III<br />
community scale, the competitive ability-tolerance of stress tra<strong>de</strong>-off cannot be <strong>de</strong>monstrated<br />
(Violle et al. 2010).<br />
Conclusion<br />
The competitive ability-stress tolerance tra<strong>de</strong>-off drives species distribution along the<br />
flooding gradient resulting in a competitive hierarchy with a ranked or<strong>de</strong>r from competitive<br />
dominant species to subordinate species. The predictability of the constraint, i.e. a regular<br />
occurrence as it is the case in our study site (occurrence at each autumn and finish at the end<br />
of spring) is a prerequisite to observe the competitive ability-stress tolerance tra<strong>de</strong>-off (Keddy<br />
1989; Costa et al. 2003). In<strong>de</strong>ed, in irregular environment characterized by large variations<br />
inducing a low persistent gradient of stress across time, competitive hierarchy does not appear<br />
as a major process structuring plant communities (Costa et al. 2003). However the non<br />
persistence of stress over time is not the only reason explaining the non-observation of the<br />
tra<strong>de</strong>-off. In<strong>de</strong>ed, the high plasticity of species allowing the adaption to local abiotic and<br />
biotic conditions may reduce the effects of interspecific competition, as it the case for<br />
Agrostis stolonifera.<br />
This study <strong>de</strong>monstrated the essential role of the <strong>de</strong>termination of fundamental niches<br />
in the assessment of competition importance against environmental conditions and in the<br />
extrapolation of results from the species scale to the community scale, thus allowing to<br />
un<strong>de</strong>rstand abundance patterns and species coexistence. Responses are species specific for<br />
both the <strong>de</strong>termination of range of tolerance to the abiotic variable and for the outcome of<br />
competition. Our results could <strong>de</strong>monstrate that the variation of competition importance could<br />
not be applied generally at the community scale because of the presence of potential<br />
competitive species in each part of the flooding gradient.<br />
In spite of a short range of hydrological conditions experimented by transplants, this<br />
study helps to un<strong>de</strong>rstand community’ distribution due to a running field experiment along a<br />
realistic range of environmental conditions and to the characterization and the quantification<br />
of the abiotic variable, expressed here by the <strong>de</strong>gree of aeration, which is less realized in<br />
studies although essential (Aerts 1999).<br />
109
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113
Appendix<br />
Material and Methods<br />
The characterization of the elevation gradient<br />
Chapitre III<br />
A precise characterization of these waterlogging and drought constraints experienced<br />
by plants was neglected before the work of Gowing et al. (1998). They proposed the<br />
characterization of these aeration and drought stresses by the <strong>de</strong>velopment of two indices: the<br />
aeration SEV (Sum Exceedance Value) and the drought SEV expressed in cm.day -1 or in<br />
m.week -1 . Each in<strong>de</strong>x consisted of the <strong>de</strong>termination of the timing and the extent of the<br />
constraint by the <strong>de</strong>termination of two thresholds from which stress occurs from March to<br />
September (Gowing et al., 1998; Barber et al., 2004; Gilbert et al., 2003). The aeration SEV<br />
reflects changes in microtopography: aeration mainly drove species success and community<br />
distribution whereas drought does not (Gowing et al., 1998).<br />
For that, dipwells were placed along the topographical gradient in the aim to survey water-<br />
table movements across time with automatic probes (Fig. 1). The SEV aeration was calculated<br />
as the difference between the water table <strong>de</strong>pth and a reference level following this equation<br />
(Swetnam et al., 1998):<br />
where<br />
where WTt is the water-table <strong>de</strong>pth at time t from March to September and RV the reference<br />
water-table <strong>de</strong>pth above which the plants are expected to be aeration-stressed. The reference<br />
water-table <strong>de</strong>pth was expressed as a threshold calculated from a soil moisture release curve<br />
as the <strong>de</strong>pth that gives 10% air-filled porosity inducing an insufficient oxygen diffusion to<br />
supply the respiratory <strong>de</strong>mands of roots during growth period (Gowing et al., 1998; Swetnam<br />
et al., 1998). The aeration threshold thus correspon<strong>de</strong>d to a water table <strong>de</strong>pth of -0.191 m:<br />
when water table level is above this threshold, aeration stress occurs.<br />
114
Chapitre III<br />
Fig 1: Hydrograph realized along the topographical sequence where field experiment ran. The<br />
black line corresponds to the threshold of aeration at -0.191m below the ground surface. In<br />
grey: the area of the hydrograph which rise over the threshold aeration value indicative of the<br />
extent and the time that plants are aeration-stressed.<br />
The SEV aeration was positively correlated with flooding duration, measured as the number<br />
of days when water is above the soil surface from March to September (Pearson correlation,<br />
r=0.862, t=27.71, df=265, P
Chapitre III<br />
Table 1: Corresponding values of duration of submergence and SEV for aeration <strong>de</strong>termined<br />
by Pearson correlation.<br />
Results<br />
Duration of submergence in days Range of SEV aeration (cm.days -1 )<br />
0-7 [0; 4]<br />
10-18 [4.3; 5]<br />
21-27 [5.8; 7.2]<br />
31 7.6<br />
43 8.3<br />
54-57 [8.8; 8.9]<br />
62-65 [9.1; 9.4]<br />
70-74 [9.7; 10.2]<br />
80-89 [10.9; 14.7]<br />
90 [14.9; 16.9]<br />
91 [17.1; 20.7]<br />
92 [20.9; 25.5]<br />
93 [25.6; 30.1]<br />
94 [34.1; 36.9]<br />
Results of AIC between GLM and GAM realized for predictions of realized niches (Table<br />
2) <strong>de</strong>monstrated that realized of the twelve species were better predicted with GAM than with<br />
quadratic GLM because of low AIC values .<br />
Table 2: AIC values of quadratic GLM and GAM predicting realized niches of species.<br />
Quadratic GLM GAM<br />
A. stolonifera 208.91 190.02<br />
B. perennis 103.34 83.88<br />
C. cristatus 115.65 109.39<br />
C. divisa 236.71 223.47<br />
G. fluitans 194.87 187.21<br />
H. secalinum 198.66 181.05<br />
J. articulatus 302.17 289.16<br />
J. gerardii 281.52 245.72<br />
L. autumnalis 67.71 58.08<br />
L. perenne 209.75 193.14<br />
M. pulegium 205.14 195.18<br />
T. fragiferum 288.81 243.95<br />
116
DONNEES COMPLEMENTAIRES<br />
La mesure <strong>de</strong> la fluorescence a le potentiel <strong>de</strong> quantifier la contrainte subie par les<br />
espèces, à travers la mesure <strong>de</strong>s capacités photosynthétiques <strong>de</strong>s plantes, car l’appareil<br />
photosynthétique peut être affecté par la contrainte (Saltmarsh et al. 2006). La fluorescence a<br />
été mesurée lors d’une expérimentation similaire que celle <strong>de</strong> l’article 1.<br />
En raison <strong>de</strong> résultats similaires, certains résultats <strong>de</strong> cette expérimentation<br />
(matériel et métho<strong>de</strong>s en Annexe 3) sont présentés uniquement si <strong>de</strong>s méthodologies<br />
différentes ont été utilisées.<br />
Les résultats <strong>de</strong> l’analyse du ren<strong>de</strong>ment photosynthétique confirment les précé<strong>de</strong>ntes<br />
tendances, c’est-à-dire que l’augmentation en hauteur et en durée <strong>de</strong>s inondations diminue les<br />
performances photosynthétiques <strong>de</strong> J. gerardii (one-way ANOVA, P=1.08e-7), celle <strong>de</strong> L.<br />
autumnalis (one-way ANOVA, P=0.015) excepté pour le <strong>de</strong>rnier traitement, et augmente pour<br />
J. articulatus (one-way ANOVA, P=0.021). En revanche, les traitements n’ont pas d’effet sur<br />
les performances photosynthétiques <strong>de</strong> M. pulegium (one-way ANOVA, P=0.36) et <strong>de</strong> L.<br />
perenne (one-way ANOVA, P=0.57) (Fig. 19). Ces résultats nous permettent <strong>de</strong> directement<br />
approcher la réponse <strong>de</strong>s espèces aux conditions d’inondation.<br />
La tolérance <strong>de</strong>s espèces à l’augmentation <strong>de</strong>s conditions d’inondation a pu être reliée<br />
à la longueur <strong>de</strong> la tige, permettant aux espèces <strong>de</strong> renouer contact avec l’atmosphère<br />
augmentant la disponibilité à la lumière. Pour Juncus articulatus, les tiges dépassaient la<br />
surface <strong>de</strong> l’eau présentant une hauteur supérieure à 50cm.<br />
La présence <strong>de</strong> compétiteurs peut être vue comme une contrainte pour les espèces en<br />
raison <strong>de</strong> la réduction <strong>de</strong> la disponibilité <strong>de</strong>s ressources par les compétiteurs. La présence <strong>de</strong><br />
compétiteurs réduit les performances photosynthétiques <strong>de</strong> L. perenne et J. articulatus pour<br />
les traitements 20cm à durée d’inondation courte (Fig. 19). Les performances <strong>de</strong> L.<br />
autumnalis sont améliorées en présence <strong>de</strong> voisins pour les traitements 20cm, et cette<br />
différence significative pour le traitement 20cm à durée longue d’inondation. Les<br />
performances photosynthétiques <strong>de</strong> M. pulegium et <strong>de</strong> J. gerardii mesurées en présence <strong>de</strong><br />
voisins ne sont pas significativement <strong>de</strong> ses performances en l’absence <strong>de</strong> voisins (two-way<br />
ANOVA, P>0.05).<br />
117
Fig. 19 : Ren<strong>de</strong>ment photosynthétique <strong>de</strong>s espèces soumises à 4 traitements expérimentaux<br />
croissant en monocultures (losanges blancs) et en présence d’un compétiteur A. stolonifera<br />
(losange noir) (moyenne ± écart-type). 10S : 10cm durée courte, 20s : 20cm durée courte,<br />
20L : 20cm durée longue ; 40L : 40cm durée longue. Les différences significatives entre les<br />
ren<strong>de</strong>ments photosynthétiques sans et avec compétiteur sont indiquées par <strong>de</strong>s lettres<br />
distinctes (résultat <strong>de</strong>s tests <strong>de</strong> Tukey).<br />
118
RESUME PARTIE 1<br />
Dans cette partie, nous avons mis en évi<strong>de</strong>nce l’intérêt <strong>de</strong> caractériser les niches<br />
fondamentales <strong>de</strong>s espèces pour comprendre le rôle <strong>de</strong> la compétition dans les patrons<br />
d’abondance <strong>de</strong>s espèces.<br />
Deux types <strong>de</strong> contraintes ont été mis en évi<strong>de</strong>nce le long du gradient d’inondation :<br />
l’augmentation <strong>de</strong> la durée d’inondation et l’absence d’inondation. L’augmentation <strong>de</strong> la<br />
durée d’inondation est une contrainte pour une gran<strong>de</strong> partie <strong>de</strong>s espèces étudiées (9 espèces<br />
sur 12). Les espèces présentent <strong>de</strong>s niches fondamentales contrastées avec différents <strong>de</strong>grés <strong>de</strong><br />
tolérance à l’augmentation <strong>de</strong> la durée d’inondation. Ces contrastes sont observés à la fois<br />
entre espèces appartenant à <strong>de</strong>s communautés végétales différentes, mais ils sont également<br />
observés entre espèces appartenant à une même communauté.<br />
Dans le premier chapitre, nous avons mis en évi<strong>de</strong>nce différents patrons <strong>de</strong> variation<br />
<strong>de</strong> la réponse à la compétition (une composante <strong>de</strong> l’intensité <strong>de</strong> la compétition) le long du<br />
gradient expérimental d’inondations. Certaines espèces (Hor<strong>de</strong>um secalinum, Juncus gerardii<br />
et Mentha pulegium) ont <strong>de</strong>s réponses à la compétition qui varient entre les traitements<br />
d’inondation ; alors que d’autres espèces (Lolium perenne, Carex divisa, Bellis perennis,<br />
Glyceria fluitans et Juncus articulatus) ont <strong>de</strong>s réponses à la compétition qui ne varient pas<br />
entre les traitements expérimentaux d’inondation. Dans ce chapitre, il a également été montré<br />
que l’importance <strong>de</strong> la compétition est négativement corrélée à l’intensité <strong>de</strong> la contrainte<br />
mesurée à l’échelle <strong>de</strong> l’espèce.<br />
La comparaison <strong>de</strong>s niches fondamentales et réalisées <strong>de</strong>s espèces met en évi<strong>de</strong>nce un<br />
déplacement <strong>de</strong>s optima <strong>de</strong>s niches pour les espèces méso-hygrophiles (Juncus gerardii,<br />
Bellis perennis et Leontodon autumnalis) et pour les espèces hygrophiles (Mentha pulegium et<br />
Trifolium fragiferum). Les espèces les moins compétitives seraient exclues vers les parties du<br />
gradient plus contraignantes pour leur développement, c’est-à-dire vers les parties inondées<br />
du gradient. Ces espèces seraient exclues par les espèces mésophiles plus compétitives. La<br />
mise en évi<strong>de</strong>nce <strong>de</strong> ce déplacement <strong>de</strong> niche, couplée à la quantification <strong>de</strong> l’importance <strong>de</strong><br />
la compétition, démontre le rôle structurant <strong>de</strong> la compétition le long du gradient<br />
d’inondation.<br />
119
120
Partie 2 :<br />
La démographie comme outil d’étu<strong>de</strong> <strong>de</strong>s<br />
filtres écologiques<br />
121
122
- Chapitre 4 -<br />
Effect of the environment on species competitive effect and<br />
importance of competition along a flooding gradient.<br />
En préparation<br />
123
124
Chapitre IV<br />
Effect of the environment on species competitive effect and importance of<br />
competition along a flooding gradient.<br />
<strong>Amandine</strong> <strong>Merlin</strong> 1,2 , Christian Damgaard 3 , François Mesléard 2,4 , Anne Bonis 1<br />
1 UMR CNRS 6553 EcoBio, <strong>Université</strong> <strong>de</strong> <strong>Rennes</strong> 1, Campus Beaulieu, 35042 <strong>Rennes</strong><br />
Ce<strong>de</strong>x, France<br />
2 Centre <strong>de</strong> recherche La Tour du Valat, Le sambuc F-13200, France<br />
3 Department of Terrestrial Ecology, NERI, Aarhus University, Vejlsøvej 25, 8600 Silkeborg,<br />
Denmark<br />
4 <strong>Université</strong> d’Avignon - IMEP, IUT site Agroparc BP 1207, F-84911 Avignon Ce<strong>de</strong>x 09,<br />
France<br />
Abstract<br />
Competition is little measured in natural communities. In this study, we examined the<br />
effect of two constraints (flooding and soil drying) observed during two different periods<br />
(during flooding period and after the end of flooding) on the competitive effect of species and<br />
importance of competition, to <strong>de</strong>termine the shaping role of competition on species<br />
distribution using a competition mo<strong>de</strong>l.<br />
The mo<strong>de</strong>l measured the competitive effect of species and the importance of<br />
competition using two ecological variables estimated from pin point measurements realized in<br />
natural communities: species cover and species vertical <strong>de</strong>nsity. The two integrative variables<br />
<strong>de</strong>scribed the plant performances: survival, colonization and biomass growth. This<br />
competition mo<strong>de</strong>l was tested for five wetland species presenting different ecological ranges<br />
along the flooding gradient.<br />
Along the gradient, the increase in flooding duration and in water availability had a<br />
positive effect on survival, colonization, biomass growth and competitive effect of Agrostis<br />
stolonifera and Juncus gerardii. These environmental conditions had no significant effect on<br />
the competitive effect of Lolium perenne and G. fluitans. For Cynosurus cristatus, the<br />
competitive effect <strong>de</strong>creased only with the availability of water resources. Along the gradient,<br />
competition importance of overall studied species increased with the <strong>de</strong>crease in soil drying.<br />
During the floo<strong>de</strong>d period, the importance of competition for L. perenne, J. gerardii and G.<br />
fluitans increased with flooding duration whereas the patterns of competition importance<br />
varied greatly along the gradient for A. stolonifera and C. cristatus.<br />
This competition mo<strong>de</strong>l, which directly measures competition in natural communities,<br />
shows the importance of flooding as a filter influencing species performances such as the<br />
shaping role of competition along the gradient. This mo<strong>de</strong>l provi<strong>de</strong>s a powerful tool to<br />
<strong>de</strong>termine processes acting on communities’ composition and structure.<br />
Key-words: competition mo<strong>de</strong>l, survival, colonization, growth, pinpoint method<br />
125
Chapitre IV<br />
126
Introduction<br />
Chapitre IV<br />
Along environmental gradients, plant species are simultaneously submitted to the<br />
presence of a constraint as well as the presence of competitors interacting for similar<br />
resources affecting species performances. Measurements of the impact of competition relative<br />
to the impact of other environmental factors on species performances are essential for<br />
<strong>de</strong>termining population dynamics and then predict their consequences at the level of the<br />
whole community. However, few studies have measured competition integrating the different<br />
measures of population dynamics (Brooker & Kikvidze 2008; Frekleton et al. 2009).<br />
Along flooding gradients, frequency and duration of flooding represent constraints for<br />
weak tolerant species reducing plant survival (Van Eck et al. 2004; Fraser & Karnesis 2005),<br />
plant recruitment (Lenssen et al. 1998; Casanova & Brock 2000; Fraser & Karnesis 2005;<br />
Kotowski et al. 2010), plant reproduction (Warwick & Brock 2003) or plant growth (Visser et<br />
al. 2003; Fraser & Karnesis 2005). Competition between species also affects these different<br />
measures of plant performances (Howard & Goldberg 2001; Fayolle et al. 2009). Competition<br />
is <strong>de</strong>scribed by two components: the intensity of competition, i.e. the absolute reduction of the<br />
fitness of a species due to the presence of neighbors (Wel<strong>de</strong>n & Slauson 1986) and the<br />
importance of competition, i.e. the reduction of the fitness of a species by the presence of<br />
competitors and by any other process and condition (Wel<strong>de</strong>n & Slauson 1986). Competition<br />
does not only regulate species performances, competition can also represent a main<br />
structuring factor regulating the community structure along environmental gradients.<br />
Studies measuring the two components of competition have been mainly realized<br />
un<strong>de</strong>r controlled conditions. Experimentally, studies have highlighted an increase in the<br />
importance of competition with the <strong>de</strong>crease in the intensity of the constraint along<br />
productivity (Gaucherand et al. 2006), salinity (Greiner la Peyre et al. 2001) and flooding<br />
gradients (Jung et al. 2009). By contrast, generalization of results is not easy for the intensity<br />
of competition. In some cases, it has been <strong>de</strong>monstrated to be constant along salinity,<br />
productivity and disturbance gradients (e.g. Greiner la Peyre et al. 2001; Gaucherand et al.<br />
2006; Carlyle et al. 2010). In other cases, it has been shown to <strong>de</strong>crease with the increase in<br />
the intensity of the constraint as well as with gradients of productivity, salinity and flooding<br />
gradients (e.g. Fraser & Keddy 2005; Pennings et al. 2005). These contrasted results may be<br />
explained by different aspects of the competitive ability of species measured: the competitive<br />
response, measuring the ability of species to tolerate the presence of competitor (Goldberg<br />
1990), or the competitive effect, measuring the ability of species to <strong>de</strong>plete resources for<br />
127
Chapitre IV<br />
others, thus reducing neighbors performances (Goldberg 1990). Competitive response informs<br />
on the species strategies implemented to tolerate the reduction of resources level due to the<br />
presence of neighbors. Competition response <strong>de</strong>pends on the type of resources available in the<br />
environment. Competition response thus appears more variable along gradients than the<br />
competitive effect (Keddy et al. 1994). However, the methodologies used as well as the<br />
distinction between the competitive response and the competitive effect appear insufficient to<br />
predict the real effect of competition experienced by species in natural communities, because<br />
species are affected by the presence of competitors and induce a <strong>de</strong>pletion of resources<br />
simultaneously.<br />
Competition mo<strong>de</strong>ls are now required to measure competition in natural communities.<br />
The <strong>de</strong>velopment of mo<strong>de</strong>ls integrating <strong>de</strong>mographic data of individual plants have a great<br />
potential for measuring competition in natural communities (Goldberg 1999; Aarssen &<br />
Keogh 2002) and providing an interesting approach to dissect out the various processes<br />
influencing the dynamics of populations. Recently, Frekleton et al. (2009) proposed a<br />
dynamic mo<strong>de</strong>l for annual species measuring the two components of competition using<br />
different measures of annual ecological success such as seed production and survival,<br />
consi<strong>de</strong>ring species <strong>de</strong>nsity. Another promising mo<strong>de</strong>l quantifying the importance of<br />
competition and <strong>de</strong>riving from the <strong>de</strong>finition of Wel<strong>de</strong>n & Slauson (1986) (previously<br />
enounced) has been <strong>de</strong>veloped by Damgaard & Fayolle (2010). They proposed a general<br />
method to calculate the two components at any measure of population or individual ecological<br />
success, consi<strong>de</strong>ring the <strong>de</strong>nsity <strong>de</strong>pen<strong>de</strong>nce of interactions and the various levels of<br />
environmental gradients. They thus <strong>de</strong>monstrated that the importance of competition relative<br />
to the level of herbici<strong>de</strong>s appears species-specific.<br />
The aim of this study is to measure the two components of competition in natural<br />
communities. We particularly aimed to: i) test the effect of the environment on the<br />
competitive effect of different species presenting different ecological ranges along a flooding<br />
gradient, ii) predict competition importance relative to environmental conditions on species<br />
ecological success, and iii) compare these results for two periods characterized by two kinds<br />
of constraint: flooding occurring between growing seasons and the reduction of the<br />
availability of water resources occurring during the growing season.<br />
To respond to these objectives, the competition mo<strong>de</strong>l <strong>de</strong>veloped by Damgaard<br />
(submitted) has been used to test the effect of the environmental conditions on the competitive<br />
effect of species and to quantify the importance of competition at species level. The<br />
originality of this approach is the non-<strong>de</strong>structive character of this method measuring two<br />
128
Chapitre IV<br />
ecological variables relevant for the perennial species ecological success: the cover and the<br />
vertical <strong>de</strong>nsity (Damgaard et al. 2009). The competition mo<strong>de</strong>l has been <strong>de</strong>veloped to<br />
estimate competition coefficients between species, i.e. estimating the competitive effect of<br />
species on neighbors’ performances in natural communities (Damgaard et al. 2009). In this<br />
mo<strong>de</strong>l, the vertical <strong>de</strong>nsity is a function of the cover of species during the growing season, as<br />
the change in vertical <strong>de</strong>nsity is assimilated to the biomass growth during the growing season.<br />
Between growing seasons, the cover of species is function of their vertical <strong>de</strong>nsity at the end<br />
of the previous growing season, as cover can be assimilated to survival and colonization<br />
abilities of species in response to the environment. In this way, the effect of the environment<br />
on the processes controlling the biomass growth during the growing season and the processes<br />
controlling the translation of biomass in cover between growing seasons can be studied.<br />
Methods<br />
Study site and vegetation analysis<br />
The study site was the productive and wet grasslands of the Marais Poitevin on the<br />
French Atlantic coast (46°28’N; 1°13’W). The climate is atlantic with a mean annual rainfall<br />
of 655 mm (Amiaud & Touzard 2004). These wet grasslands are characterized by an elevation<br />
gradient with a range of 45 cm, leading to a flooding gradient varying in duration and water<br />
<strong>de</strong>pth (Violle et al. 2007). Annual cycles of flooding start in autumn <strong>de</strong>pending on rainfall<br />
amount. Within a year, two constraints succeed across time. During the floo<strong>de</strong>d/anoxic season<br />
(from end of October to beginning of June), the main constraint is anoxia paired with<br />
submergence, particularly observed in the lower parts of the gradient. During summer-<br />
beginning of autumn (from beginning of June to end of October), the main constraint is the<br />
low availability of water for plants.<br />
Along two different topographical sequences, 70 permanent plots (25*25 cm) were<br />
placed systematically along the gradient spaced by 20 cm from each other (35 plots per<br />
sequence). Along both diagonals of each plot, relevés using the pinpoint method, were<br />
realized every 4 cm, for a total of 17 points per plot. Samples were taken at the beginning<br />
(end of October 2008) and at the end of the floo<strong>de</strong>d/anoxic period (beginning of June 2009),<br />
as well as at the end of summer, right before the beginning of the next flooding period (end of<br />
October 2009). By sampling at these dates, we are able to study competition un<strong>de</strong>r different<br />
water availability constraints.<br />
129
Chapitre IV<br />
As the topographical sequences are located in two different floo<strong>de</strong>d <strong>de</strong>pressions, we<br />
have verified whether hydrological functioning were similar. For that, we have quantified the<br />
intensity of flooding conditions during the flooding period, as well as water availability (i.e.<br />
intensity of drought constraint) during summer, following the proposition of Gowing et al.<br />
(1998) (see Appendix). This method consists in the quantification of flooding intensity<br />
represented by the Sum Excee<strong>de</strong>nce Value (SEV) for aeration (aeration SEV) and the<br />
quantification of the intensity of drought represented by the SEV for drought (soil drying<br />
SEV). The plot of the aeration SEV in relation to the measure of elevation <strong>de</strong>monstrated that<br />
for a same measure of elevation correspon<strong>de</strong>d two different intensities of flooding conditions<br />
(Fig. 1). The hydrological functioning was different between the two topographical<br />
sequences. The measure of the elevation did not represent the best variable to study<br />
competition according to the flooding conditions: we chose to analyze data according to the<br />
intensity of flooding conditions and soil drying, known to better explain species distribution<br />
(Silvertown et al. 1999; Silvertown 2004). The duration of flooding was correlated with<br />
aeration SEV (Pearson correlation, r=0.862, P
Chapitre IV<br />
hygrophilous species recor<strong>de</strong>d in the part of the gradient characterized by long duration of<br />
flooding conditions, is constrained by the absence of flooding conditions. The competition of<br />
these four species was calculated in association with another one: Agrostis stolonifera. This<br />
last species was distributed all along the elevation gradient.<br />
Competition mo<strong>de</strong>l<br />
The species competitive effect was measured for two periods: the flooding season and<br />
the growing season. In or<strong>de</strong>r to evaluate the species competitive effect during the flooding<br />
season, the samples taken after and before flooding were compared as explained below.<br />
Similarly, to evaluate the species competitive effect during the growing season, the samples<br />
taken after and before.<br />
For a given year, the vertical <strong>de</strong>nsity of a species i, Yi, represents the total number of<br />
contact points between the species and any of the 17 pinpoints in the frame. The vertical<br />
<strong>de</strong>nsity appeared to be integrative of the biomass growth of a species during the growing<br />
season (Damgaard et al. 2009). The cover of species i, Xi, is the total number of occurrence of<br />
the species at any of the 17 pinpoints of the frame. It is assumed that the vertical <strong>de</strong>nsity of a<br />
species i at time t2 is an increasing function of the cover of species i, function of the cover of<br />
species j and k at time t1, and function of the environmental gradient zr. Competitive growth<br />
of species i was mo<strong>de</strong>led as:<br />
with r the pin-point frame; the residual process variation during the growing season of species<br />
i across different years and pin-point frames. ai(zr) corresponds to the growth of species i<br />
directly affected by the relation between the cover and the vertical <strong>de</strong>nsity of the species;<br />
cj(zr) and ck(zr) correspond to the competitive effects of species j and k affecting growth of<br />
species i. The parameters a and c are functions of the environmental gradient as linear<br />
functions a0+a1z and c0+c1z.<br />
(1)<br />
131
Chapitre IV<br />
Flooding occurred at the end of autumn and en<strong>de</strong>d in spring of the following year:<br />
colonization and survival abilities of species appeared integrative of fitness of species during<br />
this period (Damgaard et al. 2009). Among growing seasons, it is assumed that the cover of<br />
species i at year t+1 is an increasing function of the vertical <strong>de</strong>nsity of species at the year t (at<br />
the end of the growing season), function of the vertical <strong>de</strong>nsity of species j and k at year t and<br />
function of the environmental gradient zr. Survival and colonization of species i at year t+1<br />
was mo<strong>de</strong>led as:<br />
with the residual process variation from one season to the next of species i among years and<br />
pin-point frames.<br />
Parameters estimation<br />
Mo<strong>de</strong>l parameters (a, b, c and d) were estimating using the Bayesian method: the joint<br />
posterior distribution of mo<strong>de</strong>l parameters were calculated using the MCMC method<br />
(Metropolis-Hastings algorithm with 120000 iterations) and a multivariate distribution. The<br />
sampled chains of all parameters were inspected to check the properties of the sampling<br />
procedure. The effect of the environmental conditions on species performances (parameter a1)<br />
and on species competitive effect (parameter c1) were assessed by the 2.5%, 50% and 97.5%<br />
percentiles taken from the marginal posterior distribution of the parameters.<br />
Measurement of competition importance<br />
From equations (1) and (2), the importance of competition was quantified along the<br />
environmental gradient for the two studied periods, i.e. during the growing season (3) and<br />
during the flooding season (4) following the proposition of Damgaard & Fayolle (2010):<br />
(3)<br />
(4)<br />
Where and represent the absolute changes in the ecological<br />
success of the species i by changing respectively the cover of species j (Xj) and the vertical<br />
(2)<br />
132
Chapitre IV<br />
<strong>de</strong>nsity of species j (Yj). and represent the absolute changes in the<br />
ecological success of the species i by changing the level of the environment z. Absolute<br />
values indicate that the importance of competition is comprised between 0 and 1. All analyses<br />
were run with Mathematica (Wolfram 2003).<br />
Results<br />
Effect of environmental conditions on species performances<br />
The effect of the environmental conditions on species performances (survival,<br />
colonization and growth) was assessed by the percentiles of the parameter a1 (Table 1).<br />
During the growing season, the <strong>de</strong>crease in the intensity of soil drying along the gradient had<br />
a positive effect on growth for L. perenne, C. cristatus, J. gerardii, G. fluitans and A.<br />
stolonifera for the four groups of species tested. For the aggregated species, the effect of the<br />
<strong>de</strong>crease in the intensity of soil drying along the gradient varied between the groups of species<br />
studied: this effect was negative for the A. stolonifera-C. cristatus group and positive only for<br />
the A. stolonifera-J. gerardii group .<br />
During the flooding season, the increase in flooding duration along the gradient had a<br />
positive effect on the survival and colonization for J. gerardii. The increase in flooding<br />
duration had a positive effect on the survival and colonization for A. stolonifera only for the<br />
A. stolonifera-L. perenne group. For the aggregated species, increase in flooding duration had<br />
a positive effect for the four groups of species tested, except the A. stolonifera-L. perenne<br />
group.<br />
Effect of environmental conditions on species competitive effect<br />
The effect of the environmental conditions on species competitive effect was assessed<br />
by the percentiles of the parameter c1 (Table 1). During the growing season, the <strong>de</strong>crease in<br />
the intensity of soil drying along the gradient had a positive effect only on the competitive<br />
effect for J. gerardii and a negative effect only on the competitive effect for C. cristatus. For<br />
A. stolonifera and others aggregated species, the <strong>de</strong>crease in the intensity of soil drying had a<br />
positive effect on their competitive effects for the four groups of species tested.<br />
133
Chapitre IV<br />
During the flooding season, the increase in flooding duration along the gradient had a<br />
positive effect only on the competitive effect for other aggregated species for all groups of<br />
species tested, except for the A. stolonifera-C. cristatus group.<br />
Table 1: Percentiles provi<strong>de</strong>d from the marginal posterior distribution of the parameters a1<br />
and c1, measuring the effect of environment on species ecological success (a1) and species<br />
competitive effect (c1), for both studied periods and for the four groups of species tested. In<br />
bold: parameters significantly <strong>de</strong>viated from zero.<br />
Mo<strong>de</strong>l parameters a1 c1<br />
Confi<strong>de</strong>nce interval 2.5% 50% 97.5% 2.5% 50% 97.5%<br />
During A. stolonifera 10.115 14.415 31.355 0.167 0.208 0.262<br />
growing<br />
season<br />
L.perenne<br />
4.453 9.610 17.900 -0.355 -0.060 0.081<br />
Other aggregated<br />
species<br />
-4.219 -2.340 1.0152 0.048 0.074 0.115<br />
During A. stolonifera 0.0001 0.003 0.009 -0.006 0.001 0.007<br />
flooding L.perenne -0.004 0.002 0.018 -0.023 -0.001 0.013<br />
season Other aggregated<br />
species<br />
-0.0006 0.030 0.103 0.003 0.018 0.034<br />
During A. stolonifera 6.819 8.855 11.196 0.190 0.211 0.223<br />
growing C. cristatus 4.971 5.022 5.694 -0.940 -0.692 -0.497<br />
season Other aggregated<br />
species<br />
-5.760 -3.886 -2.012 0.063 0.067 0.088<br />
During A. stolonifera -0.004 0.0001 0.003 -0.006 0.005 0.022<br />
flooding C. cristatus -0.005 -0.002 0.004 -0.001 0.009 0.029<br />
season Other aggregated<br />
species<br />
0.560 0.630 0.723 -0.004 0.005 0.014<br />
During A. stolonifera 20.337 23.812 27.136 0.278 0.295 0.316<br />
growing J. gerardii 2.017 2.158 2.528 0.130 0.200 0.274<br />
season Other aggregated<br />
species<br />
17.176 22.662 31.033 0.073 0.090 0.109<br />
During A. stolonifera -0.003 0.0005 0.005 -0.003 0.006 0.016<br />
flooding J. gerardii 0.194 0.296 0.446 -0.057 -0.026 0.008<br />
season Other aggregated<br />
species<br />
0.935 1.109 1.412 0.004 0.017 0.030<br />
During A. stolonifera 10.916 18.438 31.817 0.225 0.250 0.283<br />
growing G. fluitans 47.157 67.449 100.842 -0.144 0.208 0.638<br />
season Other aggregated<br />
species<br />
-4.427 1.274 8.262 0.066 0.086 0.117<br />
During A. stolonifera -0.002 0.0005 0.005 -0.018 -0.006 0.0002<br />
flooding G. fluitans -0.001 0.002 0.004 -0.037 -0.007 0.006<br />
season Other aggregated<br />
species<br />
0.906 1.047 1.149 0.002 0.581 0.973<br />
134
Importance of competition<br />
Chapitre IV<br />
The importance of competition was calculated for each group of species tested: the<br />
curves (Fig. 2 & 3) represented the proportion of change in ecological success caused by<br />
competition relative to environmental conditions.<br />
During the growing season, the patterns of competition importance along the gradient<br />
were similar between species (Fig. 2). The importance of competition increased with the<br />
<strong>de</strong>crease in soil drying for L. perenne, C. cristatus and G. fluitans: this importance of<br />
competition was lower in the higher parts of the gradient, followed by a strong increase in<br />
importance of competition and reached a plateau at high values of competition importance for<br />
the rest of the gradient (closed to 1 for each species). For J. gerardii, the importance of<br />
competition increased progressively with the <strong>de</strong>crease in soil drying. For A. stolonifera, the<br />
pattern of competition importance increased with the <strong>de</strong>crease in soil drying but not linearly.<br />
Fig. 2: Pattern of competition importance along the gradient calculated for each group of<br />
tested species during the growing season, with an initial cover of species, i, j and k: 1, 1, 0<br />
135
Chapitre IV<br />
During the flooding season, the patterns of competition importance for L. perenne, J.<br />
gerardii and G. fluitans increased with the flooding duration along the gradient (Fig. 3). The<br />
importance of competition was especially high for J. gerardii but less important for G.<br />
fluitans and L. perenne in floo<strong>de</strong>d parts of the gradient. The importance of competition for C.<br />
cristatus was the lowest for an aeration SEV value of 7cm.day -1 (Fig. 3). The importance of<br />
competition for A. stolonifera varied between the four groups of species tested (Fig. 3). The<br />
importance of competition for A. stolonifera increased with aeration SEV for the A.<br />
stolonifera-L. perenne group. The importance of competition for A. stolonifera <strong>de</strong>creased with<br />
the increase in aeration SEV for the A. stolonifera-G. fluitans group. For the A. stolonifera-J.<br />
gerardii and A. stolonifera-C. cristatus groups, the importance of competition for A.<br />
stolonifera was lowest for aeration SEV values equal to 4 and 8cm.day -1 .<br />
Fig. 3: Pattern of competition importance along the gradient calculated for each group of<br />
species tested during the flooding season with an initial vertical <strong>de</strong>nsity of species, i, j and k:<br />
1, 1, 0.<br />
136
Discussion<br />
Chapitre IV<br />
The analysis of species cover and vertical <strong>de</strong>nsity allows, on the one hand, the<br />
estimation of the effect of the environmental conditions on the competitive effect of species<br />
on neighbors, and on the other hand, the <strong>de</strong>termination whether competition represents a main<br />
factor structuring species assemblages relative to environmental conditions.<br />
Effect of the environmental conditions on species competitive effect<br />
Species performances and competitive effects varied along the gradient for the two<br />
studied periods, i.e. during the floo<strong>de</strong>d period and during the growing season. For J. gerardii<br />
and A. stolonifera, the increase in flooding duration along the gradient increases their survival<br />
and colonization abilities. However, the flooding duration has no effect on their ability to<br />
occupy space. Moreover, the increase in water availability along the gradient, i.e. during the<br />
growing season, positively impacted the biomass growth of these two species and also their<br />
competitive effect. Only performances of these two species increased along the gradient for<br />
the two studied periods. This result suggests a relationship between species performances<br />
between the two studied periods. We can hypothesize that the ability of species to respond to<br />
flooding is an advantage to be competitive afterwards. This hypothesis could be supported by<br />
the growth strategies of these two species. In<strong>de</strong>ed, in response to flooding conditions, A.<br />
stolonifera <strong>de</strong>veloped long stems along which adventitious roots grow, i.e. long above-ground<br />
runners (obs. pers.). This growth strategy is recognized to be a <strong>de</strong>terminant response to<br />
flooding conditions representing an efficient strategy to occupy space and to re-growth after<br />
the end of the flooding period (Soukupova 1994; Lenssen et al. 2004; Benot et al. 2011).<br />
In<strong>de</strong>ed, the adventitious roots allow the rooting of stems after the end of flooding events<br />
(Santamaria 2002; Lenssen et al. 2004). The ability to store resources in rhizomes during a<br />
constrained period such as flooding, known to represent resources storage organs (Dong & <strong>de</strong><br />
Kroon 1994), may explained the increase in plant survival and colonization with flooding<br />
duration highlighted for J. gerardii. The stored resources may be efficiently used when the<br />
<strong>de</strong>crease in flooding conditions starts as it was observed for rhizomatous species such as<br />
Mentha aquatica (Lenssen et al. 2000) or as Phragmites australis (Clevering 1998).<br />
In literature, a relationship between plant biomass and species competitive effect<br />
(Fraser & Miletti 2008), as well as plant size and species competitive effect (Peltzer & Kochy<br />
2001; Wang et al. 2010) has been highlighted. Few plant strategies exist to suppress<br />
neighbors: in high productive environments as ours, the increase in size and consequently in<br />
137
Chapitre IV<br />
biomass represents a strong advantage in capturing light (Grime 1977). An increase in<br />
biomass growth responsible of the increase of species competitive effect with the availability<br />
of water resource during the growing season could be expected. However, this biomass<br />
growth does not appear sufficient to allow species to be competitive during the growing<br />
season in our study. In<strong>de</strong>ed, for C. critatus, L. perenne and G. fluitans, <strong>de</strong>spite a biomass<br />
growth increased with water availability, neither colonization and survival abilities nor the<br />
ability to occupy space are impacted during the flooding period, resulting in an absence of<br />
their ability to suppress neighbors. A hypothesis concerning the growth strategy of these<br />
species may be formulated to un<strong>de</strong>rstand these results. Cynosurus critatus, L. perenne and G.<br />
fluitans present caespitose growths form, highly efficient in semi-arid environments (Derner<br />
& Briske 2001), but less efficient to store resources than the rhizomatous organs in<br />
waterlogged and floo<strong>de</strong>d environment. Thus, the lower ability of species to store resources<br />
during the constrained period may limit the ability of species to re-growth at the end of<br />
flooding event and to be strong competitive species. This may suggest the importance of the<br />
strategies implemented by species, such as the vegetative reproduction and the storage<br />
resources, to respond to flooding (Lenssen et al. 2000) and to explain the dynamic of the<br />
population over the studied periods.<br />
Importance of competition<br />
The importance of competition relative to environmental conditions varied between<br />
the studied periods. In<strong>de</strong>ed the competition has a greater impact than environmental<br />
conditions on species ecological success during the growing season for all studied species and<br />
all along the gradient. While the importance of competition explains 45% of survival and<br />
colonization abilities of L. perenne and C. cristatus during the floo<strong>de</strong>d season compared to<br />
70% in floo<strong>de</strong>d parts of the gradient for J. gerardii and G. fluitans. During the floo<strong>de</strong>d season,<br />
competition does not represent the main filter acting on species ecological success except for<br />
J. gerardii and G. fluitans in floo<strong>de</strong>d parts of the gradient, whereas competition appears to be<br />
the main process acting on species ecological success during the growing season. This then<br />
suggests an important role of flooding conditions in regulating plant populations and species<br />
distribution along the gradient and also a shaping role of competition all along the gradient<br />
after the end of flooding event.<br />
For a given period, patterns of competition importance along the gradient are similar<br />
among species. In<strong>de</strong>ed, the importance of competition increases along the gradient with the<br />
138
Chapitre IV<br />
availability of water resources for all species studied; in general the importance of<br />
competition along the gradient increases with flooding duration. However, the importance of<br />
competition does not <strong>de</strong>crease with the intensity of environmental conditions known to<br />
constrain species performances – such as the increase in flooding duration for C. cristatus, L.<br />
perenne and J. gerardii – as it could be expected (Brooker et al. 2005). Only the importance<br />
of competition increased linearly with flooding duration for G. fluitans, arguing for the<br />
expected negative relationship between the importance of competition and the constraint at<br />
species level, <strong>de</strong>monstrated using experimental data (<strong>Merlin</strong> et al. in prep). Such a linear<br />
relationship is not observed for the studied species affected by the increase in flooding<br />
duration (C. cristatus, L. perenne and J. gerardii) and for A. stolonifera. These results suggest<br />
that the importance of competition cannot always be predicted by the intensity of the<br />
environmental conditions known to reduce species performances. The results may be<br />
explained by the stages of plant life-cycle used as measures of plant performances: these<br />
stages may be differently affected by competition and abiotic factors. Moreover the species<br />
studied have been submitted to several annual cycles of flooding conditions: their response to<br />
abiotic and biotic factors and their distribution along the gradient probably represent the<br />
outcome of past and present processes (Wel<strong>de</strong>n & Slauson 1986; Violle et al. 2010) resulting<br />
in variable patterns of the competition importance.<br />
Despite a similarity in the pattern of competition importance between the species<br />
studied for a given period, the values of competition importance appear different along the<br />
gradient, especially during the floo<strong>de</strong>d period. This suggests that the patterns of competition<br />
importance along the flooding gradient are species-specific.<br />
Conclusion<br />
This study, quantifying both components of competition in natural communities using<br />
a competition mo<strong>de</strong>l, emphasizes the interest to integer the different measures of species life-<br />
cycle such as survival, colonization and biomass growth, as well as other factors such as<br />
environmental conditions, in or<strong>de</strong>r to un<strong>de</strong>rstand the role of competition on species ecological<br />
success (Brooker & Kikvidze 2008; Frekleton et al. 2009). This study thus represents a step<br />
forward the un<strong>de</strong>rstanding of the shaping role of competition on species and populations in a<br />
dynamics context. Further investigations are required for studying competition and predicting<br />
the importance of competition to explain species distribution and communities’ organization<br />
139
Chapitre IV<br />
consi<strong>de</strong>ring long term monitoring. In<strong>de</strong>ed we lack a step back to really un<strong>de</strong>rstand the patterns<br />
of competition importance.<br />
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142
Appendix<br />
Material and Methods<br />
The characterization of the gradient<br />
Chapitre IV<br />
A precise characterization of these waterlogging and drought constraints experienced<br />
by plants was neglected before the work of Gowing et al. (1998). They proposed the<br />
characterization of these aeration and drought stresses by the <strong>de</strong>velopment of two indices: the<br />
aeration SEV (Sum Exceedance Value) and the drought SEV expressed in cm.day -1 or in<br />
m.week -1 . Each in<strong>de</strong>x consisted of the <strong>de</strong>termination of the timing and the extent of the<br />
constraint by the <strong>de</strong>termination of two thresholds from which stress occurs from March to<br />
September (Gowing et al., 1998; Barber et al., 2004; Gilbert et al., 2003). The aeration SEV<br />
reflects changes in microtopography: aeration mainly drove species success and community<br />
distribution whereas drought does not (Gowing et al., 1998).<br />
For that, dipwells were placed along the topographical gradient in the aim to survey water-<br />
table movements across time with automatic probes (Fig. 1). The SEV aeration was calculated<br />
as the difference between the water table <strong>de</strong>pth and a reference level following this equation<br />
(Swetnam et al., 1998):<br />
where<br />
where WTt is the water-table <strong>de</strong>pth at time t from March to September and RV the reference<br />
water-table <strong>de</strong>pth above which the plants are expected to be aeration-stressed. The reference<br />
water-table <strong>de</strong>pth was expressed as a threshold calculated from a soil moisture release curve<br />
as the <strong>de</strong>pth that gives 10% air-filled porosity inducing an insufficient oxygen diffusion to<br />
supply the respiratory <strong>de</strong>mands of roots during growth period (Gowing et al., 1998; Swetnam<br />
et al., 1998). The aeration threshold thus correspon<strong>de</strong>d to a water table <strong>de</strong>pth of -0.191 m:<br />
when water table level is above this threshold, aeration stress occurs. Similarly, the soil<br />
drying threshold corresponds to a water table <strong>de</strong>pth of -0.42 m: when water table level is<br />
below this threshold, drought stress occurs.<br />
143
Chapitre IV<br />
144
- Chapitre 5 -<br />
The <strong>de</strong>mography of space occupancy: measuring plant<br />
colonization and survival probabilities using repeated pin-point<br />
measurements<br />
Accepté dans Methods in Ecology and Evolution<br />
MEE (2011) 2: 100-115<br />
145
146
Chapitre V<br />
The <strong>de</strong>mography of space occupancy: measuring plant colonisation and<br />
survival probabilities using repeated pin-point measurements<br />
Christian Damgaard 1 , <strong>Amandine</strong> <strong>Merlin</strong> 2,3 , François Mesléard 3,4 , Anne Bonis 2<br />
1 Department of Terrestrial Ecology, NERI, Aarhus University, Vejlsøvej 25, 8600 Silkeborg,<br />
Denmark<br />
2 UMR CNRS 6553 EcoBio, <strong>Université</strong> <strong>de</strong> <strong>Rennes</strong> 1, Campus Beaulieu, 35042 <strong>Rennes</strong><br />
Ce<strong>de</strong>x, France<br />
3 Centre <strong>de</strong> recherche La Tour du Valat, Le sambuc F-13200, France<br />
4 UMR CNRS-IRD 6116 IMEP, <strong>Université</strong> d’Avignon - IUT site Agroparc BP 1207, F-84911<br />
Avignon Ce<strong>de</strong>x 09, France<br />
Summary<br />
1. The study of plant <strong>de</strong>mography has long been an important component of plant ecological<br />
studies. However in plant communities, e.g. grasslands, where individual plants are not<br />
easily distinguished and often vary in size, a convenient method of <strong>de</strong>scribing the<br />
<strong>de</strong>mography and its ecological consequences has been lacking.<br />
2. The aim of the present paper is to discuss the potential for using the change in the<br />
probability of space occupancy as a measure of ecological success in plant population<br />
biology. This will be done by <strong>de</strong>monstrating how the change in the probability of space<br />
occupancy <strong>de</strong>pends on the processes of colonisation and survival and <strong>de</strong>monstrating how<br />
colonisation and survival probabilities may be estimated from repeated pin-point<br />
recordings at the same pin-position. Furthermore, it will be <strong>de</strong>monstrated how to calculate<br />
the sensitivity and elasticity of the change in the probability of space occupancy to<br />
colonisation and survival probabilities.<br />
3. The method is applied to a case of repeated pin-point data of the perennial grass<br />
Brachypodium phoenicoï<strong>de</strong>s that un<strong>de</strong>rwent different grazing regimes from 2001 to 2008<br />
on xero-halophytic grasslands.<br />
4. It was <strong>de</strong>monstrated that grazing affected both plant survival and colonisation probabilities<br />
so that the estimated colonisation and survival probabilities of B. phoenicoï<strong>de</strong>s were<br />
highest in the non-grazed regime. Furthermore, using the calculated elasticity of the change<br />
in space occupancy, we found that survival was more important than colonization events in<br />
<strong>de</strong>termining the ecological success of B. phoenicoï<strong>de</strong>s in the non-grazed regime, whereas<br />
colonization events were more important than survival in the grazed regime.<br />
5. Synthesis and applications. We expect that the relatively simple method may be wi<strong>de</strong>ly<br />
used on existing and future repeated pin-point recordings from the same pin-position and,<br />
consequently, that plant <strong>de</strong>mographic questions may be addressed with larger precision in<br />
plant communities where individual plants are not easily distinguished.<br />
Keywords: colonisation, <strong>de</strong>mography, elasticity, plant cover, sensitivity, survival<br />
147
Chapitre V<br />
148
Introduction<br />
Chapitre V<br />
Knowledge of recruitment and survival probabilities at different environmental<br />
conditions is important for un<strong>de</strong>rstanding fundamental plant ecological processes, which often<br />
play a significant role in the dynamics of the entire ecosystem (Harper, 1977). Recruitment<br />
and survival probabilities are, typically, estimated from <strong>de</strong>mographic data of individual<br />
plants. For example, Batista et al. (1998) studied the effect of a hurricane on the <strong>de</strong>mography<br />
of Fagus gradifolia trees by censusing individual trees. However, in many natural plant<br />
communities dominated by perennial plants, e.g. grasslands, it is often difficult to distinguish<br />
individual plants due to their vegetative growth pattern and, consequently, to obtain reliable<br />
<strong>de</strong>mographic data at the level of the individual. Furthermore, once individual plants can be<br />
distinguished, they often vary markedly in size so that the number of individuals is<br />
insufficient for <strong>de</strong>scribing the plant population. Instead, plant abundance may be <strong>de</strong>scribed by<br />
the cover, i.e. the relative area covered by different plant species, rather than using the number<br />
of individuals (Kent and Coker, 1992). Plant cover data may be used to classify the studied<br />
plant community into a vegetation type, to test different ecological hypotheses on plant<br />
abundance, and in gradient studies, where the effects of different environmental gradients on<br />
the abundance of specific plant species are investigated (Damgaard, 2008, Damgaard, 2009,<br />
Damgaard and Ejrnæs, 2009).<br />
A popular and objective method of measuring plant cover is the pin-point method, also<br />
known as the point-intercept method (Kent and Coker, 1992, Levy and Mad<strong>de</strong>n, 1933). In a<br />
pin-point analysis, a frame with a number of fixed pin positions is placed above the vegetation<br />
and a pin is inserted vertically into the vegetation at each pin position. The cover of plant<br />
species A is measured by , where is the number of pins that hit species A out of a total<br />
of n pins. Since a single pin often will hit more than a single species, the sum of the plant<br />
cover of the different species may be larger than unity when estimated by the pin-point<br />
method. The sum of the estimated plant cover is expected to increase with the number of plant<br />
species in a plot and with increasing 3-dimensional structuring of the plants in the<br />
community.<br />
Instead of calculating plant cover by estimating the mean probability of being hit by<br />
one of the n pins, it is possible to consi<strong>de</strong>r each pin-position separately and <strong>de</strong>fine the<br />
probability of space occupancy by plant species A at the specific pin-position. If the location<br />
of the pin-point frame is marked on the ground so that the pin-point frame may be placed at<br />
exactly the same location the next time the plot is visited, then it is possible to track the fate of<br />
149
Chapitre V<br />
a species at a specific pin-position through time. If the species was present at time t but absent<br />
at time t +1, we may loosely speak of a mortality event, and if the species is absent from a<br />
specific pin-position at time t and present at time t +1, we may loosely speak of a colonisation<br />
event. We have chosen to use the term “colonisation” rather than “recruitment” since the<br />
event is <strong>de</strong>fined by a novel occurrence in space (Adler et al., 2006, Adler et al., 2009).<br />
However, in the interpretation of the results, it is important to remember that the concepts of<br />
colonisation and mortality have a different meaning than usual in studies where individuals<br />
are consi<strong>de</strong>red. For perennial plant species that spread clonally by forming well <strong>de</strong>fined<br />
ramets, the concepts of colonisation and mortality at a certain pin position make apparent<br />
biological sense whereas, for species characterised by more continuous plant growth, the<br />
concepts are ina<strong>de</strong>quate <strong>de</strong>scriptions of the un<strong>de</strong>rlying biological causes of a change in plant<br />
abundance. Bearing this issue of terminology in mind, we may, generally, assert that the<br />
probability of space occupancy of a plant species will increase with colonisation and <strong>de</strong>crease<br />
with mortality.<br />
The quantitative effect of birth and <strong>de</strong>ath processes on population growth of<br />
individuals is most effectively summarised using matrix population mo<strong>de</strong>ls (Caswell, 2001),<br />
and the calculation of sensitivity and elasticity of different birth and <strong>de</strong>ath processes on<br />
population growth rates has particularly been shown to be a powerful tool to investigate the<br />
importance of different <strong>de</strong>mographic variables in <strong>de</strong>termining population growth. For<br />
example, Dalgleish et al. (2010) used elasticity calculations to conclu<strong>de</strong> that climate change<br />
was more likely to influence population growth through effects on recruitment than through<br />
survival for 10 short-lived forb species. Likewise, when studying the effects of a variable<br />
environment, which may affect both colonisation and survival probabilities, it will be<br />
important to be able to calculate the sensitivity and elasticity of the change in the probability<br />
of space occupancy as influenced by variable colonisation and survival. Such sensitivity and<br />
elasticity calculations have not been discussed before.<br />
The aim of the present paper is, therefore, to discuss the potential of using the change<br />
in the probability of space occupancy as a measure of ecological success in plant population<br />
biology. This will be done by i) showing how the change in the probability of space<br />
occupancy <strong>de</strong>pends on colonisation and survival, ii) <strong>de</strong>monstrating how colonisation and<br />
survival probabilities may be estimated from repeated recordings at the same pin-position,<br />
and iii) calculating the sensitivity and elasticity of the change in the probability of space<br />
occupancy to colonisation and survival probabilities.<br />
150
Mo<strong>de</strong>l<br />
Chapitre V<br />
If absence-presence data of species A from two successive recordings from the same<br />
pin-position are consi<strong>de</strong>red, there are four possible transition events and corresponding<br />
probabilities (Table 1). These transition probabilities <strong>de</strong>pend on i) the probability (p) that a<br />
plant of species A is present at time t, ii) survival; the probability (s) that a plant of species A<br />
is present at the pin-position at both time t and time t+1, and iii) colonisation; the probability<br />
(c) that a plant of species A is present at the pin-position at time t+1 but was absent at time t.<br />
Table 1. The four possible events of two successive recordings of presence absence data and<br />
their corresponding probabilities, p: current probability, c: colonisation probability, s: survival<br />
probability.<br />
Event Description Probability<br />
X1: At , At+1<br />
A plant of species A was present in year t and was also<br />
present in year t+1<br />
X2: nAt , nAt+1 A plant of species A was not present in year t and was<br />
X3: At , nAt+1<br />
X4: nAt , At+1<br />
also not present in year t+1<br />
A plant of species A was present in year t but was not<br />
present in year t+1<br />
A plant of species A was not present in year t but was<br />
present in year t+1<br />
The change in the probability that species A is present in year t to t +1 is <strong>de</strong>noted by<br />
and may be <strong>de</strong>fined, analogous to the population growth rate λ of individuals (Caswell, 2001),<br />
as the ratio between the probability that species A is present at time t + 1 and the<br />
probability p that species A is present at time t :<br />
where are <strong>de</strong>fined in Table 1.<br />
(1),<br />
151
Chapitre V<br />
From the above <strong>de</strong>finition, it is apparent that if , then the probability that species<br />
A is present <strong>de</strong>creases, and if , then the probability that species A is present increases.<br />
Furthermore, the change in the probability that species A is present is a function of the current<br />
probability (p), the colonisation probability and the survival probability probabilities (Fig 1).<br />
Note also, that the plant cover of species A at time t is estimated by taking the mean of p<br />
across space.<br />
Fig. 1. The change in the probability that species A is present ( ) as a function of the<br />
colonisation (c) and survival (s) probabilities. The current probability that species A is present<br />
(p) is set to 0.5.<br />
The change in the probability of being present, as <strong>de</strong>fined above in equation (1), is<br />
always positive, which assures that it is possible to calculate both the sensitivity and elasticity<br />
of the change (Caswell, 2001). The sensitivity of the change in the probability that species A<br />
is present is a function of the colonisation and survival probabilities which are <strong>de</strong>fined as:<br />
Likewise, the elasticity of the change in the probability that species A is present is a<br />
function of the colonisation and survival probabilities which are <strong>de</strong>fined as:<br />
(2).<br />
152
Estimation and statistical inferences<br />
(3).<br />
Chapitre V<br />
The colonisation and survival probabilities may be estimated from data of the four<br />
possible events of two successive recordings of presence-absence data ,<br />
which are <strong>de</strong>fined in Table 1, by maximising the likelihood function of the multinomial<br />
distribution using the transition probabilities specified in Table 1, and assuming that<br />
the plants are distributed homogenously across the site of the investigation, such that the<br />
probability that species A is present at time t may be estimated<br />
as :<br />
or by simulating the Bayesian joint posterior distribution of the colonisation and survival<br />
probabilities using MCMC methods (Carlin and Louis, 1996), where the prior distribution of<br />
the colonisation and survival probabilities may be assumed to be beta-distributed (Chew,<br />
1971).<br />
Different hypotheses on the colonisation and survival probabilities may be tested by<br />
standard hierarchical likelihood ratio procedures or by comparing different joint posterior<br />
distributions of the parameters. Furthermore, the credibility interval of the sensitivity and<br />
elasticity may be calculated from the joint posterior distribution of the colonisation and<br />
survival probabilities (Ghosh et al., 2006).<br />
Example<br />
In or<strong>de</strong>r to <strong>de</strong>monstrate the potential of the method, repeated pin-point data of the<br />
dominant perennial grass Brachypodium phoenicoï<strong>de</strong>s on xero-halophytic grasslands at the<br />
Tour du Valat estate, Rhône <strong>de</strong>lta, Southern France (43°29’ N, 4°40’ E) were analysed. The<br />
xero-halophytic grasslands, which are a priority habitat (co<strong>de</strong> 6220) in the European Union<br />
(4),<br />
153
Chapitre V<br />
Habitats Directive (1992) and the richest habitat of the Camargue (Molinier and Tallon,<br />
1968), are managed by extensive livestock grazing using traditional methods (Otero and<br />
Bailey, 2003). The xero-halophytic grasslands are distributed along an elevation gradient with<br />
variable salinity, and they experience large variation in annual rain fall (Fig 2).<br />
Fig 2 : Histogram reprensenting the mean probability of space occupancy (= plant cover) of B.<br />
phoenicoï<strong>de</strong>s in non-grazed (dark grey) and grazed regimes (light gray) from 2001 to 2008<br />
plotted together with the annual rainfall (black line) at Tour du Valat Station, Camargue,<br />
France.<br />
In autumn 2001, an experimental <strong>de</strong>sign of paddocks with three different grazing<br />
regimes was set up in or<strong>de</strong>r to study the effect of domestic grazing on space occupancy and<br />
population persistence. Two of those grazing regimes are consi<strong>de</strong>red in this example: (i) an<br />
extensive grazing pressure that has been applied continuously since 1970 and (ii) a treatment<br />
where grazing was prevented by fencing at the start of the experiment in 2001. Twenty-one<br />
permanent quadrates of 0.16 m² were placed along the elevation gradient in each grazing<br />
regime. In these permanents quadrates, a set of crossing lines was used to position 36 pins so<br />
that the distance between the pins along the crossing line was three centimetres. Pin-point<br />
cover data were collected in April over an eight year period, from 2001 to 2008.<br />
Two different plant community types may be discriminated along the elevation<br />
gradient (Mesléard et al., 1991), which loosely may be categorised as either grassland or salt<br />
marches, and it is the grassland plant community that is dominated by B. phoenicoï<strong>de</strong>s, which<br />
is investigated in this example with five quadrates in the grazed regime and sixteen quadrates<br />
in the non-grazed regime.<br />
154
Chapitre V<br />
For both grazing regimes the colonisation and survival probabilities were estimated<br />
from two successive yearly recordings of B. phoenicoï<strong>de</strong>s and the sensitivities and elasticities<br />
of the change in space occupancy were calculated (Table 2).<br />
Table 2. The estimated colonisation and survival probabilities of B. phoenicoï<strong>de</strong>s, the change<br />
in the probability of space occupancy ( ), and the calculated sensitivities and elasticities of<br />
the change in the probability of space occupancy to colonisation and survival in the two<br />
grazing regimes.<br />
Grazing<br />
regime<br />
Non-<br />
grazed<br />
Grazed<br />
Year Colonisation Survival Colonisation<br />
sensitivity<br />
Colonisation<br />
elasticity<br />
Survival<br />
sensitivity<br />
Survival<br />
elasticity<br />
2001/2002 0.35 0.66 1.51 2.42 0.56 0.65 0.28<br />
2002/2003 0.65 0.85 1.62 1.19 0.47 0.36 0.19<br />
2003/2004 0.20 0.69 0.80 0.57 0.14 0.80 0.68<br />
2004/2005 0.33 0.71 1.00 0.86 0.29 0.67 0.47<br />
2005/2006 0.57 0.91 1.29 0.66 0.29 0.43 0.30<br />
2006/2007 0.22 0.84 0.93 0.37 0.09 0.78 0.71<br />
2007/2008 0.27 0.73 0.89 0.58 0.18 0.73 0.60<br />
mean 0.37 0.77 1.15 0.95 0.29 0.63 0.46<br />
CV 46.43 12.36 28.15 73.41 60.24 27.45 45.18<br />
2001/2002 0.20 0.44 0.86 2.06 0.49 0.80 0.41<br />
2002/2003 0.38 0.77 1.58 2.14 0.52 0.62 0.30<br />
2003/2004 0.20 0.45 0.72 1.38 0.37 0.81 0.51<br />
2004/2005 0.20 0.51 0.92 2.03 0.45 0.80 0.44<br />
2005/2006 0.23 0.56 1.08 2.21 0.48 0.77 0.40<br />
2006/2007 0.20 0.29 0.74 2.29 0.62 0.80 0.31<br />
2007/2008 0.27 0.44 1.27 3.02 0.65 0.73 0.25<br />
mean 0.24 0.49 1.02 2.16 0.51 0.76 0.37<br />
CV 28.05 29.58 30.35 22.26 18.77 8.93 23.89<br />
Grazing affected plant survival and colonisation probabilities, and the estimated<br />
colonisation and survival probabilities of B. phoenicoï<strong>de</strong>s were highest in the non-grazed<br />
regime. Moreover, the estimated colonisation probabilities were more variable across years<br />
than the estimated survival probabilities, particularly in the non-grazed regime, and it was<br />
hypothesised that these variations in the colonisation probabilities may be linked to variation<br />
in annual rainfall (Fig. 2). It was expected that high water availability would increase<br />
colonisation probabilities, but there was no significant correlation between annual rainfall and<br />
155
Chapitre V<br />
colonisation probability (Spearman correlations; non-grazed regime: P = 0.302 and grazed<br />
regime: P=0.435). This unexpected result may be due to <strong>de</strong>nsity-<strong>de</strong>pen<strong>de</strong>nce which may<br />
induce a temporal variation in the estimated probabilities (Grant and Benton, 2000) due to<br />
overlapping generations.<br />
Based on the calculated elasticity of the change in space occupancy (Table 2), it was<br />
found that survival was more important than colonization events in <strong>de</strong>termining the ecological<br />
success of B. phoenicoï<strong>de</strong>s in the non-grazed regime, i.e. low mortality played a major role in<br />
the increase of the cover of B. phoenicoï<strong>de</strong>s from 2001 to 2008 (Fig. 2). Oppositely,<br />
colonization events were found to be more important than survival in the grazed regime.<br />
In the non-grazed regime, sensitivity and elasticity of estimated probabilities were<br />
inconsistent, i.e. colonisation seemed to be more important than mortality according to the<br />
sensitivity calculations in the first years after cessation of grazing, whereas the contrary was<br />
observed when elasticity was used.<br />
Discussion<br />
Space occupancy in <strong>de</strong>nse vegetation is a sign of successful colonisation and survival<br />
of individual plants, and the probability of space occupancy, or plant cover, is, therefore, a<br />
relevant measure of the ecological success of plant species (Damgaard et al., 2009).<br />
Furthermore, an increased emphasis on the empirical study of the spatial dynamics of plant<br />
populations is in excellent agreement with the recent <strong>de</strong>velopment in the theoretical<br />
un<strong>de</strong>rstanding on the spatial dynamics of plant communities (e.g. Law et al., 1997, Bolker and<br />
Pacala, 1999, Dieckmann et al., 2000). Here, theoretical and empirical plant ecology are<br />
linked by showing how a change in the probability of space occupancy <strong>de</strong>pends on the<br />
ecological processes of colonisation and survival, and by introducing methods for calculating<br />
the sensitivity and elasticity of the change in the probability of space occupancy on the<br />
ecological processes. The two measures to compare the effect of colonisation and survival on<br />
space occupancy complement each other, but since sensitivity is an absolute measure and is<br />
less robust than elasticity; elasticity is often the favoured measure when comparing the effect<br />
of <strong>de</strong>mographic parameters between and within species (<strong>de</strong> Kroon et al., 1986, van<br />
Groenendael et al., 1994).<br />
As stated in the introduction, the concepts of mortality and colonisation are only<br />
loosely connected to the usual biological <strong>de</strong>finitions of the two concepts that are tightly linked<br />
156
Chapitre V<br />
to the <strong>de</strong>scription of the fate of individual plants. For example, the estimated survival<br />
probability inclu<strong>de</strong>s both true survival as well as mortality followed by colonisation, and the<br />
colonisation probability aggregates both vegetative and reproductive growth. For some<br />
applications and for some species, such an aggregation of the <strong>de</strong>mographic processes may<br />
obscure a more mechanistic un<strong>de</strong>rstanding of the ecological processes occurring at the level<br />
of individual plants.<br />
In the presented methodology, it is <strong>de</strong>monstrated how colonisation and survival<br />
probabilities may be estimated from repeated recordings at the same pin-position, and it is<br />
implicitly assumed that the plant is either present or absent at the pin-position. However,<br />
consi<strong>de</strong>r a plant that is rooted some distance away for the pin-position; in favourable years,<br />
the leaves of such a plant may grow sufficiently to be hit by the pin, whereas in unfavourable<br />
years, growth may be insufficient to be hit by the pin. Such a plant will be misclassified in the<br />
pin-point analysis and this misclassification will lead to an upwards biased estimate of<br />
colonisation probability and a downwards biased estimate of survival probability. A similar<br />
effect will arise if the pin-point frame is not situated exactly the same place each year. The<br />
possible effect of such misclassification is expected to <strong>de</strong>pend on the phenology and growth<br />
pattern of the specific plant species and the consequences for the ecological interpretation will<br />
have to be evaluated for each species and may be estimated with confi<strong>de</strong>nce intervals of space<br />
occupancy calculation using re-sampling methods as proposed by Valver<strong>de</strong> and Silvertown<br />
(1998).<br />
The method may be generalised so that the survival probability varies with plant age,<br />
i.e. the survival probability is assumed to <strong>de</strong>pend on the uninterrupted number of years the<br />
species is present at the position. Such a generalisation of the method may be especially<br />
important when the effect of e.g. extreme climatic events or other forms of disturbances on<br />
plant mortality and recruitment <strong>de</strong>pend on plant age or size (e.g. O'Connor, 1993).<br />
In the present study, it is assumed that the plants are distributed homogenously across<br />
the site of investigation, but if this assumption is known to not be approximately true, then it<br />
may be appropriate to partition the site of investigation into areas with approximately equal<br />
local <strong>de</strong>nsities and estimate the colonisation and survival probabilities for each area.<br />
Additionally, it may be beneficial to characterise the spatial variation using methods of spatial<br />
point pattern analysis (Diggle, 2003, Wiegand et al., 2007, Law et al., 2009). Furthermore, in<br />
the present analysis, the possible competitive effects among neighbouring plants are inclu<strong>de</strong>d<br />
in the estimated probabilities of survival and colonisation for each species, instead of<br />
examining the spatial intra- and inter-specific interactions among the studied plant species<br />
157
Chapitre V<br />
(e.g. Rees et al., 1996, Law et al., 1997, Adler et al., 2009). The mo<strong>de</strong>l choice whether to<br />
inclu<strong>de</strong> the competitive effects among neighbouring plants into “non-spatial” probabilities of<br />
mortality and colonisation, or to examine the competitive interaction in a spatial competition<br />
mo<strong>de</strong>l, hinges on i) the spatial <strong>de</strong>sign, for example, if the pin-point data arise from linear<br />
transects, as in the present case, or if the distance between the pins is relatively large, then the<br />
spatial information is not sufficiently <strong>de</strong>tailed to allow a spatial analysis of the effect of<br />
neighbouring plants; ii) statistical power, since the spatial mo<strong>de</strong>ls need more parameters than<br />
the non-spatial mo<strong>de</strong>l, the size of the <strong>de</strong>sign may be insufficient to mo<strong>de</strong>l the spatial inter-<br />
specific interactions; iii) the number of species, if the number of species in the plant<br />
community is relatively high and the species-specific competition kernels are expected to vary<br />
among species, then it may be unmanageable to mo<strong>de</strong>l the spatial inter-specific interaction<br />
and instead it may be useful, as in the present study, to consi<strong>de</strong>r the neighbouring vegetation<br />
as a variable matrix that influences mortality and colonisation; iv) the scientific question, for<br />
some scientific question species, specific probabilities of survival and colonisation may be the<br />
most relevant measure of the ecological success.<br />
In this manuscript, space occupancy data are assumed to be sampled by the pin-point<br />
method, which is especially relevant for plant species with a relatively high cover. However,<br />
for a long time, it has been technically possible to measure plant cover in continuous two-<br />
dimensional space by using a pantograph (Adler et al., 2006, Adler et al., 2009), but since the<br />
pantograph method is cumbersome, such plant cover data are relatively rare compared to pin-<br />
point data. In the near future, the use of digital imagery for measuring the change in plant<br />
cover through time is expected to become much more important (e.g. Seefeldt and Booth,<br />
2006) and the forthcoming of such plant cover data will allow a more sophisticated mo<strong>de</strong>lling<br />
of plant <strong>de</strong>mography, e.g. by including the effect of competitive interactions (Adler et al.,<br />
2006, Adler et al., 2009).<br />
Acknowledgements<br />
This work is a publication from the DIVHERB Project (French national program ECOGER<br />
fun<strong>de</strong>d by the Institut National <strong>de</strong> la Recherche Agronomique) and a contribution to GDR<br />
2574 “TRAITS”. Also thanks to College Doctoral International <strong>de</strong> Bretagne for providing a<br />
mobility grant, and two anonymous reviewers and Charlotte Elisabeth Kler for comments on a<br />
previous version of the manuscript.<br />
158
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heritage. Friends of the countrysi<strong>de</strong>pp. 463-467. The European Estate Friends of the Country<br />
si<strong>de</strong>, Brussel.<br />
Rees, M., Grubb, P. J. & Kelly, D. (1996) Quantifying the impact of competition and spatial<br />
heterogeneity on the structure and dynamics of a four-species guild of winter annuals.<br />
American Naturalist, 147, 1-32.<br />
Seefeldt, S. S. & Booth, D. T. (2006) Measuring plant cover in sagebrush steppe rangelands:<br />
A comparison of methods. Environmental Management, 37, 703-711.<br />
Valver<strong>de</strong>, T. & Silvertown, J. (1998) Variation in the <strong>de</strong>mography of a woodland un<strong>de</strong>rstorey<br />
herb (Primula vulgaris) along the forest regeneration cycle: projection matrix analysis.<br />
Journal of Ecology, 86, 545-562.<br />
van Groenendael, J., <strong>de</strong> Kroon, H., Kalisz, S. & Tuljapurkar, S. (1994) Loop analysis:<br />
evaluating life history pathways in population projection matrices. . Ecology, 75, 2410-2415.<br />
Wiegand, T., Gunatilleke, S., Gunatilleke, N. & Okuda, T. (2007) Analysing the spatial<br />
structure of a Sri Lankan tree species with multiple scales of clustering. Ecology, 88, 3088–<br />
3102.<br />
160
RESUME PARTIE 2<br />
Dans ces <strong>de</strong>ux chapitres, la métho<strong>de</strong> <strong>de</strong>s points-contact a été utilisée pour déterminer<br />
les probabilités <strong>de</strong> survie et <strong>de</strong> colonisation <strong>de</strong>s espèces, ainsi que la croissance en biomasse, à<br />
partir du recouvrement d’une espèce et du volume occupé par l’espèce.<br />
Dans les prairies humi<strong>de</strong>s du Marais Poitevin (Chapitre 4), un modèle a été construit<br />
afin d’estimer <strong>de</strong>ux composantes <strong>de</strong> la compétition : l’effet compétitif <strong>de</strong>s espèces et<br />
l’importance <strong>de</strong> la compétition. Ces <strong>de</strong>ux composantes ont été estimées sur <strong>de</strong>ux pério<strong>de</strong>s<br />
d’étu<strong>de</strong> : la pério<strong>de</strong> d’inondation et la saison estivale faisant suite à la pério<strong>de</strong> d’inondation.<br />
Cette étu<strong>de</strong> montre l’intérêt <strong>de</strong> tenir compte <strong>de</strong>s paramètres démographiques <strong>de</strong>s espèces pour<br />
comprendre la capacité <strong>de</strong>s espèces à être compétitive face aux voisins. En effet l’importance<br />
<strong>de</strong> la survie <strong>de</strong>s espèces aux inondations et à leur reprise via la colonisation du milieu est<br />
montrée pour que l’espèce puisse être compétitive notamment après la fin <strong>de</strong>s inondations.<br />
Cette étu<strong>de</strong> met en évi<strong>de</strong>nce le rôle structurant <strong>de</strong>s inondations qui affectent la survie et la<br />
colonisation <strong>de</strong>s espèces, ainsi que le rôle structurant <strong>de</strong> la compétition qui augmente avec<br />
l’arrêt <strong>de</strong>s inondations.<br />
Le chapitre 5 propose une analyse d’élasticité <strong>de</strong>s paramètres démographiques dans le<br />
but <strong>de</strong> comprendre quel paramètre démographique influence le plus fortement la dynamique<br />
<strong>de</strong>s populations, et plus particulièrement la probabilité <strong>de</strong>s espèces pérennes à occuper<br />
l’espace. Cette analyse d’élasticité réalisée sur une espèce pérenne méditerranéenne,<br />
Brachypodium phoenicoï<strong>de</strong>s, a mis en évi<strong>de</strong>nce un effet <strong>de</strong> la gestion <strong>de</strong> pâturage sur la<br />
probabilité <strong>de</strong> cette espèce à occuper l’espace. En effet, en milieu non pâturé, la survie <strong>de</strong><br />
cette espèce expliquerait son succès à occuper l’espace et par conséquent son augmentation en<br />
abondance au cours <strong>de</strong>s années. En milieu pâturé, la probabilité <strong>de</strong> colonisation expliquerait<br />
plus la dynamique <strong>de</strong> cette espèce. Ces résultats suggèrent un effet négatif <strong>de</strong> la présence <strong>de</strong>s<br />
herbivores sur la survie <strong>de</strong> cette espèce.<br />
161
162
Partie 3 :<br />
La part <strong>de</strong>s filtres abiotiques et biotiques dans<br />
la structure <strong>de</strong>s communautés végétales<br />
163
164
- Chapitre 6 -<br />
Detecting community assembly patterns in floo<strong>de</strong>d and productive<br />
grasslands<br />
En préparation<br />
165
166
Chapitre VI<br />
Detecting community assembly patterns in floo<strong>de</strong>d and productive<br />
grasslands.<br />
<strong>Amandine</strong> <strong>Merlin</strong> 1, 2 , Jan-Bernard Bouzillé 1 , François Mesléard 2, 3 , Anne Bonis 1<br />
1 UMR CNRS 6553 EcoBio, <strong>Université</strong> <strong>de</strong> <strong>Rennes</strong> 1, Campus Beaulieu, F-35042 <strong>Rennes</strong><br />
Ce<strong>de</strong>x, France<br />
2 Centre <strong>de</strong> recherche La Tour du Valat, Le sambuc, F-13200, France<br />
3 UMR CNRS-IRD 6116 Institut Méditerranéen d’Ecologie et <strong>de</strong> Paléoécologie, <strong>Université</strong><br />
d’Avignon IUT site Agroparc BP 1207, F-84911 Avignon Ce<strong>de</strong>x 09, France<br />
Abstract<br />
Trait-based approaches are currently <strong>de</strong>veloped to study mechanisms un<strong>de</strong>rlying<br />
species coexistence. Few studies have investigated the distribution of functional traits with<br />
respect to theoretical predictions formulated according to a given environmental gradient. In<br />
wet and productive grasslands located on the French Atlantic coast, we investigated whether<br />
plant species assembly show non-random organization with respect to Specific Leaf Area<br />
(SLA) and plant height, two functional traits associated with tolerance to flooding and<br />
resource acquisition. We used a null mo<strong>de</strong>l approach to test: (i) convergent trait values<br />
resulting from habitat filtering process in response to flooding conditions, (ii) divergent trait<br />
values resulting from niche differentiation process, and (iii) convergent trait values attributed<br />
to competition in highly competitive environments.<br />
Significant patterns of mean traits values were <strong>de</strong>tected for SLA and plant height;<br />
traits values increased with flooding conditions. Significant patterns of divergent trait values<br />
between nearest neighbors were <strong>de</strong>tected for SLA and plant height. This divergence in plant<br />
height was higher in flooding environments than in non-floo<strong>de</strong>d environments.<br />
Simultaneously, significant pattern of traits values convergence between nearest neighbors<br />
was found along the gradient for plant height in non-floo<strong>de</strong>d environment.<br />
Our results may be interpreted as evi<strong>de</strong>nce for habitat filtering selecting species<br />
according to their ability to tolerate flooding conditions. Competition between coexisting<br />
species may have led to a functional niche differentiation along the gradient, but coexisting<br />
species require a certain <strong>de</strong>gree of similarity to coexist in highly competitive environments.<br />
Our findings support hypotheses tested concerning traits distribution and reveal<br />
mechanisms responsible for species assembly along the flooding gradient.<br />
Keywords: Community assembly, habitat filtering, niche differentiation, functional<br />
convergence, plant height, Specific Leaf Area<br />
167
Chapitre VI<br />
168
Introduction<br />
Chapitre VI<br />
Un<strong>de</strong>rstanding processes un<strong>de</strong>rlying community assembly along environmental<br />
gradients and predicting which subset of the regional species pool occurs in a specified<br />
habitat, <strong>de</strong>fying the assembly rules, are one of the main aims of community ecology<br />
(Diamond 1975; Keddy 1992; Weiher & Keddy 1995; McGill et al. 2006). The <strong>de</strong>velopment<br />
of trait-based approach, studying the traits organization within and among communities, has a<br />
great potential to reveal general processes (Weiher et al. 1998; Mc Gill et al. 2006; Ackerly &<br />
Cornwell 2007; Violle & Jiang 2009). The hypothesis of the trait-based approach is that<br />
functional traits reflect species responses to environmental factors, i.e. response traits<br />
(Lavorel & Garnier 2002; Wright et al. 2004; McGill et al. 2006; Ackerly & Cornwell 2007;<br />
Diaz et al. 2007). Despite the recent and strong <strong>de</strong>velopment of trait-based approaches to test<br />
assembly rules, few studies investigated the distribution of functional traits with respect to<br />
theoretical predictions formulated according to a given environmental gradient (Pillar et al.<br />
2009; Schamp et al. 2010; Pakeman et al. 2011).<br />
Along environmental gradients, two main processes un<strong>de</strong>rline assembly rules; habitat<br />
filtering and niche differentiation. However, they do not have the same importance according<br />
to the location along the gradient (Weiher & Keddy 1995). In<strong>de</strong>ed, habitat filtering is<br />
predicted in the constrained parts of the gradient which consist in a convergence of species<br />
traits associated with the tolerance to abiotic factors among coexisting species (Weiher &<br />
Keddy 1995). Accordingly, species are selected according to their trait values allowing them<br />
to persist un<strong>de</strong>r given environmental conditions. This was notably <strong>de</strong>monstrated for Specific<br />
Leaf Area (SLA) and plant height in wetlands (Weiher et al. 1998; Jung et al. 2010), and for<br />
SLA and canopy height in coastal forest (Cornwell & Ackerly 2009). In the less constrained<br />
parts of the gradient, the distribution of traits values is expected to be mainly driven by<br />
competition for limiting resources (Weiher & Keddy 1995). Accordingly, divergence in traits<br />
associated with competition is interpreted as evi<strong>de</strong>nce of niche differentiation among co-<br />
occurring species, thus reflecting the concept of limiting similarity (Mac Arthur & Levins<br />
1967). This process exclu<strong>de</strong>s the most similar species (Connell 1980), thereafter reducing<br />
competition and permitting species coexistence. Such a niche differentiation has been<br />
<strong>de</strong>monstrated, for instance, consi<strong>de</strong>ring water-use strategies in sand dunes community (Stubbs<br />
& Wilson 2004), growth forms in tropical forests (Kraft et al. 2008) and plant height and SLA<br />
in wetlands (Weiher et al. 1998; Jung et al. 2010).<br />
169
Chapitre VI<br />
In contrast to Weiher & Keddy’s point of view, Grime (2006) predicts a convergence<br />
in traits within productive and undisturbed communities, illustrating by the similarities in<br />
resource dynamics. To persist in high productive environment, i.e. in a competitive<br />
environment, an efficient capture of resources is nee<strong>de</strong>d: coexisting species consequently<br />
present similar strategies to capture resources. Accordingly, convergence in traits associated<br />
with competition is expected in these habitats due to the increasing effect of competition as a<br />
filtering process (Pakeman et al. 2011). This i<strong>de</strong>a of the effect of competition on trait<br />
convergence was supported by a recent study in tropical forest (Swenson & Enquist 2009).<br />
However, the two former points of view, suggesting different patterns of traits distribution<br />
associated with competition are not necessarily opposite. In<strong>de</strong>ed coevolution led to the<br />
divergence between species allowing coexistence through niche differentiation, but coexisting<br />
species may be similar in or<strong>de</strong>r to compete relatively equally (Scheffer & van Nes 2006).<br />
The aim of this study is to <strong>de</strong>termine assembly rules acting on community assembly<br />
along a flooding gradient in wet and productive grasslands. We aimed to test Weiher &<br />
Keddy’s and Grime’s points of view according to the effect of competition on traits dispersion<br />
within three plant communities distributed along a flooding gradient. As the selection of<br />
functional traits is fundamental to address the assembly mechanisms (Götzenberger et al.<br />
2011), two relevant traits have been chosen: Specific Leaf Area (SLA) and plant height.<br />
These two functional traits reflect species tolerance to flooding conditions: they increased<br />
with flooding conditions to improve gas diffusion un<strong>de</strong>rwater (Blom & Voesenek 1996;<br />
Clevering et al. 1998; Lenssen et al. 1998; Mommer et al. 2004). These traits are also related<br />
to species strategies to capture resources and species competitive ability (Westoby 1998;<br />
Weiher et al. 1999; Violle et al. 2011).<br />
To reach our goal, measurements of functional diversity, i.e. the range and value of<br />
species traits within a community, present a crucial importance to reveal community assembly<br />
functioning (Diaz & Cabido 2001; Tilman 2001; Petchey & Gaston 2002; Mouillot et al.<br />
2005). Three statistics, measuring trait distribution, have been used to test theoretical<br />
predictions of habitat filtering, niche differentiation and convergence due to competition<br />
previously enounced. These statistics allow the distinction between convergence in the trait<br />
distribution due to habitat filtering or due to competition. Habitat filtering restricts the mean<br />
of traits values (Kraft et al. 2008; Jung et al. 2010). Niche differentiation, through the concept<br />
of limiting similarity, affects traits spacing, increasing the mean distance between nearest<br />
neighbors and reducing the variance in distance between nearest neighbors (Schamp et al.<br />
2010). Convergence of traits values associated with competition reduces the mean distance in<br />
170
Chapitre VI<br />
the nearest neighbor (Schamp et al. 2010). Null mo<strong>de</strong>ls approach will be used to compare<br />
observed traits distribution in the field with randomly traits distribution (Gotelli & McCabe<br />
2002).<br />
Following Weiher & Keddy’s (1995) predictions, we hypothesized that in the floo<strong>de</strong>d<br />
parts of the gradient, a convergence in trait values will be observed resulting in habitat<br />
filtering, while in the non-floo<strong>de</strong>d parts the traits distribution will result in niche<br />
differentiation, signs of limiting similarity due to competition. In our study site, the<br />
productivity is high and constant all along the gradient (Violle et al. 2011): we cannot test the<br />
variation in the level of productivity on trait distribution. However, we hypothesized that the<br />
high productivity of the study site will result in convergence in traits associated with<br />
competition all along the gradient.<br />
Methods<br />
Study site<br />
The studied site is a floo<strong>de</strong>d grasslands of the Marais Poitevin on the French Atlantic<br />
coast (46°28’N; 1°13’W), characterized by an atlantic climate with an annual mean of the<br />
amount of rainfall equal to 655 mm (Amiaud & Touzard 2004). The floo<strong>de</strong>d grasslands are<br />
characterized by an elevation gradient with a mean range of 45 cm. The elevation was related<br />
to the duration of flooding (r²=0.776; P
Chapitre VI<br />
abundance of species was noted. Along the elevation, a total of 38 species were sampled.<br />
Measurements of the elevation of each plot were realized using a Trimble (Trimble M3, Ohio,<br />
USA) which is an integrative measure of flooding events that species experienced.<br />
Community level is consi<strong>de</strong>red here as a proxy to approach the environmental conditions that<br />
species experienced.<br />
Within each community and for each species, two functional traits were measured: the<br />
Specific Leaf Area (SLA) and the plant height. These two functional traits reflect species<br />
response to flooding conditions due to the facilitation of gas exchanges in oxygen-<strong>de</strong>ficient<br />
conditions (e.g. Blom & Voesenek 1996; Insausti et al. 2001; Mommer et al. 2004). They<br />
reflect also species strategies to capture resources (Weiher et al. 1999) and plant height can<br />
assess species competitive ability (Westoby 1998; Violle et al. 2011). Ten replicates of each<br />
trait were measured by species and by community when abundance of species was sufficient<br />
to sample it. These traits were measured following the standardized protocols (Weiher et al.<br />
1999; Cornelissen et al. 2001): SLA was measured on the youngest fully grown leaf in the<br />
light using a leaf area meter (Li-Cor, Lincoln, Nebraska, USA) and plant height as the<br />
difference between the elevation of the highest photosynthetic leaf and the base of the plant.<br />
Data analysis<br />
To test the hypotheses of habitat filtering and niche differentiation, we consi<strong>de</strong>red the<br />
distribution of functional traits within communities, taking into account intra-specific<br />
variability in trait values. In<strong>de</strong>ed, species may adjust their trait values to respond to changing<br />
environments and promoting species coexistence, highlighted in recent studies (Violle &<br />
Jiang 2009; Albert et al. 2010; Jung et al. 2010). Accordingly, 10 replicates were measured<br />
for each species within each community and the mean trait value was calculated per<br />
community. Then, species occurring in two different communities may then have different<br />
mean trait values.<br />
Three statistics were calculated in this study to test the dispersion of trait values within<br />
communities and test the hypotheses of habitat filtering and niche differentiation (Table 1): i)<br />
the mean of trait values <strong>de</strong>monstrating habitat filtering when observed values are higher than<br />
the expected values coming from null distribution (Kraft et al. 2008), ii) the mean nearest<br />
Eucli<strong>de</strong>an trait distance (meanNTD), measuring how traits are spaced within plots (Weiher et<br />
al. 1998; Schamp et al. 2010; Decaens et al. 2011), <strong>de</strong>monstrating competitive convergence<br />
when observed values are lower than the expected values, and <strong>de</strong>monstrating limiting<br />
172
Chapitre VI<br />
similarity when observed values are higher than the expected values coming the null<br />
distribution (Schamp et al. 2008; Schamp et al. 2010) and iii) the variance in nearest<br />
Eucli<strong>de</strong>an trait distance (varNTD) between each species measuring both how species are<br />
regularly spaced within plots (Kraft et al. 2008; Schamp et al. 2008; Schamp et al. 2010),<br />
<strong>de</strong>monstrating limiting similarity when observed values are lower than the expected values<br />
coming from the null distribution.<br />
Table 1: Theoretical prediction of processes un<strong>de</strong>rlying trait dispersion according to the<br />
direction of <strong>de</strong>viation for each statistic relatively to the null distribution.<br />
Test Statistic Habitat filtering Limiting similarity<br />
Convergence due to<br />
competition<br />
Mean Observed>Expected - -<br />
MeanNTD - Observed>Expected Observed
Chapitre VI<br />
where O is the observed value of trait, M and S are, respectively, the mean and the standard<br />
<strong>de</strong>viation for each test across the 1000 randomizations. A positive value indicates a <strong>de</strong>viation<br />
above the mean of the null distribution, a negative value a <strong>de</strong>viation below the mean of null<br />
distribution.<br />
To <strong>de</strong>termine whether variations in elevation within each community influence<br />
statistics values, regressions have been calculated between elevation and SES value of each<br />
metric in each community. To test changes of these patterns along the environmental gradient,<br />
regressions were realized between SES value of each trait and elevation all along the gradient.<br />
Results<br />
Community assembly within each community<br />
Coexisting species within the mesophilous community were more similar in SLA<br />
(MeanNTD; Table 2) and were more regularly distributed with respect to SLA than expected<br />
by chance (VarNTD; Table 2). Coexisting species showed higher SLA values on average<br />
(Mean; Table 3) but were less similar with respect to plant height than expected by chance<br />
(MeanNTD; Table 3). For both functional trait, the three statistics did not vary with the<br />
elevation range occurring within community (non significant regressions, P>0.05), except for<br />
the Mean SLA (Adjusted r²= 0.105, P=0.045).<br />
Coexisting species within the mesohygrophilous community were more similar in<br />
SLA than expected by chance (MeanNTD; Table 2). These coexisting species showed higher<br />
SLA values on average than expected by chance (Mean; Table 3). They were less similar with<br />
respect to plant height (MeanNTD; Table 3) but were more regularly distributed with respect<br />
to plant height than expected by chance (VarNTD; Table 3). For each functional trait, the<br />
three statistics did not vary with the elevation range occurring within community (non<br />
significant regressions, P>0.05), except for the VarNTD SLA (Adjusted r²= 0.525; P
Chapitre VI<br />
the elevation range occurring within community (non significant regressions, P>0.05), except<br />
for the VarNTD SLA (Adjusted r²= 0.106; P=0.05).<br />
Table 2: Results from the null distribution for the SLA trait for the three studied communities.<br />
Mesophilous<br />
community<br />
Mesohygrophilous<br />
community<br />
Hygrophilous<br />
community<br />
Test Observed>Expected ObservedExpected Observed
Chapitre VI<br />
Fig. 1: Regressions analysis showing changes in the Standardized Effect Size (SES) of each<br />
metric across the elevation gradient: the mean of plant height (a), the MeanNTD of plant<br />
height (b) and VarNTD in plant height (c), all negatively correlated with elevation gradient.<br />
Significant boundaries (-1.96; 1.96) are indicated by dashed horizontal lines.<br />
Mean SLA increased with the duration of flooding (<strong>de</strong>crease in elevation) (r²=0.409,<br />
P
Chapitre VI<br />
in accordance with predictions ma<strong>de</strong> by Weiher & Keddy (1995), i.e. coexisting species show<br />
convergence in their trait values. This convergence in trait values represents the species<br />
response to environmental conditions. In<strong>de</strong>ed in the hygrophilous community, the mean<br />
values for SLA and plant height are higher than expected by chance, providing an evi<strong>de</strong>nce of<br />
the habitat filtering. Coexisting species in the hygrophilous community are filtered with<br />
respect to traits allowing tolerance to flooding conditions, high plant height and high SLA<br />
facilitating gas exchanges among individuals un<strong>de</strong>rwater (Blom & Voesenek 1996; Clevering<br />
et al. 1998; Lenssen et al. 1998; Mommer et al. 2004; Violle et al. 2011). Such an increase in<br />
these two morphological traits has already been highlighted for other wetlands species<br />
(Weiher et al. 1998; Jung et al. 2010). At intermediate positions along the elevation gradient,<br />
on the slopes, habitat filtering also occurred but only for plant height. A high SLA together<br />
with tall plants occurs un<strong>de</strong>r long and floo<strong>de</strong>d conditions, while a high plant height appears<br />
sufficient to tolerate short and shallow flooding (Colmer & Voesenek 2009). The intensity of<br />
the constraint thus drive the range of traits involved in the filtering processes.<br />
Following the predictions of Weiher & Keddy (1995), a divergence in the trait values<br />
was expected for traits associated with species competitive abilities in the higher parts of the<br />
gradient. The metrics calculated indicated that competition process leads to an increase in the<br />
mean of the nearest neighbors among co-occuring species (MeanNTD) and a reduction in the<br />
variance of the nearest neighbors among co-occuring species (VarNTD). Our results<br />
<strong>de</strong>monstrate that in the higher elevations of the gradient, traits values for plant height and<br />
SLA are regularly spaced among mesophilous coexisting species. This provi<strong>de</strong>s an evi<strong>de</strong>nce<br />
of niche differentiation for these two functional traits, in accordance with the predictions of<br />
Weiher & Keddy (1995). The niche differentiation, following the concept of limiting<br />
similarity, allows a share of resources between coexisting species. This share of resources is<br />
realized according to both traits, which are related to resources acquisition (Weiher et al.<br />
1999; Wright et al. 2004). The process of niche differentiation has been <strong>de</strong>monstrated in other<br />
wetland communities (Weiher et al. 1998; Jung et al. 2010), in sand dunes (Stubbs & Wilson<br />
2004) and in tropical forests (Kraft et al. 2008).<br />
The divergence in traits values is also <strong>de</strong>monstrated in our communities submitted to<br />
flooding conditions. These morphological traits, and especially plant height, are filtered by<br />
environmental conditions to tolerate flooding conditions, and are also involved in resource<br />
partitioning between species. In these floo<strong>de</strong>d parts of the gradient, where the productivity is<br />
high and equal to the productivity in the non-floo<strong>de</strong>d parts (Violle et al. 2011), the niche<br />
differentiation is <strong>de</strong>monstrated. This suggests that competition between species impacts<br />
177
Chapitre VI<br />
species assemblage in floo<strong>de</strong>d habitats, probably because of the high productivity of the<br />
community leading to increased competition for light among species.<br />
Because of the high availability of soil resources, vegetation is <strong>de</strong>nse all along the<br />
gradient. Following Grime’s proposition (2006), we expect an increasing effect of<br />
competition as a filtering process in productive environments, thus leading to a convergence<br />
in traits values (Grime 2006; Pakeman et al. 2011). Our results indicate a <strong>de</strong>crease in the<br />
mean distance between the nearest neighbors (MeanNTD) involving SLA, a convergence<br />
accentuated by the increase of the elevation. Simultaneously, a <strong>de</strong>crease in the mean distance<br />
between the nearest neighbors (MeanNTD) for plant height is showed, and this convergence<br />
in traits values is higher in the non-floo<strong>de</strong>d parts of the gradient. Our results support Grime’s<br />
proposition (2006) about traits related to competition. To persist in high competitive<br />
environment, an efficient capture of resources is nee<strong>de</strong>d: coexisting species consequently<br />
present similar strategies to capture resources, and this could involve close SLA and plant<br />
height values. These similarities in resource dynamics within coexisting species have been<br />
highlighted in other vegetation types (Swenson & Enquist 2009; Pakeman et al. 2011).<br />
However, traits involved in convergence are different <strong>de</strong>pending on the location along<br />
the gradient. In the mesophilous community, niche differentiation of coexisting species was<br />
possible through the differentiation in SLA, and a convergence due to competition involving<br />
plant height was observed suggesting a competition for light. In the hygrophilous community,<br />
niche differentiation of coexisting species was possible through the differentiation in plant<br />
height and simultaneously a convergence in traits distribution due to competition for SLA are<br />
observed. As SLA is a proxy of vegetative growth (Westoby et al. 1998), species may coexist<br />
through different growth abilities during and after flooding. These results are consistent with<br />
the proposition of Scheffer & van Nes (2006) that for traits linked to competition, a<br />
convergence and divergence may be simultaneously observed, supporting the points of view<br />
of Weiher & Keddy (1995) and Grime (2006). The coevolution un<strong>de</strong>r competitive<br />
environment leads to species sufficiently divergent to coexist through niche differentiation,<br />
but species coexistence requires a <strong>de</strong>gree of similarity to persist in competitive environments.<br />
Conclusion<br />
Our results show that flooding and competition represent two filtering processes, and<br />
competition represents a filtering process throughout the flooding gradient. We show a<br />
178
Chapitre VI<br />
simultaneous effect of habitat filtering and niche differentiation, as it has been already<br />
highlighted in wetlands (Weiher et al. 1998; Jung et al. 2010), in Californian foothills<br />
(Cornwell & Ackerly 2009), and in tropical forests (Kraft et al. 2008). We also show a<br />
simultaneous effect of niche differentiation and convergence due to competition. In the plant<br />
communities studied here, the niche differentiation and the response to competition appear to<br />
involve different traits (at the community level) leading to the possibility of <strong>de</strong>monstrating it.<br />
These results are in contrast with Schamp et al. (2008) using the same metrics, but who<br />
supposed that the studied traits contribute simultaneously to niche differentiation and to the<br />
response to competition, leading to an indistinguishable pattern from pattern expected by<br />
chance. The choice of the functional traits to study thus appears crucial to test these<br />
mechanisms (Kraft et al. 2008; Götzenberger et al. 2011). In our study, SLA and plant height<br />
appear as relevant response traits to abiotic and biotic environments to <strong>de</strong>tect assembly rules<br />
in wet grasslands.<br />
Different limits may be observed in this study. First, the species abundance has not<br />
been consi<strong>de</strong>red, while its incorporation in analyses may address processes more precisely<br />
(Götzenberger et al. 2011). Moreover, the community level has been chosen to test assembly<br />
rules. However, some statistics appear sensitive to variation in elevation within a community<br />
as VarNTD. Taking into account the abiotic constraints at the plot level and adopting a null<br />
mo<strong>de</strong>l consi<strong>de</strong>ring the structure of each plot (richness, species abundance) could allow to test<br />
precisely assembly rules. We are currently testing these two elements.<br />
179
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184
- Chapitre 7 -<br />
Stress level influences the effect of grazing regime and of<br />
variability of rainfall patterns on xero-halophytic communities<br />
Soumis à Plant Ecology<br />
185
186
Chapitre VII<br />
Stress level influences the effect of grazing regime and of variability of<br />
rainfall patterns on xero-halophytic communities<br />
<strong>Amandine</strong> <strong>Merlin</strong> 1,2 , Anne Bonis 1 & François Mesléard 2,3<br />
1 UMR CNRS 6553 EcoBio, <strong>Université</strong> <strong>de</strong> <strong>Rennes</strong> 1, Campus Beaulieu, F-35042 <strong>Rennes</strong><br />
Ce<strong>de</strong>x, France<br />
2 Centre <strong>de</strong> recherche La Tour du Valat, Le sambuc, F-13200, France<br />
3 UMR CNRS-IRD 6116 Institut Méditerranéen d’Ecologie et <strong>de</strong> Paléoécologie, <strong>Université</strong><br />
d’Avignon IUT site Agroparc BP 1207, F-84911 Avignon Ce<strong>de</strong>x 09, France<br />
Abstract<br />
Questions: What are the respective roles of grazing and rainfall pattern in <strong>de</strong>termining plant<br />
community structure over 8 years? Does the abiotic stress level mediate the effects of grazing<br />
and of variability in rainfall patterns?<br />
Location: Mediterranean xero-halophytic grasslands of the Camargue (Southern France).<br />
Methods: Yearly monitoring was conducted for eight years on two vegetation communities:<br />
(1) xeric meadows, (2) Mediterranean salt-marshes. The two communities were subjected to<br />
three grazing treatments: (1) Status Quo treatment (control); (2) Short Grazing treatment and<br />
(3) the Exclusion of grazing.<br />
Results: Rainfall patterns controlled the structure of the two communities in the controls. The<br />
cessation of grazing led to a loss of diversity in xeric meadows through the increase of the<br />
dominant species Brachypodium phoenicoï<strong>de</strong>s. This cessation of grazing had no effect on the<br />
structure of the salt-marshes. The change in the modality of grazing had a weak effect mainly<br />
noticeable in xeric meadows: it increased the contribution of Bromus madritensis. The<br />
consequence is an increase in the similarity of the two communities.<br />
Conclusions: The level of abiotic stress buffers the impact of changes in grazing<br />
management. In the most highly environmentally constrained community, the structure<br />
(species diversity) remains unchanged over the 8 years studied <strong>de</strong>spite changes in grazing<br />
modality. While in the least environmentally constrained community, grazing represents the<br />
main structuring factor. The level of stress experienced by community should be consi<strong>de</strong>red<br />
before using grazing as a management tool.<br />
Keywords: Diversity; Dominant species, Evenness; NMDS; Response pathways<br />
Nomenclature: Tutin, T.G. et al. 1964-1980, Flora Europea, Cambridge University Press,<br />
Cambridge.<br />
187
Chapitre VII<br />
188
Introduction<br />
Chapitre VII<br />
Most of the time, grazing plays a <strong>de</strong>terminant role in the organization of herbaceous<br />
communities. Grazing management represents a biotic filter selecting species through<br />
<strong>de</strong>foliation and trampling actions, in relation with the respective value of their traits.<br />
According to the intensity and the frequency of grazing, different plant growth forms and<br />
functional groups are selected (Diaz et al. 1998; Díaz et al. 2001; Díaz et al. 2007; Golo<strong>de</strong>ts et<br />
al. 2010; Milchunas and Noy-Meir 2002; Noy-Meir et al. 1989; Sternberg et al. 2000)<br />
modifying community structure (Peco et al. 1983; Peco et al. 2006).<br />
Grazing represents a disturbance because it induces a partial or total loss of plant<br />
biomass (Grime 1979; White and Pickett 1985). Following the Intermediate-Disturbance<br />
Hypothesis <strong>de</strong>scribing a hump-shaped relationship (Grime 1973), a high level of diversity is<br />
to be expected for intermediate levels of disturbance due to the reduction of competitive<br />
interactions between species and due to the creation of gaps of bare soil representing vacant<br />
niches for species (Johnstone 1986), favoring the spread of a large number of species.<br />
However, at high levels of disturbance, selected species present morphological and<br />
anatomical traits allowing them to persist un<strong>de</strong>r these conditions (Golo<strong>de</strong>ts et al. 2010; Noy-<br />
Meir et al. 1989; Noy-Meir 1995; Olff and Ritchie 1998; Peco et al. 2005), while competitive<br />
exclusion by competitive species operates in the absence of disturbance (Milchunas et al.<br />
1988; Noy-Meir et al. 1989; Peco et al. 2005; Sternberg et al. 2000): both extreme levels of<br />
grazing pressure lead to a loss of diversity.<br />
Grazing can modify the environmental conditions in three ways (Cingolani et al.<br />
2003). Firstly, grazing can lead to an amplification of environmental conditions that<br />
communities are subjected to, increasing the existing divergence between communities.<br />
Secondly, grazing can reduce contrasts in environmental conditions between communities,<br />
leading to a convergence between them. Finally, grazing can preserve the contrasting<br />
environmental conditions between communities, but can lead to species replacement<br />
preserving the difference between communities, but not amplifying this difference.<br />
Alternatively, these three effects of grazing may be mediated by abiotic conditions, such as<br />
the level of stress. Highly environmentally constrained environments have been shown to<br />
reduce the magnitu<strong>de</strong> of floristic changes by grazing (Cingolani et al. 2003) especially in salt<br />
constrained environments (Allison 1992; Callaway et al. 1994; Milchunas et al. 1988). Soil<br />
salinity appears to be a <strong>de</strong>termining environmental filter (Callaway et al. 1990; Callaway and<br />
Sabraw 1994), which makes plants thus selected tolerant to other stresses. In<strong>de</strong>ed, in systems<br />
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Chapitre VII<br />
characterized by a low availability of water resources, species’ strategies implemented to<br />
tolerate the presence of salt may also help species to tolerate other stresses or disturbances<br />
such as grazing (Milchunas et al. 1988).<br />
Despite the evi<strong>de</strong>nce of interactions between grazing management and environmental<br />
conditions, such as the variability in rainfall patterns and the soil salinity reducing community<br />
productivity, few studies have attempted to evaluate their respective importance with regard<br />
to community structure (Alday et al. 2010). The aim of this study is first to evaluate the<br />
respective roles of grazing and rainfall pattern and secondly to <strong>de</strong>termine whether the eventual<br />
effects are mediated by the abiotic stress level affecting the structure of Mediterranean<br />
grasslands. In particular, we aimed to <strong>de</strong>termine the impact of changes in grazing<br />
management on the composition and structure of the xero-halophytic grasslands over 8 years,<br />
exposed to different levels of abiotic stress: the xeric-meadows and the Mediterranean salt<br />
marshes.<br />
We hypothesized that (i) community composition responds strongly to the inter-annual<br />
variability in rainfall patterns due to the presence of a large proportion of annual species, (ii)<br />
a cessation of grazing leads to a strong modification of community structure and composition,<br />
with strong <strong>de</strong>velopment of perennial species reducing diversity, (iii) a change in the modality<br />
of grazing, from grazing by herbivores during six months to grazing during three days,<br />
representing a disturbance, leads to an increase in species diversity (iv) in the constrained<br />
community (i.e. the Mediterranean salt marshes), the impact of inter-annual rainfall<br />
variations and grazing changes on the community are weak due to the previous selection of<br />
species tolerant to various stresses.<br />
Methods<br />
Study site<br />
The study was carried out on the xero-halophytic grasslands of the Tour du Valat<br />
estate (Rhône <strong>de</strong>lta, Southern France, 43°29’ N, 4°40’ E). The xero-halophitic grasslands are<br />
characterized by a wi<strong>de</strong> diversity in annual species (Molinier and Tallon 1968) and represent<br />
the most diverse habitat of the Camargue. They are a priority habitat (co<strong>de</strong> 6220) un<strong>de</strong>r the<br />
terms of the European Union Habitats Directive (1992). They have been drastically reduced<br />
over the last <strong>de</strong>ca<strong>de</strong>s due to intensive agriculture (Lemaire et al. 1987).<br />
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Chapitre VII<br />
The Camargue is characterized by a Mediterranean climate with cold winters and<br />
warm dry summers (Heurteaux 1970). Precipitation occurs mainly during spring and autumn.<br />
Average annual precipitation (1988-2008) is 570 mm, with strong inter-annual variations:<br />
from 252 mm (1989) up to 1049 mm (1996). Over the eight years studied, P-PET<br />
(Precipitation minus Potential Evapotranspiration), quantifying the water resource availability<br />
for plants (Scarnati et al. 2009), was variable (Fig. 1). We found three years which can be<br />
qualified as dry: 2001, 2006 and 2007. Spring was always dry, particularly in 2006, whereas<br />
the amount of autumn rainfall was variable but low for two years out of the eight years (2007<br />
and 2008) (see data in supplementary material).<br />
Fig 1 Annual P minus ETP (P-ETP) values for the 8 years of the study. Continuous line<br />
represents the mean of annual P-ETP values of 20 years (1989-2008) and dotted lines<br />
represent the limits of 95% confi<strong>de</strong>nce interval<br />
The xero-halophitic grasslands extend along a weak elevation gradient with the soil<br />
salinity <strong>de</strong>creasing with the increase in elevation, due to the influence of a superficial salty<br />
water table (Heurteaux 1970). Floristic composition varies along the gradient in response to<br />
the variation in soil salinity. Two different communities were discriminated according to a<br />
Correspon<strong>de</strong>nce analysis (CA) (Fig. 2) run on the first year dataset: the community occurring<br />
at higher elevations is dominated by Brachypodium phoenicoï<strong>de</strong>s L., Crepis vesicaria subsp.<br />
taraxacifolia Thuill., Scorpiorus muricatus L., Crepis sancta L., Brachypodium distachyon<br />
L., Psilurus aristatus L., Evax pygmaea L., Galium murale L. and other characteristic species<br />
of xeric meadows (habitat directive co<strong>de</strong> 6220-2). The second community occurring at lower<br />
elevations is composed of Halimione portulacoï<strong>de</strong>s Aellen, Limonium narbonense Mill,<br />
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Chapitre VII<br />
Bromus hor<strong>de</strong>aceus L., Hymenolobus procumbens L., Parapholis filiformis Roth, Bupleurum<br />
semicompositum L. that can be assimilated to a Mediterranean salt-marsh community (habitat<br />
directive co<strong>de</strong> 1310-4).<br />
Fig. 2 Correspon<strong>de</strong>nce analysis (CA) run on vegetation records for 2001 discriminated two<br />
variations of composition of the xero-halophytic grasslands. Two communities are<br />
discriminated along axis 1, corresponding to a salinity gradient. In the left hand part of the<br />
plot are grouped samples presenting the strong contribution of Brachypodium phoenicoï<strong>de</strong>s,<br />
Crepis vesicaria, Vulpia sp. characterizing xeric meadows. In the right hand part of the plot,<br />
samples are grouped according to the contribution of Halimione portulacoï<strong>de</strong>s, Limonium<br />
narbonense, Bromus madritensis characterizing salt-marshes<br />
Community productivity was found to be significantly different between the two<br />
communities, with productivity significantly higher in the xeric meadows with a mean<br />
productivity in 2007 equal to 3.97 t/ha compared to 1.74 t/ha in Mediterranean salt-marshes.<br />
According to the <strong>de</strong>finition of stress sensu Grime (1979), the Mediterranean salt-marshes<br />
seemed to be more highly constrained than xeric meadows, because are less productive.<br />
In situ grazing treatment<br />
In autumn 2001, an experimental <strong>de</strong>sign was set up which consisted of three grazing<br />
treatments. The first treatment corresponds to extensive grazing pressure with a continuous<br />
annual stocking rate of 0.07 ha -1 .year -1 applied during six months. This treatment was applied<br />
from 1970 and it can be consi<strong>de</strong>red as the control: the Status Quo treatment. The second<br />
192
Chapitre VII<br />
grazing treatment consisted of the same annual stocking rate as the previous treatment but was<br />
applied during only two days at the beginning of February, representing a strong<br />
instantaneous disturbance, hereafter called the Short Grazing treatment. The third treatment<br />
consisted of a cessation of domestic grazing through fencing: the Exclusion treatment. Three<br />
replicates (plots) were set up for each treatment (4ha), except for the Status Quo treatment,<br />
applied over a 350 ha area. Because of the large size of the study area, the replicates of each<br />
grazing treatment could be well spaced out in or<strong>de</strong>r to avoid too much spatial <strong>de</strong>pen<strong>de</strong>nce<br />
between them.<br />
Twenty-one permanent 40 cm × 40 cm quadrats were established randomly in each of<br />
the three grazing treatment plots. Before any analysis, the plot effect was tested to verify<br />
whether there was any confusion between the plot and grazing treatment effects. First year<br />
floristic samplings were un<strong>de</strong>rtaken at random without taking into account the slight elevation<br />
gradient. CA of our first year floristic surveys was used to classify each quadrat as one of the<br />
two communities. Intermediate quadrats were exclu<strong>de</strong>d from further analyses. This results in<br />
an unbalanced number of quadrats between grazing treatments and between the two<br />
communities (Table 1).<br />
Table 1 Number of quadrats classified by CA realized on floristic samplings of 2001;<br />
intermediate quadrats were exclu<strong>de</strong>d for Exclusion treatment and for Short Grazing treatment<br />
Exclusion Status Quo Short Grazing<br />
treatment treatment treatment<br />
Xeric meadows 15 5 9<br />
Salt-marshes 5 16 10<br />
Data were collected using the pinpoint method: in each permanent quadrat, a set of<br />
two crossing lines was established materializing 36 pins, three cm apart from each other.<br />
Species presence was recor<strong>de</strong>d at each of the 36 pins per quadrat resulting in species<br />
frequencies which were obtained per quadrat for each grazing treatment and for each<br />
community. Data were collected every year in the two last weeks of April over an eight year<br />
period, from 2001 to 2008.<br />
193
Characterization of community structure and composition<br />
Chapitre VII<br />
Community structure was <strong>de</strong>scribed by the Shannon diversity in<strong>de</strong>x (H’) (Shannon<br />
and Weaver 1949) and species evenness (J’) (Pielou 1975) with:<br />
These indices were calculated firstly at quadrat scale. Secondly a mean of these indices was<br />
calculated for each year and for each grazing treatment within each community. But the wi<strong>de</strong><br />
difference in the number of quadrats sampled within each grazing treatment affects the<br />
number of species within each quadrat. Therefore, to calculate the mean, a bootstrap<br />
procedure was performed to select randomly a balanced number of quadrats for each<br />
community and for each grazing treatment (n = 5).<br />
Data analyses<br />
Characterization of the communities’ pathways<br />
Distance matrices ordination was used to <strong>de</strong>tect species composition changes in each<br />
community in response to grazing treatment and to climatic variability, because this<br />
ordination is useful to quantify vegetation changes across time and to express the<br />
community’s displacement after disturbance (Halpern 1988). Bray-Curtis similarity matrix<br />
was calculated as the sum of contacts by species in each year and in a given grazing treatment<br />
plot relative to the total number of contacts: then dissimilarities among communities were<br />
calculated on the basis of species composition (Halpern 1988; Myers and Harms 2011). The<br />
ordination of these similarity matrices was performed using a Non-Metric Dimensional<br />
Scaling ordination (NMDS, isoNMDS function) using the vegan package in R software (R<br />
version 2.7.2., 2008, R Foundation for Statistical Computing, Vienna, Austria). Using this<br />
technique, slow compositional changes in response to variation of environmental factors are<br />
translated into annual samples plotted close together. The robustness of the NMDS was<br />
assessed by measuring the discrepancy between the inter-point distances on the NMDS plot<br />
versus distances in the original distance matrix called “stress” (Legendre and An<strong>de</strong>rson 1999;<br />
Lock et al. 2010). The NMDS analysis was run simultaneously for the two communities,<br />
allowing comparison of their response pattern to grazing and climatic pattern over the years.<br />
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Chapitre VII<br />
To test changes in community composition over time for each pathway plotted on the NMDS,<br />
distance-based permutational multivariate analyses of variance (PERMANOVA, version 1.6,<br />
2005, Department of Statistics, University of Auckland, New Zealand) were performed for<br />
each community and grazing treatment.<br />
Dynamics of plant life forms<br />
Changes in plant life form cover (i.e. annual and perennial forbs, annual and perennial<br />
grasses and annual legumes) in each community with grazing treatment and years were<br />
addressed by mixed effect mo<strong>de</strong>ls, with community, grazing treatment and time as fixed<br />
factors, and replicates (quadrats) as random factor.<br />
Dynamics of dominant species<br />
Dominant species, <strong>de</strong>termined as species with abundance higher than 10% (Grime<br />
1998), are likely to drive community dynamics over time (Del Pozo et al. 2006). Changes in<br />
cover of dominant species were addressed by mixed effect mo<strong>de</strong>ls, with community, grazing<br />
treatment and time as fixed factors, and replicates (quadrats) as random factor.<br />
Effect of dominant species on community structure<br />
Changes in species diversity and evenness in each community relative to the cover of<br />
dominant species were addressed by mixed effect mo<strong>de</strong>ls. In these mo<strong>de</strong>ls, grazing treatment,<br />
cover of dominant species and time were consi<strong>de</strong>red as fixed factors and replicates (quadrats)<br />
as random factor.<br />
Relationship between community structure and rainfall patterns<br />
To investigate whether species diversity and evenness were influenced by the<br />
variability and the seasonality of rainfall, three regression analyses were run between<br />
community diversity, evenness and P-PET values for both communities and for each grazing<br />
treatment: first consi<strong>de</strong>ring the spring period only (from February until the end of April),<br />
secondly consi<strong>de</strong>ring the autumn period only (from September until the end of November),<br />
while the third regression consi<strong>de</strong>red P-PET during the period between autumn yearn-1 and<br />
spring yearn.<br />
The use of mixed effects mo<strong>de</strong>ls consi<strong>de</strong>ring quadrats as random factor allows the<br />
highlighting of the effects of tested factors on the studied variables (e.g. indices of diversity,<br />
195
Chapitre VII<br />
evenness, cover of dominant species and of plant life-forms) relative to the structure of the<br />
<strong>de</strong>sign (Bennington and Thayne 1994).<br />
Statistical tests and multivariate analyses were performed using the a<strong>de</strong>4, vegan and<br />
MASS packages of the R software (R version 2.7.2., 2008, R Foundation for Statistical<br />
Computing, Vienna, Austria).<br />
Results<br />
Directional response pathways<br />
The NMDS gave a good representation of the un<strong>de</strong>rlying similarity matrix with a<br />
stress factor of 8.9% (0.089) for the first two axes. The first axis of the NMDS biplot (Fig. 3)<br />
represented the elevation gradient, i.e. the salinity gradient with the higher salt concentrations<br />
in the right part of the biplot, distinguishing the two communities. The second axis was<br />
related to changes in the proportion of perennials, <strong>de</strong>creasing along this axis. The analysis<br />
showed that significant changes in community composition occurred between years for both<br />
communities, except with the exclusion treatment in the Mediterranean salt-marshes<br />
(PERMANOVA, xeric meadows: exclusion treatment: F=1.51, P=0.016; Short Grazing<br />
treatment: F=1.64, P=0.005; Status Quo treatment: F=1.96, P=0.002; salt-marshes: Exclusion<br />
treatment: F=1.33, P=0.111; Short Grazing treatment: F=2.80, P=0.001; Status Quo<br />
treatment: F=3.30, P=0.001). We recor<strong>de</strong>d weak changes in xeric meadows with the Short<br />
Grazing treatment and these changes followed axis one of the NMDS biplot. Changes in<br />
composition in the Status Quo treatment did not appear to follow a strictly linear trajectory,<br />
contrary to the Cessation of grazing. In Mediterranean salt-marshes, the range of composition<br />
changes was similar between grazing treatments, except for the Short Grazing treatments<br />
where the changes were weak.<br />
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Chapitre VII<br />
Fig. 3 Non-Metric Dimensional 10Scaling ordination (NMDS) on the basis of Eucli<strong>de</strong>an<br />
distance measures for centroids of years. Xeric meadows are plotted in the left part of the<br />
biplot, and salt-marshes in the right part of the biplot<br />
Changes in plant life forms in response to changes in grazing pressure<br />
Mixed effect mo<strong>de</strong>ls showed a significant impact of grazing, years and their<br />
interaction on the relative abundance of the various groups of plant life forms (Table 2) and<br />
on dominant species except for Halimoine portulacoï<strong>de</strong>s for which grazing showed no effect<br />
(Table 3). The perennial grasses (Brachypodium phoenicoï<strong>de</strong>s, Dactylis hispanica Roth)<br />
increased un<strong>de</strong>r the three grazing treatments over years, but especially un<strong>de</strong>r the exclusion<br />
treatment (from around 20% in 2001 to around 70% in 2008; Fig. 4a). This increase in the<br />
perennial grasses abundance mainly resulted from the increase in the dominant Brachypodium<br />
phoenicoï<strong>de</strong>s (Fig. 4c). Perennial forbs (Halimione portulacoï<strong>de</strong>s, Aetheorhiza bulbosa L.,<br />
Crepis vesicaria, Limonium narbonense, Limonium virgatum Willd), abundant in salt-<br />
marshes, also increased un<strong>de</strong>r the three grazing treatments (Fig. 4b), in particular the<br />
dominant species Halimione portulacoï<strong>de</strong>s (Fig 4d). Annual grasses showed clear responses<br />
to changes in grazing regime, <strong>de</strong>creasing in cover un<strong>de</strong>r grazing cessation treatment in both<br />
communities and un<strong>de</strong>r Status Quo treatment in salt-marshes, while increasing un<strong>de</strong>r the<br />
197
Chapitre VII<br />
Short Grazing treatment in both communities. However, the annual grass Bromus madritensis,<br />
the second most dominant species of salt-marshes, was negatively affected by Short Grazing<br />
and Exclusion treatments in salt-marshes (Fig. 4e).<br />
Table 2 Mixed effect mo<strong>de</strong>ls for differences in plant life forms between communities and<br />
grazing treatments across time, taking into account interactions between factors<br />
Source of variation<br />
Annual Perennial<br />
df MS F P df MS F P<br />
Community 1 828 9.12 0.003 1 3045 56.82 < 0.001<br />
Grazing 2 1213 13.37 < 0.001 2 182 3.4 0.03<br />
Year 7 1164 12.82 < 0.001 7 401.4 7.49 < 0.001<br />
Community×grazing 2 410 4.51 0.01 2 3.9 0.073 0.93<br />
Community×year 7 822 9.05 < 0.001 7 101.4 1.89 0.07<br />
Grazing×year 14 59 0.65 0.82 14 72.2 1.35 0.18<br />
Community×grazing×year 14 92 1.02 0.43 14 40.2 0.75 0.72<br />
Residuals 431 91 431 53.6<br />
Community 1 57369 456.57 < 0.001 1 46293 1229.4 < 0.001<br />
Grazing 2 1242 9.88 < 0.001 2 2222 59.01 < 0.001<br />
Year 7 1632 12.99 < 0.001 7 174 4.62 < 0.001<br />
Community×grazing 2 2848 22.67 < 0.001 2 1250 33.2 < 0.001<br />
Community×year 7 185 1.47 0.17 7 204 5.41 < 0.001<br />
Grazing×year 14 245 1.95 0.02 14 101 2.68 < 0.001<br />
Community×grazing×year 14 73 0.58 0.88 14 73 1.94 0.02<br />
Residuals 431 126 431 38<br />
Community 1 1503.9 71.52 < 0.001<br />
Grazing 2 126.3 6.01 0.03<br />
Year 7 1683.2 80.04 < 0.001<br />
Community×grazing 2 354.3 16.85 < 0.001<br />
Community×year 7 605.1 28.77 < 0.001<br />
Grazing×year 14 0.8 0.04 0.96<br />
Community×grazing×year 14 14.4 0.68 0.51<br />
Residuals 431 21<br />
df: <strong>de</strong>gree of freedom, MS: Mean-square, F: F-value, P: P-value<br />
Forbs<br />
Grasses<br />
Legumes<br />
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Chapitre VII<br />
Table 3 Mixed effect mo<strong>de</strong>ls for differences in cover of Brachypodium phoenicoï<strong>de</strong>s<br />
(dominant species of xeric meadows), Halimoine portulacoï<strong>de</strong>s and Bromus madritensis<br />
(dominant species of salt-marshes) between communities and grazing treatments across time,<br />
taking into account interactions between factors<br />
Source of variation df MS F P<br />
Brachypodium<br />
phoenicoï<strong>de</strong>s<br />
Community<br />
1 43539<br />
1312.<br />
< 0.001<br />
9<br />
Grazing 2 2520 76.0 < 0.001<br />
Year 7 200 6.0 < 0.001<br />
Community×grazing 2 1409 42.5 < 0.001<br />
Community×year 7 219 6.6 < 0.001<br />
Grazing×year 14 113 3.4 < 0.001<br />
Community×grazing×year 14 72 2.2 < 0.01<br />
Residuals 43<br />
1<br />
33<br />
Halimoine Community 1 19022 509.9 < 0.001<br />
portulacoï<strong>de</strong>s Grazing 2 45 1.2 0.30<br />
Year 7 108.9 2.9 < 0.01<br />
Community×grazing 2 27.5 0.7 0.48<br />
Community×year 7 84 2.3 0.03<br />
Grazing×year 14 13.1 0.4 0.99<br />
Community×grazing×year 14 11.9 0.3 0.99<br />
Residuals 43<br />
1<br />
37.3<br />
Bromus<br />
madritensis<br />
Community<br />
1<br />
9962.<br />
4<br />
193.4 < 0.001<br />
Grazing<br />
2<br />
1780.<br />
8<br />
34.6 < 0.001<br />
Year 7 342.6 6.7 < 0.001<br />
Community×grazing 2 1022 19.8 < 0.001<br />
Community×year 7 184.4 3.6 < 0.001<br />
Grazing×year 14 69.4 1.3 0.18<br />
Community×grazing×year 14 28.7 0.6 0.90<br />
Residuals 43<br />
1<br />
51.5<br />
df: <strong>de</strong>gree of freedom, MS: Mean-square, F: F-value, P: P-value<br />
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Chapitre VII<br />
Fig. 4 Dynamics of cover across the 8 years for a) perennial grasses, b) perennial forbs, c)<br />
Brachypodium phoenicoï<strong>de</strong>s, d) Halimoine portulacoï<strong>de</strong>s and e) Bromus madritensis in the<br />
two communities<br />
Effect of grazing on community structure<br />
Mixed effect mo<strong>de</strong>ls showed a significant impact of grazing on diversity in both<br />
communities and on evenness in xeric meadows (Table 4). In xeric meadows, diversity and<br />
evenness <strong>de</strong>creased in the Exclusion treatment (Fig. 5). Across the eight years, the diversity<br />
did not vary significantly per grazing treatment in salt-marshes, but a ten<strong>de</strong>ncy of a <strong>de</strong>crease<br />
of diversity was observed with the Short Grazing and the Status Quo treatments.<br />
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Chapitre VII<br />
Table 4 Mixed effect mo<strong>de</strong>ls for differences in diversity and evenness between grazing<br />
treatments and in relation with cover of dominant species across time, taking into account<br />
interactions between factors<br />
Source of variation Shannon diversity Evenness<br />
df MS F P df MS F P<br />
Grazing 2 7.7 80.9 < 0.001 2 0.2 95.0 < 0.001<br />
Dominant cover 1 56.8 598.7 < 0.001 1 1.3 723.8 < 0.001<br />
Year 7 1.3 13.5 < 0.001 7 0.01 6.0 < 0.001<br />
Grazing ×dominant 2 1.4 15.1 < 0.001 2 0.04 21.3 < 0.001<br />
Grazing ×year 14 0.2 1.9 0.03 14 0.001 0.8 0.71<br />
Dominant ×year 7 0.2 1.8 0.10 7 0.004 2.0 0.06<br />
Grazing × dominant<br />
×year<br />
14 0.1 1.4 0.16 14 0.004 2.3 0.007<br />
Residuals 183 183<br />
Grazing 2 2.4 14.6 < 0.001 2 0.01 2.2 0.11<br />
Dominant cover 1 41.6 257.8 < 0.001 1 0.7 218.2 < 0.001<br />
Year 7 1.1 6.6 < 0.001 7 0.01 2.3 0.03<br />
Grazing ×dominant 2 0.2 1.4 0.24 2 0.01 3.6 0.03<br />
Grazing ×year 14 0.1 0.7 0.74 14 0.003 0.8 0.67<br />
Dominant ×year 7 0.3 2 0.06 7 0.02 4.5 < 0.001<br />
Grazing × dominant<br />
×year<br />
14 0.1 0.8 0.65 14 0.004 1.2 0.25<br />
Residuals 199 199 0.003<br />
df: <strong>de</strong>gree of freedom, MS: Mean-square, F: F-value, P: P-value<br />
Xeric meadows<br />
Salt-marshes<br />
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Chapitre VII<br />
Fig. 5 Dynamics of Shannon diversity and evenness across the 8 years in the two<br />
communities and in each grazing treatments<br />
Relationships between community structure and cover of dominant species<br />
Mixed effect mo<strong>de</strong>ls showed a significant impact of the abundance of dominant<br />
species on diversity and evenness in xeric meadows and in Mediterranean salt-marshes (Table<br />
4). The interaction term Grazing treatment*Dominant cover*time was significant only in<br />
xeric meadows for evenness.<br />
202
Relationships between rainfall patterns and community structure<br />
Chapitre VII<br />
The diversity and evenness indices varied only slightly between years, both in the<br />
xeric meadows and in the Mediterranean salt-marshes. In xeric meadows, species diversity or<br />
evenness were not correlated with any of the seasonal variations of P-ETP consi<strong>de</strong>red. In the<br />
Mediterranean salt-marshes, diversity and evenness were positively correlated with the<br />
autumn P-PET value (F=5.679, P=0.055; F=8.169, P=0.029 respectively) for the Status Quo<br />
treatment. Plant diversity was positively correlated with both spring and autumn P-PET<br />
values for the Short Grazing and Status Quo treatments (F=6.986, P=0.036; F=79.44,<br />
P=0.0002 respectively).<br />
Discussion<br />
A response to variability of rainfall patterns stronger in the community the most exposed to<br />
stress.<br />
At the time of our eight year experiment, the grazing pressure in the control<br />
treatments had been constant and unchanged for over 40 years. The changes observed<br />
occurring in vegetation were therefore attributable to effects of the variability of rainfall<br />
patterns. During the experiment, we observed an effect of variability in rainfall patterns on<br />
structure in the two communities studied, confirming our hypothesis of the role of variability<br />
of rainfall patterns. This effect concerns species composition and functional groups, but does<br />
not alter species diversity and evenness. This effect is more marked in xeric meadows but<br />
follows a more strictly linear pathway in salt-marshes, especially for the last years of the<br />
study. This strong influence of the amount of rainfall characterizing the whole of the period<br />
studied on species composition is in accordance with other studies conducted in<br />
Mediterranean and semi-arid environments (e.g. Allen et al. 1995; Clarke et al. 2005; Kutiel<br />
et al. 2000; Sternberg et al. 2000).<br />
The strong increase in perennial species observed during the spring of 2008 seems to<br />
be largely attributable to the high amount of rainfall, which probably increased the water<br />
resource availability leading to the increase of competition between species. Such an increase<br />
in the contribution of perennials was especially observed in our salt-marshes, for Halimione<br />
portulacoï<strong>de</strong>s which is known to be a strong competitive species (Bouchard et al. 2003; Keihl<br />
et al. 1996). Despite the non-significant inter-annual variation of species diversity, we<br />
203
Chapitre VII<br />
observed a correlation between autumn rainfall and species diversity, with lower species<br />
diversity obtained for the autumns of 2006 and 2007, the least rainy during the experiment.<br />
A contrasting effect of the cessation of grazing between the two communities<br />
The cessation of grazing strongly altered the structure of xeric meadows, leading to a<br />
linear pathway, thus validating our hypothesis. Over years, a strong increase in perennial<br />
species is observed, in particular in Brachypodium phoenicoï<strong>de</strong>s clearly reducing the<br />
contribution of annual species by competition, due to the growth in height. This effect<br />
negatively affects the species diversity and evenness of xeric meadows, a result reported in<br />
many others studies (Allen et al. 1995; Díaz et al. 2001; Firincioglu et al. 2007; McIntyre and<br />
Lavorel 2001; Milchunas et al. 1988; Noy-Meir et al. 1989; Noy-Meir 1995; Peco et al. 2005;<br />
Sternberg et al. 2000).<br />
In our Mediterranean salt-marshes, we observed weak changes in species composition,<br />
and no change in diversity and evenness, showing a low impact of the cessation of grazing,<br />
and validating our hypothesis of a lesser role of grazing in communities subjected to abiotic<br />
stress. This result fits with observations ma<strong>de</strong> on Atlantic salt marshes where the contribution<br />
of Halimione portulacoï<strong>de</strong>s is reduced by cattle grazing (Tessier et al. 2003), as we were able<br />
to test the effect of the presence of grazing with the grazing treatment.<br />
The cessation of grazing in the two communities maintains their structural divergence,<br />
but the divergence only concerns a species replacement in the xeric meadows (a replacement<br />
of annuals by perennials).<br />
Response to change in extensive grazing modality is stronger in the community the most<br />
exposed to stress.<br />
The application of the same annual pressure but over a short period (i.e. from 6<br />
months to 3 days), unexpectedly leads to mo<strong>de</strong>rate changes in community structure in the two<br />
communities. Our hypothesis of a strong increase in species diversity in the xeric meadows<br />
was not confirmed. Changes in species composition are weak and the proportion of annual<br />
species is only maintained mainly by forbs and grasses, known to be grazing tolerant<br />
(Golo<strong>de</strong>ts et al. 2010; Illius and O’Connor 1999; Noy-Meir et al. 1989; Noy-Meir 1995; Peco<br />
et al. 2005). Although our two days un<strong>de</strong>r grazing application can be assimilated to<br />
disturbance (Grime 1979; White and Pickett 1985) by creating large gaps of bare soil (pers.<br />
obs.) increasing the vacant niches for species, it did not lead to an increase in species<br />
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Chapitre VII<br />
diversity. Our hypothesis that a strong instantaneous grazing pressure will lead to an increase<br />
of species diversity is clearly ruled out; but the other hypothesis that the impact of change in<br />
grazing modality will be less perceptible for the community subjected to the greatest stress is<br />
confirmed. The tolerance of the water shortage generated by stress, like salt, minimizes the<br />
effect of the addition of a new stress or disturbance, such as grazing in our study, particularly<br />
for succulent species (Milchunas et al. 1988). In dry environments, succulent plants show<br />
high tolerance of stress such as salt (Hale and Orcutt 1987). This is the case for Halimione<br />
portulacoï<strong>de</strong>s, a dominant species in our salt-marshes.<br />
Compositional changes over the 8 years were less marked un<strong>de</strong>r this Short Grazing<br />
treatment than with the cessation of grazing, which was to be expected as the species pool<br />
was already filtered by the occurrence of long lasting grazing in the system studied (Metzger<br />
et al. 2005; Ward et al. 1998). However a change in floristic cortege in xeric meadows occurs<br />
along with the increase of the contribution of salt-tolerant species such as Bromus<br />
madritensis, a dominant species of the Mediterranean salt-marshes, leading to an increase in<br />
similarity between the two communities. The augmentation of such salt-tolerant species may<br />
result in an increase in soil salt content due to an increase of instantaneous pressure,<br />
particularly of trampling, as suggested by Amiaud et al. (1998). It highlights one of the three<br />
types of grazing impact <strong>de</strong>scribed by Cingolani et al. (2003) which consists in the<br />
amplification of the similarity between grazed communities due to the alteration of local<br />
abiotic conditions.<br />
Conclusion<br />
Our experiment was carried out over 8 years: this was a necessary precaution since<br />
the communities we studied may have shown slow responses. Probably, we need to un<strong>de</strong>rtake<br />
long term monitoring to be sure of characterizing the communities’ response to rainfall and<br />
changes in grazing management.<br />
Despite that, the annual variation of rainfall patterns induces an immediate response to<br />
this variability although species have been selected by these environmental conditions, due to<br />
the high proportion of annual species. Our results suggest that abiotic conditions, such as soil<br />
salinity resulting in salt-tolerant vegetation, buffer the effects of grazing. In consequence, the<br />
impact of grazing, shown to be <strong>de</strong>terminant in Mediterranean pastures, is mediated in<br />
environmentally constrained communities, even if subjected to a long history of grazing. The<br />
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Chapitre VII<br />
effect of high instantaneous grazing pressure is not as strong as expected, although we know<br />
that this instantaneous pressure is an efficient management approach to control colonization<br />
by shrubs in xeric meadows (Mesléard et al. 2004; Mesléard et al. 2010).<br />
Acknowledgements<br />
We acknowledge the help of N. Yavercovski and L. Willm for technical assistance. This work<br />
is a publication from the DIVHERB Project (French national program ECOGER fun<strong>de</strong>d by<br />
the Institut National <strong>de</strong> la Recherche Agronomique) and a contribution to GDR 2574<br />
‘TRAITS’. This study was supported by a doctoral research grant from the Ministère <strong>de</strong><br />
l’Enseignement Supérieur et <strong>de</strong> la Recherche, the Fondation PRO-VALAT and the Fondation<br />
MAVA.<br />
206
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Chapitre VII<br />
210
RESUME PARTIE 3<br />
Dans les prairies du Marais Poitevin, l’utilisation <strong>de</strong>s modèles nuls nous a permis <strong>de</strong><br />
montrer que l’assemblage <strong>de</strong>s espèces le long du gradient résulterait <strong>de</strong> processus<br />
déterministes. En effet, une convergence <strong>de</strong>s valeurs <strong>de</strong> traits (SLA et hauteur) est mise en<br />
évi<strong>de</strong>nce : elle résulterait <strong>de</strong>s conditions abiotiques <strong>de</strong> l’habitat et plus particulièrement <strong>de</strong>s<br />
conditions d’inondation. De plus, il est à la fois observé une divergence dans les écarts <strong>de</strong>s<br />
valeurs <strong>de</strong> traits entre espèces qui coexistent et une convergence dans ces écarts au sein <strong>de</strong><br />
chaque communauté. Cependant cette divergence et cette convergence ne concernent pas les<br />
mêmes traits suivant la communauté. Dans la communauté mésophile par exemple, une<br />
divergence est plutôt observée pour la distribution <strong>de</strong>s valeurs <strong>de</strong> SLA et une convergence<br />
pour les valeurs <strong>de</strong> hauteur ; alors que l’inverse est observé dans la communauté hygrophile.<br />
Ces résultats nous indiquent que la compétition agirait comme un filtre écologique suivant les<br />
capacités compétitives <strong>de</strong>s espèces leur permettant <strong>de</strong> persister dans un milieu compétitif.<br />
Ainsi, les conditions d’inondations et la compétition entre plantes seraient responsables <strong>de</strong><br />
l’assemblage <strong>de</strong>s espèces tout le long du gradient.<br />
Les pelouses strictes méditerranéennes, c’est-à-dire la communauté la moins<br />
contrainte, apparaissent être contrôlées par les patrons <strong>de</strong> pluviosité dans le traitement témoin<br />
correspondant à 40 années d’une charge extensive <strong>de</strong> pâturage. En revanche, un changement<br />
dans la modalité <strong>de</strong> pâturage apparaît être un filtre écologique important, contrôlant la<br />
structure <strong>de</strong> la communauté. En effet, un arrêt du pâturage conduit à une perte <strong>de</strong> diversité, à<br />
travers l’augmentation en abondance d’une graminée pérenne Brachypodium phoenicoï<strong>de</strong>s.<br />
En revanche, le niveau <strong>de</strong> stress plus élevé dans les prés salés apparaît être un facteur qui<br />
module la réponse <strong>de</strong> cette communauté aux patrons <strong>de</strong> pluviosité et aux effets du<br />
changement dans les modalités <strong>de</strong> pâturage. L’espèce dominante <strong>de</strong>s prés salés, Halimione<br />
portulacoï<strong>de</strong>s, apparaît contrôler la structure <strong>de</strong> cette communauté.<br />
211
212
DISCUSSION GENERALE<br />
L<br />
’objectif <strong>de</strong> ce travail <strong>de</strong> recherche était d’i<strong>de</strong>ntifier les mécanismes structurant les<br />
communautés végétales le long <strong>de</strong> gradients environnementaux et d’évaluer leur<br />
importance. Dans cet objectif, nous avons privilégié <strong>de</strong>s méthodologies différentes,<br />
car aucune méthodologie ne peut, à elle seule, décrire les patrons <strong>de</strong> variation <strong>de</strong> composition<br />
et <strong>de</strong> structure <strong>de</strong>s communautés naturelles et expliquer les mécanismes sous-jacents (Noy-<br />
Meir & Whittaker 1977).<br />
Dans un premier temps, nous nous sommes intéressés à quantifier la part <strong>de</strong> la<br />
compétition relativement aux conditions d’inondation pour expliquer la structuration <strong>de</strong>s<br />
communautés végétales <strong>de</strong>s prairies humi<strong>de</strong>s du Marais Poitevin, en cherchant à répondre aux<br />
questions suivantes :<br />
- Les espèces sont-elles toutes contraintes par les mêmes conditions d’inondation ?<br />
- La compétition est-elle structurante le long du gradient d’inondation ? Comment<br />
varient les <strong>de</strong>ux composantes <strong>de</strong> la compétition (intensité et importance) le long du<br />
gradient d’inondation ?<br />
Pour cela, <strong>de</strong>s approches expérimentales en conditions contrôlées et en conditions<br />
naturelles ont été utilisées pour établir précisément si un déplacement <strong>de</strong>s niches est observé<br />
en réponse à la compétition (Silvertown 2004) (Chapitres 1 à 3). Ces approches ont été<br />
conduites au niveau <strong>de</strong> l’espèce. Ces approches, où la présence <strong>de</strong> voisins est contrôlée,<br />
limitent cependant les généralisations <strong>de</strong>s résultats <strong>de</strong>s interactions plante-plante testées: ces<br />
limites sont liées à la pério<strong>de</strong> d’étu<strong>de</strong> ainsi qu’à l’i<strong>de</strong>ntité et l’abondance <strong>de</strong>s voisins<br />
(Goldberg 1996). En complément, une approche fondée sur l’analyse <strong>de</strong> données<br />
démographiques a été utilisée. Les <strong>de</strong>ux composantes <strong>de</strong> la compétition (intensité et<br />
importance) ont été estimées à travers l’estimation <strong>de</strong>s probabilités <strong>de</strong> survie et <strong>de</strong><br />
colonisation <strong>de</strong>s espèces ainsi que l’estimation <strong>de</strong> la croissance en biomasse, le long du<br />
gradient d’inondation naturel (Chapitre 4). En effet, considérer une perspective dynamique<br />
<strong>de</strong>s populations ou <strong>de</strong>s communautés est nécessaire pour initier une meilleure compréhension<br />
du rôle <strong>de</strong> la compétition dans l’organisation <strong>de</strong> celles-ci (Aarssen & Keogh 2002 ; Frekleton<br />
et al. 2009). Enfin, une approche fonctionnelle a été utilisée pour évaluer l’effet <strong>de</strong>s filtres<br />
abiotique et biotique sur l’assemblage <strong>de</strong>s communautés, et ce, à l’échelle <strong>de</strong>s communautés<br />
(Chapitre 6).<br />
213
Dans un second temps, nous nous sommes intéressés aux mécanismes responsables <strong>de</strong><br />
la structure <strong>de</strong>s communautés végétales méditerranéennes <strong>de</strong> Camargue soumises à <strong>de</strong>ux<br />
niveaux <strong>de</strong> contraintes abiotiques (salinité du sol), en cherchant à répondre aux questions<br />
suivantes :<br />
- Quels sont les rôles respectifs <strong>de</strong> la variabilité <strong>de</strong>s patrons <strong>de</strong> pluviosité (hétérogénéité<br />
<strong>de</strong> la ressource en eau) et <strong>de</strong> la gestion par pâturage sur la structure <strong>de</strong>s<br />
communautés ?<br />
- Le niveau <strong>de</strong> productivité <strong>de</strong> la communauté, proxy du niveau <strong>de</strong> stress à l’échelle <strong>de</strong><br />
la communauté sensu Grime (1979), influence-t-il les effets <strong>de</strong> la variabilité <strong>de</strong>s<br />
patrons <strong>de</strong> pluviosité et du pâturage?<br />
La dynamique <strong>de</strong> ces <strong>de</strong>ux communautés a été étudiée sur une pério<strong>de</strong> <strong>de</strong> 8 années par<br />
une approche multivariée (Chapitre 7). Elle a été couplée à une approche démographique axée<br />
sur les analyses d’élasticité <strong>de</strong>s probabilités <strong>de</strong> survie et <strong>de</strong> colonisation, pour apprécier<br />
l’importance du changement <strong>de</strong> modalité <strong>de</strong> pâturage sur le succès écologique d’une espèce<br />
dominante (Chapitre 5).<br />
1. La caractérisation <strong>de</strong>s contraintes<br />
La première étape pour comprendre les mécanismes qui structurent les communautés<br />
végétales consiste en la caractérisation et la quantification <strong>de</strong> la contrainte environnementale<br />
majeure. Il est en effet nécessaire <strong>de</strong> savoir comment les conditions environnementales<br />
peuvent influencer la performance <strong>de</strong>s espèces, afin <strong>de</strong> pouvoir replacer le rôle <strong>de</strong> la<br />
compétition parmi d’autres filtres écologiques affectant eux-mêmes la performance <strong>de</strong>s<br />
espèces.<br />
La notion <strong>de</strong> stress diffère si on considère le stress à l’échelle <strong>de</strong> la communauté ou à<br />
l’échelle <strong>de</strong> l’individu. A l’échelle <strong>de</strong> la communauté, le stress correspond à tout facteur qui<br />
réduit la productivité <strong>de</strong> la communauté (Grime 1979); alors qu’à l’échelle <strong>de</strong> l’individu, il<br />
correspond à la réponse <strong>de</strong> l’individu aux facteurs abiotiques ou « strain » (Wel<strong>de</strong>n & Slauson<br />
1986). Ces <strong>de</strong>ux niveaux d’organisation ont été considérés pour caractériser la contrainte dans<br />
les <strong>de</strong>ux milieux étudiés.<br />
Nous nous sommes placés au niveau <strong>de</strong> la communauté dans les milieux<br />
méditerranéens en suivant la définition <strong>de</strong> Grime (1979) énoncée précé<strong>de</strong>mment, afin <strong>de</strong><br />
214
comparer les effets <strong>de</strong> la pluviosité et du pâturage sur la structure <strong>de</strong> <strong>de</strong>ux communautés<br />
végétales soumises à <strong>de</strong>s niveaux <strong>de</strong> productivité différents en raison d’une salinité du sol<br />
différente. En revanche, afin <strong>de</strong> déterminer la part réelle <strong>de</strong>s conditions d’inondation et <strong>de</strong> la<br />
compétition dans le succès écologique <strong>de</strong>s espèces <strong>de</strong>s prairies humi<strong>de</strong>s du Marais Poitevin,<br />
nous avons choisi l’échelle <strong>de</strong> l’individu pour mesurer le « strain » ; ce travail fait l’objet <strong>de</strong>s<br />
sections 1.1 et 1.2 suivantes.<br />
1.1. Deux contraintes rencontrées le long du gradient d’inondation<br />
Dans les prairies humi<strong>de</strong>s du Marais Poitevin, <strong>de</strong>ux types <strong>de</strong> contraintes sont mises en<br />
évi<strong>de</strong>nce le long du gradient <strong>de</strong> durée d’inondation. Pour une gran<strong>de</strong> majorité <strong>de</strong>s espèces, et<br />
plus particulièrement pour les espèces décrites dans les communautés mésophiles et méso-<br />
hygrophiles, l’augmentation <strong>de</strong> la durée d’inondation, couplée à une augmentation <strong>de</strong> la<br />
hauteur d’eau, représentent la contrainte majeure limitant la performance <strong>de</strong>s individus<br />
(Chapitres 1 et 2). Cet effet négatif <strong>de</strong>s inondations est mis en évi<strong>de</strong>nce sur différentes<br />
mesures <strong>de</strong> performances <strong>de</strong>s espèces : leur survie, leur croissance en biomasse, et également<br />
sur leurs capacités photosynthétiques, résultats fréquemment rapportés dans la littérature<br />
(Baruch 1994 ; Grimoldi 1999 ; van Eck et al. 2004 ; Fraser & Karnesis 2005 ; Jung et al.<br />
2009). En revanche, l’absence d’inondation représente une contrainte majeure pour <strong>de</strong>ux<br />
espèces : Glyceria fluitans et Juncus articulatus. Les niches fondamentales <strong>de</strong> ces 12 espèces<br />
(β-niche, Wilson 1999) le long du gradient <strong>de</strong> durée d’inondation par conséquent se<br />
chevauchent, avec <strong>de</strong>s optima <strong>de</strong> niches fondamentales observés majoritairement dans les<br />
parties non inondées du gradient (Chapitre 2).<br />
Les <strong>de</strong>grés <strong>de</strong> tolérance aux inondations sont différents entre les 12 espèces étudiées, à<br />
la fois entre espèces appartenant à <strong>de</strong>ux communautés différents mais également entre espèces<br />
d’une même communauté (Chapitres 1 et 2). Par exemple, parmi les espèces <strong>de</strong> la<br />
communauté mésophile, Cynosurus cristatus apparaît moins tolérant à l’inondation que<br />
Lolium perenne. Un autre exemple est celui <strong>de</strong>s espèces <strong>de</strong> la communauté hygrophile avec<br />
Trifolium fragiferum, qui est contraint par l’augmentation <strong>de</strong> la durée d’inondation alors que<br />
Juncus articulatus est favorisé par les inondations.<br />
Les conclusions quant à l’impact <strong>de</strong>s conditions d’inondation sur les performances <strong>de</strong>s<br />
espèces en monocultures sont similaires entre l’expérimentation réalisée en conditions<br />
contrôlées (Chapitre 1) et celle réalisée in situ (Chapitre 2). Néanmoins, les mesures <strong>de</strong><br />
« strain » indiquent qu’en conditions naturelles, les individus transplantés sont plus contraints<br />
215
qu’en conditions contrôlées. Il semblerait donc qu’in situ d’autres facteurs environnementaux<br />
que les conditions d’inondation limitent également les performances <strong>de</strong>s individus. Pour<br />
l’expérimentation en conditions naturelles, les individus ont été transplantés en Octobre, avant<br />
l’hiver. Il est possible que les conditions hivernales, avec les gelées observées au cours <strong>de</strong>s<br />
mois <strong>de</strong> janvier et février, aient affecté les performances <strong>de</strong>s espèces.<br />
Notons qu’une seule espèce répond différemment aux conditions d’inondation<br />
naturelles et aux conditions expérimentales : il s’agit <strong>de</strong> Mentha pulegium. En conditions<br />
contrôlées, cette espèce est plutôt contrainte par l’absence d’inondation (Chapitre 1) alors<br />
qu’en conditions naturelles les durées longues d’inondation apparaissent être un facteur <strong>de</strong><br />
contrainte important (Chapitre 2). Il est possible que les fluctuations du niveau d’eau au cours<br />
d’une saison d’inondation, qui interviennent in natura (voir partie Modèles biologiques,<br />
section 2.1), représentent une contrainte supplémentaire pour cette espèce. Les fluctuations du<br />
niveau d’eau sont reconnues pour affecter négativement les performances <strong>de</strong>s espèces<br />
(Casanova & Brock 2000), comme cela pourrait être le cas pour Mentha pulegium.<br />
1.2. Les traits reliés à la tolérance aux inondations<br />
Avec l’augmentation <strong>de</strong> la durée d’inondation est observée une élongation <strong>de</strong>s tiges<br />
(Chapitres 1 et 6) ainsi qu’une augmentation <strong>de</strong> la surface spécifique foliaire ou SLA<br />
(Chapitre 6). Ces <strong>de</strong>ux traits fonctionnels facilitent les échanges gazeux au sein <strong>de</strong>s plantes<br />
sous <strong>de</strong>s conditions inondées, comme cela a été observé pour Juncus articulatus, Glyceria<br />
fluitans et Agrostis stolonifera. Ces résultats sont communément observés pour une gran<strong>de</strong><br />
diversité d’espèces (Blom & Voesenek 1996 ; Clevering et al. 1998 ; Lenssen et al. 1998 ;<br />
Insausti et al. 2001 ; Mommer et al. 2004 ; Voesenek et al. 2004 ; Mommer et al. 2006 ;<br />
Colmer & Voesenek 2009). Nous avons également observés l’existence d’aérenchyme dans<br />
les tiges pour Glyceria fluitans et Juncus articulatus, permettant également <strong>de</strong> faciliter ces<br />
échanges gazeux (Blom & Voesenek 1996 ; Mommer et al. 2004 ; Voesenek 2006 ; Colmer &<br />
Voesenek 2009) ; cet aérenchyme est probablement présent dans leurs racines. De plus, un<br />
développement <strong>de</strong> racines adventices a été observé en conditions inondées pour Mentha<br />
pulegium et Agrostis stolonifera.<br />
216
2. Variation <strong>de</strong> la compétition le long du gradient d’inondation<br />
Cet axe <strong>de</strong> travail montre clairement l’intérêt <strong>de</strong> différencier l’intensité et l’importance<br />
<strong>de</strong> la compétition, qui sont <strong>de</strong>ux composantes différentes <strong>de</strong> la compétition (Chapitre 1). Ce<br />
résultat est en accord avec la démarche scientifique actuelle proposant la différenciation <strong>de</strong><br />
ces <strong>de</strong>ux composantes pour une meilleure compréhension <strong>de</strong> la variation <strong>de</strong> la compétition le<br />
long <strong>de</strong> gradients environnementaux (Brooker et al. 2005 ; Brooker & Kikvidze 2008 ;<br />
Kikvidze et al. 2011). En effet, l’intensité <strong>de</strong> la compétition mesure l’effet <strong>de</strong> la présence <strong>de</strong><br />
compétiteurs per se sur les performances <strong>de</strong> l’espèce cible alors que l’importance <strong>de</strong> la<br />
compétition mesure l’effet <strong>de</strong> la présence <strong>de</strong> compétiteurs sur les performances <strong>de</strong>s espèces<br />
cibles relativement aux autres facteurs environnementaux qui impactent également les<br />
performances <strong>de</strong>s espèces. Jusqu’à récemment, l’importance <strong>de</strong> la compétition était évaluée à<br />
travers <strong>de</strong>s mesures d’intensité <strong>de</strong> la compétition permettant <strong>de</strong> conclure si oui ou non la<br />
compétition jouait un rôle important dans les patrons d’espèces. Actuellement et grâce au<br />
développement d’indices permettant <strong>de</strong> mesurer l’importance <strong>de</strong> la compétition, il est possible<br />
<strong>de</strong> déterminer si la compétition est structurante par rapport à d’autres facteurs<br />
environnementaux. Nous passons alors d’une approche binaire à une approche quantitative<br />
(Kikvidze et al. 2011), comme cela a été entrepris dans cette thèse.<br />
2. 1. Intensité <strong>de</strong> la compétition le long du gradient d’inondation<br />
2.1.1. Ségrégation spatiale <strong>de</strong>s niches le long du gradient d’inondation en réponse à la<br />
présence <strong>de</strong> compétiteurs<br />
En réponse à la présence <strong>de</strong> compétiteurs, un déplacement <strong>de</strong> niche, et plus<br />
particulièrement <strong>de</strong> l’optimum <strong>de</strong> développement, est observé pour certaines espèces (Juncus<br />
gerardii, Bellis perennis, Leontodon autumnalis, Trifolium fragiferum et Mentha pulegium)<br />
validant l’hypothèse d’un compromis entre les capacités compétitives <strong>de</strong>s espèces et la<br />
tolérance à la contrainte inondation (Chapitre 3). Ce déplacement <strong>de</strong> niche est mis en évi<strong>de</strong>nce<br />
par un changement dans la hiérarchie compétitive entre espèces le long du gradient<br />
expérimental d’inondation (Chapitre 1 ; Lenssen et al. 2004).<br />
Suivant l’hypothèse du compromis entre capacités compétitives <strong>de</strong>s espèces et la<br />
tolérance à la contrainte, les espèces exclues <strong>de</strong>s niveaux les moins contraints (non inondés)<br />
vers <strong>de</strong>s niveaux plus contraints (vers les parties inondées), seraient <strong>de</strong>s espèces faiblement<br />
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compétitives par rapport aux espèces mésophiles ; ce serait le cas <strong>de</strong> Juncus gerardii ou Bellis<br />
perennis par exemple. En effet, les espèces mésophiles caractéristiques <strong>de</strong>s niveaux non-<br />
inondés du gradient (Lolium perenne, Hor<strong>de</strong>um secalinum, Cynosurus cristatus), ne subissent<br />
pas <strong>de</strong> déplacement d’optimum <strong>de</strong> développement. Ces espèces présentent <strong>de</strong>s stratégies<br />
d’acquisition rapi<strong>de</strong> <strong>de</strong>s ressources, c’est-à-dire <strong>de</strong> fortes valeurs <strong>de</strong> surfaces spécifiques<br />
foliaires ainsi qu’une croissance en hauteur importante (Chapitres 2 et 6). Les valeurs <strong>de</strong> la<br />
surface spécifique foliaire sont faibles pour Juncus gerardii (moyenne <strong>de</strong> 10m²/kg dans les<br />
niveaux les plus hauts du gradient) contrairement à celles <strong>de</strong> Lolium perenne (moyenne <strong>de</strong><br />
31.04 m²/kg dans les niveaux les plus hauts du gradient). Le port en rosette <strong>de</strong> Leontodon<br />
autumnalis et Bellis perennis ne leur permet pas croître en hauteur et serait par conséquent un<br />
facteur limitant pour leur développement et leur aptitu<strong>de</strong> compétitive dans les parties non<br />
inondées où la végétation est <strong>de</strong>nse et haute. Ces traits pourraient expliquer leur exclusion<br />
compétitive <strong>de</strong> la communauté mésophile.<br />
Cependant, ces <strong>de</strong>ux mêmes traits ne peuvent pas expliquer l’exclusion <strong>de</strong> toutes les<br />
espèces. Par exemple, l’espèce hygrophile Trifolium fragiferum présente <strong>de</strong>s capacités<br />
d’acquisition rapi<strong>de</strong> <strong>de</strong>s ressources (SLA en moyenne <strong>de</strong> 37.87 m²/kg dans les parties non<br />
inondées) et une croissance en hauteur équivalente aux espèces mésophiles. Globalement, il<br />
apparaît difficile <strong>de</strong> relier expérimentalement le <strong>de</strong>gré <strong>de</strong> similitu<strong>de</strong> entre le compétiteur et<br />
l’espèce cible pour ces <strong>de</strong>ux traits et l’intensité <strong>de</strong> la compétition (Chapitre 1). D’autres traits<br />
fonctionnels sont sans doute impliqués dans la réponse à la compétition, c’est-à-dire pour être<br />
capable <strong>de</strong> tolérer la réduction <strong>de</strong> la disponibilité <strong>de</strong>s ressources. Il existe en effet différentes<br />
stratégies <strong>de</strong> réponse à la compétition, et chacune implique différents ensembles <strong>de</strong> traits<br />
fonctionnels (Keddy et al. 1998 ; Wang et al. 2010). Les espèces peuvent soit présenter <strong>de</strong>s<br />
stratégies <strong>de</strong> conservation <strong>de</strong>s ressources, soit prélever rapi<strong>de</strong>ment ces ressources ou décaler<br />
ce prélèvement dans le temps (Keddy et al. 1998 ; Carlyle & Fraser 2006).<br />
La stratégie mise en place par une espèce pour répondre à la présence <strong>de</strong> compétiteurs<br />
peut varier suivant les conditions environnementales (Keddy et al. 1994 ; Wang et al. 2010).<br />
En conséquence, on peut s’attendre à <strong>de</strong>s variations dans la réponse à la compétition <strong>de</strong><br />
l’espèce selon les conditions environnementales. L’existence <strong>de</strong> compromis entre traits<br />
fonctionnels pourrait expliquer les variations <strong>de</strong> stratégie mise en place par l’espèce, car ces<br />
compromis entre traits peuvent varier suivant les conditions environnementales induisant <strong>de</strong>s<br />
capacités <strong>de</strong> réponse à la compétition différentes, modifiant alors les capacités d’une espèce à<br />
être capable <strong>de</strong> persister dans un environnement donné (Lortie et al. 2004). Cette sensibilité à<br />
l’environnement <strong>de</strong> la réponse à la compétition pourrait expliquer les variations <strong>de</strong> réponse à<br />
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la compétition observées pour trois espèces le long du gradient expérimental d’inondation<br />
(Chapitre 1, Hor<strong>de</strong>um secalinum, Juncus gerardii et Mentha pulegium). Parallèlement, cette<br />
étu<strong>de</strong> a également montré que d’autres espèces (Lolium perenne, Carex divisa, Bellis<br />
perennis, Glyceria fluitans et Juncus articulatus) ont une réponse à la compétition qui ne<br />
varie pas suivant les conditions d’inondation. Ce résultat suggère une stabilité <strong>de</strong> la<br />
performance <strong>de</strong>s espèces, qui peut néanmoins s’accompagner d’une variation dans l’allocation<br />
<strong>de</strong>s ressources vers les racines ou les parties aériennes suivant les conditions (Tilman 1988),<br />
ce qui n’a pas été mesurée dans ce travail. On peut émettre l’hypothèse que les patrons <strong>de</strong><br />
réponse à la compétition observés (Chapitre 1) sont la résultante d’une stratégie mise en place<br />
par l’espèce dans un milieu donné suivant la disponibilité <strong>de</strong>s ressources, stratégie qui serait<br />
dépendante <strong>de</strong>s conditions locales où l’espèce se trouve, et également du besoin <strong>de</strong>s plantes en<br />
ressources dans un environnement donné comme le propose Taylor et al. (1990). De plus,<br />
l’i<strong>de</strong>ntité du voisin serait également une variable qui influencerait la réponse à la compétition<br />
<strong>de</strong>s espèces (Gau<strong>de</strong>t & Keddy1995 ; Fraser & Keddy 2005 ; Engel & Weltzin 2008). Tout<br />
cela pourrait alors expliquer la diversité <strong>de</strong> résultats concernant la variation <strong>de</strong> la réponse à la<br />
compétition d’espèces le long <strong>de</strong> gradients environnementaux observée dans ce travail et dans<br />
la littérature.<br />
Les méthodologies utilisées pour mesurer la réponse à la compétition dans les<br />
chapitres 1 et 3, c’est-à-dire le contrôle <strong>de</strong> voisins en conditions contrôlées et naturelles, sont<br />
complémentaires. La comparaison précise <strong>de</strong>s niches fondamentales et réalisées in situ<br />
(Chapitre 3) par une caractérisation précise du gradient d’inondation, couplée à une<br />
expérimentation en conditions contrôlées, ont permis <strong>de</strong> réellement établir une analyse <strong>de</strong> la<br />
ségrégation spatiale <strong>de</strong>s niches <strong>de</strong>s espèces en réponse à la compétition (α-niche, Wilson<br />
1999) le long du gradient d’inondation (Silvertown et al. 1999 ; Silvertown 2004).<br />
Il faut tout <strong>de</strong> même noter les limites <strong>de</strong> ce travail <strong>de</strong> comparaison <strong>de</strong>s niches<br />
fondamentales et réalisées <strong>de</strong>s espèces à partir <strong>de</strong>s données récoltées en conditions naturelles.<br />
La comparaison entre les niches fondamentales et réalisées ne s’est faite que sur une part<br />
restreinte du gradient naturel d’inondation. Nous n’avions tout simplement pas toutes les<br />
données <strong>de</strong> caractérisation <strong>de</strong>s inondations disponibles au moment <strong>de</strong> la mise en place <strong>de</strong><br />
l’expérimentation, pour transplanter idéalement les plantes sur une plus large gamme<br />
d’inondation. Il apparaît maintenant nécessaire d’avoir une gamme plus large pour déterminer<br />
si le déplacement <strong>de</strong> la niche en réponse à la compétition reste dans la gamme <strong>de</strong>s conditions<br />
que peuvent tolérer physiologiquement les espèces ; auquel cas les résultats pourraient<br />
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indiquer <strong>de</strong> la facilitation car la largeur <strong>de</strong> la niche réalisée serait plus gran<strong>de</strong> que celle <strong>de</strong> la<br />
niche fondamentale (Bruno et al. 2003). Néanmoins, l’extrapolation <strong>de</strong>s niches fondamentales<br />
<strong>de</strong>s espèces méso-hygrophiles et <strong>de</strong> certaines hygrophiles tend à conclure que le déplacement<br />
<strong>de</strong> l’optimum <strong>de</strong> la niche reste dans la gamme <strong>de</strong> conditions environnementales que tolère<br />
physiologiquement l’espèce.<br />
2.1.2. Importance <strong>de</strong> la survie et <strong>de</strong> la colonisation pendant les inondations pour être<br />
compétitif<br />
A travers l’approche démographique, les probabilités <strong>de</strong> survie et <strong>de</strong> colonisation ont<br />
été estimées en réponse à l’inondation ainsi que la croissance en biomasse après la fin <strong>de</strong>s<br />
inondations (Chapitre 4). La modélisation <strong>de</strong> ces données nous a permis <strong>de</strong> déterminer le rôle<br />
essentiel <strong>de</strong>s espèces à occuper l’espace durant la phase inondée, via une augmentation <strong>de</strong> la<br />
survie et <strong>de</strong> la colonisation, pour être compétitif et réduire la performance <strong>de</strong>s autres espèces,<br />
notamment après les inondations. Nous avons pu ainsi relier les probabilités <strong>de</strong> survie et <strong>de</strong><br />
colonisation <strong>de</strong> <strong>de</strong>ux espèces dans les parties inondées du gradient à leurs stratégies <strong>de</strong><br />
croissance clonale. L’effet <strong>de</strong>s conditions d’inondation comme filtre écologique sélectionnant<br />
les espèces suivant leur stratégie clonale a d’ailleurs été mise en évi<strong>de</strong>nce dans les prairies<br />
humi<strong>de</strong>s du Marais Poitevin (Benot et al. 2011, Annexe 4).<br />
Tout d’abord, Agrostis stolonifera a un fort effet compétitif sur ces voisins, en raison<br />
<strong>de</strong> la survie et <strong>de</strong> la colonisation améliorées pendant les inondations (Chapitre 4). Ce succès<br />
peut s’expliquer par sa forte croissance clonale lui permettant à la fois <strong>de</strong> tolérer les<br />
inondations et <strong>de</strong> coloniser l’espace. En effet, sa croissance clonale, par le développement <strong>de</strong><br />
stolons photosynthétiques flottants, est reconnue pour être une stratégie importante pour<br />
répondre aux inondations (Soukupova 1994). Après la fin <strong>de</strong>s inondations ces stolons sont<br />
capables <strong>de</strong> s’implanter grâce aux racines adventives, représentant une stratégie d’occupation<br />
<strong>de</strong> l’espace et ainsi permettant d’être compétitif après la fin <strong>de</strong>s inondations (Santamaria<br />
2002 ; Lenssen et al. 2004). Ces stolons peuvent être assez longs : expérimentalement sous<br />
une profon<strong>de</strong>ur d’eau <strong>de</strong> 40 cm, nous avons mesuré <strong>de</strong>s longueurs <strong>de</strong> stolons supérieurs à 1m<br />
avec <strong>de</strong>s racines adventives espacées régulièrement le long <strong>de</strong> ces stolons.<br />
Cette approche permet également <strong>de</strong> comprendre comment une espèce qui est exclut<br />
en réponse à la compétition vers <strong>de</strong>s zones qui lui sont physiologiquement plus contraignantes<br />
(Chapitre 3), est tolérance aux inondations et compétitif dans ces parties inondées du gradient<br />
(Chapitre 4). C’est le cas <strong>de</strong> Juncus gerardii, qui voit sa survie et ses capacités <strong>de</strong> colonisation<br />
220
améliorées sous <strong>de</strong>s conditions d’inondation. Cette espèce est caractérisée par la présence <strong>de</strong><br />
rhizomes, connus pour être <strong>de</strong>s organes <strong>de</strong> réserve (Dong & <strong>de</strong> Kroon 1994). Cet organe <strong>de</strong><br />
stockage permet une utilisation rapi<strong>de</strong> <strong>de</strong>s ressources lorsqu’intervient la fin <strong>de</strong>s inondations,<br />
lui donnant un avantage pour la reprise <strong>de</strong> croissance, augmentant par conséquent sa<br />
colonisation, et ainsi être compétitif.<br />
2.2. Importance <strong>de</strong> la compétition le long du gradient d’inondation<br />
2.2.1. Importance <strong>de</strong> la compétition prédite par le « strain »<br />
La compétition a une gran<strong>de</strong> importance pour expliquer le succès écologique <strong>de</strong>s<br />
espèces car elle est responsable <strong>de</strong> la ségrégation spatiale <strong>de</strong>s niches <strong>de</strong>s espèces, à travers un<br />
compromis entre leurs capacités compétitives <strong>de</strong>s espèces et leur tolérance au « strain »<br />
(Chapitre 3).<br />
Par une approche quantitative, nous avons mis en évi<strong>de</strong>nce expérimentalement que<br />
l’importance <strong>de</strong> la compétition, mesurée à l’échelle <strong>de</strong> l’espèce, est inversement<br />
proportionnelle à l’intensité <strong>de</strong> la contrainte subie par cette espèce, c’est-à-dire l’intensité du<br />
« strain » (Chapitres 1 et 3). Cela signifie que la part <strong>de</strong> la compétition dans le succès<br />
écologique d’une espèce augmente quand cette espèce se trouve dans <strong>de</strong>s conditions<br />
environnementales proches <strong>de</strong> son optimum physiologique <strong>de</strong> développement, comme attendu<br />
par Wel<strong>de</strong>n & Slauson (1986) et Brooker et al. (2005).<br />
Dans les prairies humi<strong>de</strong>s du Marais poitevin, il est ainsi possible <strong>de</strong> conclure que les<br />
limites inférieures <strong>de</strong>s niches <strong>de</strong>s espèces mésophiles, <strong>de</strong>s espèces méso-hygrophiles et <strong>de</strong><br />
Trifolium fragiferum sont déterminées principalement par les inondations (Shipley et al.<br />
1991). Les limites supérieures <strong>de</strong>s niches <strong>de</strong>s espèces méso-hygrophiles et <strong>de</strong> Trifolium<br />
fragiferum sont déterminées principalement par la compétition. Parallèlement, les limites<br />
supérieures <strong>de</strong>s niches <strong>de</strong> Glyceria fluitans et <strong>de</strong> Juncus articulatus seraient d’abord<br />
déterminées par l’absence d’inondation. De plus, il a été mis en évi<strong>de</strong>nce l’importance <strong>de</strong> la<br />
compétition dans les parties faiblement inondées du gradient pour expliquer leur succès<br />
(Chapitre 3). En effet, la présence <strong>de</strong> compétiteurs réduit la gamme <strong>de</strong>s niches <strong>de</strong> ces <strong>de</strong>ux<br />
espèces vers <strong>de</strong>s zones longuement inondées. Ainsi les limites supérieures <strong>de</strong>s niches <strong>de</strong> ces<br />
espèces sont déterminées par l’absence d’inondations et par la compétition, exliquant ainsi<br />
leur distribution dans les parties longuement inondées du gradient.<br />
221
Nous remarquons que moitié moins d’espèces suivent la relation entre l’importance <strong>de</strong><br />
la compétition et l’intensité <strong>de</strong> la contrainte à l’échelle <strong>de</strong> l’espèce (« strain ») en conditions<br />
naturelles (Chapitre 3) qu’en conditions contrôlées (Chapitre 1), suggérant que d’autres<br />
facteurs interviennent in natura dans le succès écologiques <strong>de</strong>s espèces.<br />
2.2.2. Variation <strong>de</strong> l’importance <strong>de</strong> la compétition - apport <strong>de</strong> l’approche<br />
démographique<br />
La mesure <strong>de</strong> l’importance <strong>de</strong> la compétition in natura confirme la part importante <strong>de</strong>s<br />
inondations dans le succès écologique <strong>de</strong>s espèces mais également celle <strong>de</strong> la compétition<br />
(Chapitre 4). La mesure d’importance <strong>de</strong> la compétition a été réalisée à partir <strong>de</strong> la<br />
modélisation <strong>de</strong> différentes mesures <strong>de</strong> performances <strong>de</strong>s espèces : les probabilités <strong>de</strong> survie<br />
et <strong>de</strong> colonisation <strong>de</strong>s espèces durant la saison d’inondation, ainsi que la croissance en<br />
biomasse <strong>de</strong>s espèces estimée après la fin <strong>de</strong>s inondations. La prise en compte <strong>de</strong>s différentes<br />
mesures <strong>de</strong> performances <strong>de</strong>s espèces montrent que les probabilités <strong>de</strong> survie et <strong>de</strong><br />
colonisation sont largement impactées par les inondations que par la compétition pendant la<br />
pério<strong>de</strong> d’inondation. Les inondations ont une influence forte dans le succès écologique <strong>de</strong>s<br />
espèces et plus particulièrement dans leur capacité à occuper l’espace après la fin <strong>de</strong>s<br />
inondations. En revanche, après la fin <strong>de</strong>s inondations, la croissance en biomasse <strong>de</strong>s espèces<br />
est plus largement impactée par la compétition que par tout autre facteur environnemental, y<br />
compris la faible disponibilité en eau en été. Ces résultats montrent ainsi une variation<br />
temporelle <strong>de</strong> l’importance <strong>de</strong> la compétition.<br />
A cette variation temporelle <strong>de</strong> l’importance <strong>de</strong> la compétition entre saisons s’ajoute<br />
une variation spatiale <strong>de</strong> l’importance <strong>de</strong> la compétition le long du gradient. Ces patrons <strong>de</strong><br />
variation <strong>de</strong> l’importance <strong>de</strong> la compétition le long du gradient sont différents selon les<br />
espèces étudiées, spécialement durant la saison d’inondation, reflétant les différentes<br />
capacités <strong>de</strong> survie et <strong>de</strong> colonisation <strong>de</strong>s espèces suivant les changements <strong>de</strong> voisins et <strong>de</strong><br />
conditions environnementales.<br />
Cette approche considère différents paramètres démographiques et place la<br />
quantification in natura <strong>de</strong> l’importance <strong>de</strong> la compétition dans un cadre <strong>de</strong> dynamique <strong>de</strong>s<br />
populations et communautés : cela représente un complément dans la compréhension du rôle<br />
<strong>de</strong> la compétition dans l’organisation <strong>de</strong>s communautés (Goldberg et al. 1999 ; Frekleton et<br />
al. 2009). Nos résultats vont dans le sens <strong>de</strong>s propositions récentes <strong>de</strong> Frekleton et al. (2009)<br />
222
suggérant <strong>de</strong> ne pas se contenter d’une seule mesure <strong>de</strong> performances <strong>de</strong>s espèces pour<br />
déterminer le rôle <strong>de</strong> la compétition. En effet, en considérant seulement la mesure <strong>de</strong><br />
biomasse, nous avons mis en évi<strong>de</strong>nce une relation linéaire entre importance <strong>de</strong> la<br />
compétition et intensité <strong>de</strong> la contrainte (Chapitre 1). Or en considérant d’autres mesures <strong>de</strong><br />
performances, cette relation linéaire <strong>de</strong>vient moins évi<strong>de</strong>nte (Chapitre 4). Cela suggère que la<br />
compétition peut affecter différemment les différents sta<strong>de</strong>s du cycle <strong>de</strong> vie <strong>de</strong>s plantes<br />
(Howard & Goldberg 2001), mesurés dans cette thèse par les probabilités <strong>de</strong> colonisation et<br />
<strong>de</strong> survie <strong>de</strong>s espèces, et qu’il est nécessaire d’en tenir compte pour mesurer l’importance <strong>de</strong><br />
la compétition.<br />
2.3. Intensité et importance <strong>de</strong> la compétition le long du gradient d’inondation<br />
Aucune relation linéaire significative n’a été mise en évi<strong>de</strong>nce entre l’intensité et<br />
l’importance <strong>de</strong> la compétition, qui varient indépendamment le long du gradient expérimental<br />
d’inondation (Chapitre 1). Néanmoins, les résultats ten<strong>de</strong>nt à montrer que ces <strong>de</strong>ux<br />
composantes <strong>de</strong> la compétition sont tout <strong>de</strong> même reliées. En effet, pour les espèces qui sont<br />
affectées par une forte intensité <strong>de</strong> la compétition par les voisins, comme par exemple pour<br />
Juncus gerardii, Mentha pulegium, ou encore Leontodon autumnalis dans les parties non<br />
inondées du gradient (Chapitre 3), la compétition semble avoir une importance décisive pour<br />
leur succès et déterminante <strong>de</strong> leur limite <strong>de</strong> niches supérieures (i.e. dans les zones non<br />
inondées). Une intensité forte <strong>de</strong> la compétition apparaît donc être un pré-requis à une<br />
importance <strong>de</strong> la compétition forte (Bartelheimer et al. 2010 ; Kikvidze et al. 2011).<br />
3. Les traits fonctionnels comme outils d’étu<strong>de</strong> <strong>de</strong>s règles d’assemblage<br />
La ségrégation spatiale <strong>de</strong>s (α)-niches <strong>de</strong>s espèces le long du gradient d’inondation<br />
permet <strong>de</strong> comprendre l’assemblage <strong>de</strong>s espèces et <strong>de</strong> la structure <strong>de</strong>s communautés dans les<br />
prairies humi<strong>de</strong>s du Marais Poitevin. Néanmoins, il existe un chevauchement <strong>de</strong>s niches<br />
réalisées entre espèces le long du gradient (Chapitre 3). L’approche fonctionnelle analysant la<br />
distribution <strong>de</strong>s traits fonctionnels mesurés in situ (Chapitre 6) permet <strong>de</strong> comprendre<br />
comment les espèces peuvent coexister.<br />
L’analyse <strong>de</strong>s distributions <strong>de</strong>s valeurs <strong>de</strong>s <strong>de</strong>ux traits fonctionnels mesurés, le SLA et<br />
la hauteur, montre qu’elles sont significativement différentes d’une distribution aléatoire.<br />
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L’assemblage <strong>de</strong>s espèces se fait donc suivant <strong>de</strong>s processus déterministes. L’analyse <strong>de</strong>s<br />
distributions <strong>de</strong> ces <strong>de</strong>ux traits met en évi<strong>de</strong>nce un effet du filtre abiotique (l’inondation) ainsi<br />
qu’un effet du filtre biotique (la compétition) sur ces distributions. Ces <strong>de</strong>ux filtres agissent<br />
simultanément sur l’assemblage <strong>de</strong>s espèces, et ce, au sein <strong>de</strong>s trois communautés végétales<br />
existant dans ces prairies (mésophile, méso-hygrophile et hygrophile). L’utilisation <strong>de</strong><br />
différentes métriques mesurant la distribution <strong>de</strong>s traits permet <strong>de</strong> différencier l’effet <strong>de</strong>s<br />
inondations agissant sur la valeur moyenne du trait <strong>de</strong> celui <strong>de</strong> la compétition agissant sur les<br />
écarts <strong>de</strong>s valeurs <strong>de</strong> traits entre espèces qui coexistent.<br />
Tout d’abord, l’analyse <strong>de</strong>s distributions <strong>de</strong> ces traits confirme que <strong>de</strong>s individus<br />
possédant <strong>de</strong>s traits associés à la tolérance à l’inondation, tels une hauteur et un SLA élevés,<br />
sont sélectionnés dans les parties inondées du gradient, conformément aux attendus <strong>de</strong><br />
l’« habitat filtering » (Weiher & Keddy 1995). Ce résultat confirme ceux concernant<br />
l’importance <strong>de</strong> ces traits reliés à la tolérance aux inondations (cf. discussion partie 1.2 ;<br />
Weiher et al. 1998 ; Mommer et al. 2006 ; Jung et al. 2010 ; Violle et al. 2011).<br />
Les distributions <strong>de</strong> <strong>de</strong>ux traits observées confirment également les prédictions du<br />
concept <strong>de</strong> « limiting similarity » (Mac Arthur & Levins 1967). Une maximisation <strong>de</strong>s écarts<br />
<strong>de</strong>s valeurs <strong>de</strong> traits entre espèces coexistantes est mise en évi<strong>de</strong>nce, limitant la similitu<strong>de</strong><br />
fonctionnelle entre voisins à l’échelle locale (α). Ainsi les résultats indiquent qu’une<br />
différenciation <strong>de</strong>s niches <strong>de</strong>s espèces semble avoir lieu. Cette différenciation <strong>de</strong>s niches se<br />
produit pour ces <strong>de</strong>ux traits mesurés, suggérant <strong>de</strong>s stratégies d’acquisition et <strong>de</strong> conservation<br />
<strong>de</strong>s ressources différentes entre espèces (Weiher et al. 1998; Kraft et al. 2008 ; Jung et al.<br />
2010). Le SLA et la hauteur apparaissent être <strong>de</strong>s traits <strong>de</strong> réponse à la fois aux conditions<br />
abiotiques, mais également à l’environnement biotique (Weiher et al. 1999 ; Wright et al.<br />
2004 ; Violle et al. 2011).<br />
Suivant les propositions <strong>de</strong> Weiher & Keddy (1995), une maximisation <strong>de</strong>s écarts <strong>de</strong>s<br />
valeurs <strong>de</strong> traits entre les espèces qui coexistent serait principalement attendue dans les<br />
niveaux non contraints d’un gradient environnemental, c’est-à-dire dans les parties non-<br />
inondées du gradient d’inondation étudié. Les résultats d’expérimentations montrent<br />
cependant que toutes les espèces ne sont pas contraintes par les inondations, et que la<br />
compétition est impliquée dans le succès écologique <strong>de</strong>s espèces à différentes positions le<br />
long du gradient (Partie 1). La compétition influencerait par conséquent la distribution <strong>de</strong>s<br />
traits le long du gradient d’inondation. Ceci est d’autant plus vraisemblable que les prairies<br />
humi<strong>de</strong>s du Marais Poitevin sont <strong>de</strong>s milieux productifs, où le niveau <strong>de</strong> ressources fort est<br />
propice à un fort niveau d’interactions compétitives. Il est donc envisageable que ces<br />
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interactions compétitives influencent les écarts <strong>de</strong>s valeurs <strong>de</strong> traits entre les espèces<br />
coexistantes le long du gradient, et pas seulement dans les parties non-inondées. Nous<br />
mettons, en effet, en évi<strong>de</strong>nce une augmentation <strong>de</strong> la moyenne <strong>de</strong>s écarts <strong>de</strong>s valeurs <strong>de</strong>s<br />
<strong>de</strong>ux traits entre espèces coexistantes (MeanNTD) et une réduction <strong>de</strong> leur variance<br />
(VarNTD), suggérant une différenciation <strong>de</strong>s niches opérant tout le long du gradient<br />
d’inondation.<br />
Les prairies étudiées étant productives, la proposition <strong>de</strong> Grime (2006) formulée dans<br />
les milieux productifs concernant la distribution <strong>de</strong>s traits doit être prise en compte, c’est-à-<br />
dire une convergence dans la distribution <strong>de</strong>s traits d’acquisition <strong>de</strong>s ressources entre les<br />
espèces qui coexistent. La convergence dans les distributions <strong>de</strong> ces traits résulte <strong>de</strong><br />
l’augmentation <strong>de</strong> l’effet <strong>de</strong> la compétition comme filtre sélectionnant les espèces sur la base<br />
<strong>de</strong> leur aptitu<strong>de</strong> compétitive (Pakeman et al. 2011). Dans notre cas d’étu<strong>de</strong>, cette convergence<br />
est attendue le long du gradient d’inondation. Cette convergence tend à réduire les écarts <strong>de</strong>s<br />
valeurs <strong>de</strong>s traits entre espèces coexistantes. Le long du gradient d’inondation, nous avons<br />
ainsi pu mettre en évi<strong>de</strong>nce une réduction <strong>de</strong>s écarts <strong>de</strong>s valeurs <strong>de</strong>s traits entre espèces qui<br />
coexistent, impliquant la hauteur dans la communauté mésophile et impliquant le SLA dans la<br />
communauté hygrophile, suggérant une convergence due à la compétition. Grâce à<br />
l’utilisation <strong>de</strong> la métrique MeanNTD (distance moyenne entre plus proches voisins), il a été<br />
possible <strong>de</strong> différencier la convergence due à l’« habitat filtering » <strong>de</strong> la convergence due à la<br />
compétition. Néanmoins, en raison <strong>de</strong> l’utilisation <strong>de</strong> métriques différentes pour détecter ces<br />
<strong>de</strong>ux sources <strong>de</strong> convergence fonctionnelle entre traits, il n’est pas possible <strong>de</strong> mesurer leur<br />
importance respective.<br />
Concernant les métriques mesurant les écarts <strong>de</strong>s valeurs <strong>de</strong> traits entre voisins, nos<br />
résultats indiquent que dans les parties non inondées, une faible similitu<strong>de</strong> entre espèces<br />
implique plutôt une différenciation <strong>de</strong>s valeurs <strong>de</strong> SLA et une plus forte similitu<strong>de</strong> <strong>de</strong>s valeurs<br />
<strong>de</strong> hauteur entre espèces coexistantes due à la compétition. A l’inverse dans les niveaux<br />
inondés, une faible similitu<strong>de</strong> implique plutôt une différence <strong>de</strong>s valeurs <strong>de</strong> hauteur et une<br />
plus forte similitu<strong>de</strong> <strong>de</strong>s valeurs <strong>de</strong> SLA entre espèces coexistantes due à la compétition. Ces<br />
résultats soutiennent une similitu<strong>de</strong> dans la dynamique <strong>de</strong>s ressources : pour coexister dans<br />
<strong>de</strong>s environnements compétitifs comme les milieux productifs, une différenciation <strong>de</strong> niche<br />
est certes nécessaire, mais elle ne doit cependant pas être trop importante pour être compétitif<br />
face à <strong>de</strong>s voisins également compétitifs pour persister (Scheffer & van Nes 2006).<br />
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Ces résultats nous indiquent que les inondations représentent un filtre abiotique<br />
important sélectionnant les individus suivant leur capacité à tolérer les inondations, et que la<br />
compétition agit le long du gradient permettant la coexistence <strong>de</strong>s espèces.<br />
4. Le stress à l’échelle <strong>de</strong> la communauté module les effets du pâturage et <strong>de</strong><br />
la pluviosité dans les milieux méditerranéens<br />
L’étu<strong>de</strong> <strong>de</strong> l’effet <strong>de</strong> différents facteurs abiotiques (variabilité <strong>de</strong>s patrons <strong>de</strong><br />
pluviosité) et biotiques (pâturage) dans les milieux méditerranéens met en évi<strong>de</strong>nce qu’en<br />
fonction du niveau <strong>de</strong> productivité <strong>de</strong> la communauté – proxy du stress à l’échelle <strong>de</strong> la<br />
communauté (Grime 1977, 1979) – les réponses à la variabilité <strong>de</strong>s patrons <strong>de</strong> pluviosité et au<br />
pâturage sont différentes (Chapitre 7).<br />
Les communautés végétales méditerranéennes sont <strong>de</strong>s communautés très riches en<br />
espèces. Les espèces annuelles sont attendues à répondre très fortement à la pluviosité,<br />
variable déterminante <strong>de</strong> leurs patrons <strong>de</strong> germination et <strong>de</strong> croissance (Pake & Venable<br />
1996). Dans la communauté la plus productive (les pelouses strictes), c’est-à-dire la moins<br />
contrainte, la composition en espèces varie fortement entre années. Ces changements en<br />
composition sont attribués à la variabilité <strong>de</strong>s patrons <strong>de</strong> pluviosité observée sur la pério<strong>de</strong><br />
2001-2008 étudiée. Des espèces xériques s’expriment plutôt les années sèches : on observe<br />
qu’un remplacement d’espèces intervient probablement entre années humi<strong>de</strong>s et années plus<br />
sèches, expliquant la diversité en espèces <strong>de</strong> ces pelouses strictes inchangée au cours <strong>de</strong> la<br />
pério<strong>de</strong> d’étu<strong>de</strong>. L’effet <strong>de</strong> la variabilité <strong>de</strong>s patrons <strong>de</strong> pluviosité est moins marqué dans les<br />
prés salés, c’est-à-dire la communauté la moins productive caractérisée par une salinité du sol<br />
forte: la dynamique <strong>de</strong> la composition est linéaire. Néanmoins, on remarque une forte<br />
augmentation <strong>de</strong> la proportion <strong>de</strong>s espèces pérennes lors <strong>de</strong>s années humi<strong>de</strong>s, et ce dans les<br />
<strong>de</strong>ux communautés.<br />
Un <strong>de</strong>sign expérimental in situ contrôlant la modalité <strong>de</strong> pâturage a été mise en place<br />
après 40 années <strong>de</strong> charge <strong>de</strong> pâturage extensive. En plus <strong>de</strong> la modalité témoin <strong>de</strong> pâturage<br />
correspondant à ces 40 années <strong>de</strong> pâturage, les <strong>de</strong>ux communautés ont été soumises à : (i)<br />
l’arrêt du pâturage et (ii) un changement dans la pério<strong>de</strong> où le bétail est présent sur le site<br />
(<strong>de</strong>ux jours au lieu <strong>de</strong> 6 mois) mais sans aucun changement <strong>de</strong> la charge annuelle, appelé le<br />
pâturage instantané et caractérisé par une forte charge instantanée <strong>de</strong> pâturage.<br />
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Dans les pelouses strictes, un arrêt du pâturage conduit à la dominance d’une espèce<br />
pérenne, Brachypodium phoenicoï<strong>de</strong>s : une fermeture du milieu est observée par cette espèce<br />
qui serait compétitive, et confirme les résultats fréquemment rapportés dans la littérature<br />
(Peco et al. 1983; Noy-Meir et al. 1989; Noy-Meir 1995; Díaz et al. 2001; Peco et al. 2005).<br />
L’analyse fine <strong>de</strong>s probabilités <strong>de</strong> colonisation et <strong>de</strong> survie <strong>de</strong> cette espèce nous indique que la<br />
survie <strong>de</strong> cette espèce est affectée par la présence <strong>de</strong>s herbivores (Chapitre 5). Le pâturage<br />
limite les capacités <strong>de</strong> survie <strong>de</strong> cette espèce (Fig. 20a) alors qu’en l’absence <strong>de</strong> pâturage, ces<br />
capacités <strong>de</strong> survie expliquent son succès et l’augmentation <strong>de</strong> son abondance, conduisant à<br />
une perte <strong>de</strong> diversité en espèces dans les pelouses strictes (Fig. 20b). Cette étu<strong>de</strong><br />
démographique met en évi<strong>de</strong>nce l’importance <strong>de</strong> la survie dans le succès <strong>de</strong>s espèces pérennes<br />
dans les milieux méditerranéens et surtout la sensibilité <strong>de</strong> ce paramètre démographique à la<br />
gestion par le pâturage, résultat observé dans d’autres milieux (Ehrlen 1995 ; Oliva et al.<br />
2005). Quelle que soit la modalité <strong>de</strong> pâturage, la présence d’herbivores limite l’augmentation<br />
en abondance <strong>de</strong> cette espèce dominante. Néanmoins, le pâturage instantané n’affecte pas la<br />
structure <strong>de</strong>s pelouses, suggérant que le pool d’espèces a déjà été filtrée par les 40 années <strong>de</strong><br />
pâturage extensif (Ward et al. 1998 ; Metzger et al. 2005). Une augmentation <strong>de</strong> la<br />
contribution d’annuelles caractéristiques <strong>de</strong>s prés salés, suggère une modification <strong>de</strong>s<br />
conditions abiotiques <strong>de</strong> salinité du sol, augmentant la similitu<strong>de</strong> entre les pelouses strictes et<br />
les prés salés en termes <strong>de</strong> composition (Cingolani et al. 2003).<br />
Fig 20 : Dans les pelouses strictes : a) Perte <strong>de</strong> diversité en espèces en raison <strong>de</strong> l’arrêt du<br />
pâturage favorisant le succès écologique <strong>de</strong> Brachypodium phoenicoï<strong>de</strong>s b) Maintien <strong>de</strong> la<br />
diversité en espèces par le pâturage limitant le succès écologique <strong>de</strong> l’espèce.<br />
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Dans les prés salés, un changement dans le régime <strong>de</strong> pâturage, c’est-à-dire<br />
l’application <strong>de</strong> la forte charge instantanée ou l’arrêt du pâturage, n’affecte pas la diversité en<br />
espèces. Au cours <strong>de</strong> la pério<strong>de</strong> étudiée 2001-2008, la dynamique <strong>de</strong> composition en espèces<br />
est similaire entre ces <strong>de</strong>ux traitements et le traitement témoin. Une espèce en particulier<br />
apparaît contrôler cette dynamique <strong>de</strong> composition : l’espèce dominante Halimione<br />
portulacoï<strong>de</strong>s. Il s’agit d’une espèce succulente possédant les caractéristiques physiologiques<br />
lui permettant <strong>de</strong> se développer sous <strong>de</strong>s conditions <strong>de</strong> salinité fortes. Cette capacité à tolérer<br />
le sel est notée comme permettant <strong>de</strong> répondre à d’autres <strong>de</strong> contraintes et perturbation<br />
(Milchunas et al. 1988). On peut ainsi supposer que la tolérance au sel permet à cette espèce<br />
<strong>de</strong> tolérer les différentes modalités <strong>de</strong> pâturage, en contrôlant la composition en espèces<br />
probablement en raison <strong>de</strong> fortes capacités compétitives.<br />
Le niveau <strong>de</strong> stress à l’échelle <strong>de</strong> la communauté limite les effets du pâturage mis en<br />
évi<strong>de</strong>nce dans les pelouses. Ce résultat peut s’expliquer par l’espèce dominante <strong>de</strong>s prés salés,<br />
qui elle ne serait pas contrainte par la salinité du sol. En effet, un niveau <strong>de</strong> contrainte à<br />
l’échelle <strong>de</strong> la communauté n’implique pas forcément que les espèces <strong>de</strong> cette communauté<br />
sont contraintes (Lortie et al. 2004). Ces résultats suggèrent que se contenter du niveau <strong>de</strong><br />
stress à l’échelle <strong>de</strong> la communauté ne permet pas, à lui seul, <strong>de</strong> comprendre précisément les<br />
mécanismes impliqués dans la structure et la dynamique <strong>de</strong>s communautés végétales.<br />
5. Conclusion & Perspectives<br />
La contrainte apparaît déterminante dans les patrons d’espèces et <strong>de</strong> structure <strong>de</strong>s<br />
communautés. Le niveau <strong>de</strong> contrainte module l’importance <strong>de</strong>s autres filtres écologiques,<br />
comme la compétition dans les prairies humi<strong>de</strong>s du Marais Poitevin, ou le pâturage dans les<br />
prés-salés méditerranéens. De plus, le choix du niveau d’organisation considéré pour<br />
caractériser la contrainte, c’est-à-dire à l’échelle <strong>de</strong> l’espèce ou à l’échelle <strong>de</strong> la communauté,<br />
est déterminant pour comprendre les effets <strong>de</strong>s différents filtres écologiques. En effet, une<br />
contrainte mesurée à l’échelle <strong>de</strong> la communauté végétale n’est pas forcément une contrainte<br />
pour les espèces qui la composent. Mais ces <strong>de</strong>ux niveaux d’organisation apparaissent<br />
complémentaires pour comprendre les assemblages d’espèces et la structure <strong>de</strong>s communautés<br />
végétales dans les <strong>de</strong>ux milieux étudiés.<br />
Dans les prairies humi<strong>de</strong>s du Marais Poitevin, les inondations représentent un filtre<br />
abiotique majeur qui sélectionne les espèces suivant leur capacité à tolérer ces conditions. A<br />
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ce filtre s’ajoute la compétition entre plantes, est particulièrement importante pendant les<br />
pério<strong>de</strong>s où les prairies ne sont plus inondées. Dans les milieux méditerranéens, la contrainte<br />
à l’échelle <strong>de</strong> la communauté limite les effets <strong>de</strong>s autres filtres écologiques, que sont la<br />
variabilité <strong>de</strong>s patrons <strong>de</strong> pluviosité et du pâturage. La réponse <strong>de</strong>s communautés à ces<br />
différents filtres écologiques est due aux espèces dominantes qui contrôlent la dynamique <strong>de</strong>s<br />
communautés et qui ne sont pas contraintes par les conditions <strong>de</strong> leur habitat respectif.<br />
Afin <strong>de</strong> compléter ce travail <strong>de</strong> compréhension <strong>de</strong>s mécanismes structurant les <strong>de</strong>ux<br />
modèles d’étu<strong>de</strong>, différentes perspectives <strong>de</strong> recherche sont envisageables.<br />
5.1. La mesure <strong>de</strong> la compétition en conditions naturelles<br />
La mesure <strong>de</strong> la compétition en conditions naturelles est une approche prometteuse car<br />
l’effet compétitif <strong>de</strong>s espèces sur les voisins et l’importance <strong>de</strong> la compétition par rapport aux<br />
autres facteurs environnementaux pouvant également contrôler la dynamique <strong>de</strong>s populations,<br />
ont été quantifiés dans un cadre dynamique considérant différentes mesures du cycle <strong>de</strong> vie<br />
<strong>de</strong>s plantes. Cette analyse a cependant été réalisée sur une seule saison d’inondation. Or une<br />
précé<strong>de</strong>nte étu<strong>de</strong> (Violle et al. 2007) a mis en évi<strong>de</strong>nce la nécessité <strong>de</strong> tenir compte <strong>de</strong><br />
plusieurs années successives, caractérisées par une variabilité <strong>de</strong>s durées d’inondation, pour<br />
comprendre le succès <strong>de</strong>s espèces et leurs patrons d’abondance. Grâce la réplication dans le<br />
temps <strong>de</strong>s relevés point contact, l’effet <strong>de</strong> la variabilité <strong>de</strong>s conditions d’inondation sur la<br />
dynamique <strong>de</strong>s populations pourrait être prise en compte, en corrélant ces probabilités à la<br />
variable durée d’inondation par exemple (Torang et al. 2010). Les effets <strong>de</strong> la variation <strong>de</strong>s<br />
conditions d’inondation sur l’importance <strong>de</strong> la compétition pourront ainsi être approchés et on<br />
pourrait mieux comprendre la distribution <strong>de</strong>s espèces et la structure <strong>de</strong>s communautés.<br />
5.2. Vers le développement <strong>de</strong>s tests <strong>de</strong>s règles d’assemblage<br />
Avec le test <strong>de</strong>s règles d’assemblage, nous avons pu montrer que l’assemblage <strong>de</strong>s<br />
espèces dans les prairies humi<strong>de</strong>s du Marais Poitevin se faisait suivant <strong>de</strong>s processus<br />
déterministes. Or cette analyse ne nous permet pas <strong>de</strong> connaître l’importance relative <strong>de</strong><br />
chacun <strong>de</strong> ces processus, notamment celui <strong>de</strong> la compétition, en raison <strong>de</strong> l’utilisation <strong>de</strong><br />
métriques différentes pour tester ces différents processus, et <strong>de</strong> généraliser les résultats.<br />
Avec ce type d’analyses, différents patrons <strong>de</strong> distribution <strong>de</strong>s traits peuvent être<br />
observés pour un même facteur (Podani 2009). En effet, une divergence dans la distribution<br />
<strong>de</strong>s traits peut être attendue par l’effet <strong>de</strong> la compétition ainsi qu’une convergence dans cette<br />
229
distribution ; cela dépend <strong>de</strong>s conditions environnementales <strong>de</strong> l’habitat (milieu productif ou<br />
non). De la même manière, il peut-être attendu à la fois une convergence dans la distribution<br />
<strong>de</strong>s traits ainsi qu’une divergence en réponse au facteur abiotique ; cela dépend <strong>de</strong> la nature<br />
du facteur abiotique, c’est-à-dire s’il s’agit ou non d’une perturbation. Cela amène à <strong>de</strong>s<br />
difficultés d’interprétation <strong>de</strong>s résultats et surtout à une difficulté à généraliser ces résultats,<br />
apparaissant comme une limite à l’utilisation <strong>de</strong> cette approche fonctionnelle <strong>de</strong>s règles<br />
d’assemblage.<br />
Suivant la proposition <strong>de</strong> Navas & Violle (2009), cette généralisation pourrait être<br />
possible en travaillant le long <strong>de</strong> gradients d’importance <strong>de</strong> la compétition. Relier la diversité<br />
fonctionnelle à l’importance <strong>de</strong> la compétition permettrait <strong>de</strong> déterminer le poids <strong>de</strong> cette<br />
importance dans l’assemblage <strong>de</strong>s espèces. Calculer un indice à l’échelle <strong>de</strong> la communauté<br />
en agrégeant les valeurs d’importance <strong>de</strong> la compétition <strong>de</strong>s espèces composant la<br />
communauté (Greiner la Peyre et al. 2001) permettrait d’avoir une valeur d’importance <strong>de</strong> la<br />
compétition à l’échelle <strong>de</strong> la communauté. Il serait ainsi possible <strong>de</strong> relier les mesures<br />
d’importance <strong>de</strong> la compétition aux mesures <strong>de</strong> diversité fonctionnelle pour déterminer plus<br />
précisément son rôle dans l’assemblage <strong>de</strong>s espèces et ainsi avoir une méthodologie plus<br />
prédictive et généralisable.<br />
5.3. De la facilitation observée ?<br />
Certains résultats expérimentaux suggèrent <strong>de</strong> la facilitation entre espèces dans les<br />
parties inondées du gradient (Chapitre 1, valeurs <strong>de</strong> lnRR(interaction) positives pour<br />
Leontodon autumnalis et performances photosynthétiques améliorées en présence <strong>de</strong> voisins<br />
sous <strong>de</strong>s conditions d’inondation). Les étu<strong>de</strong>s récentes sur le sujet montrent que la facilitation<br />
n’intervient pas seulement dans les milieux fortement contraints mais serait dépendante du<br />
type <strong>de</strong> contraintes (cf introduction 2.1) et <strong>de</strong> la stratégie <strong>de</strong> l’espèce (Michalet et al. 2006 ;<br />
Maestre et al. 2009). Il serait intéressant <strong>de</strong> regar<strong>de</strong>r ces résultats <strong>de</strong> plus près pour déterminer<br />
si réellement <strong>de</strong>s interactions positives peuvent intervenir dans notre milieu et ainsi expliquer<br />
la coexistence <strong>de</strong> certaines espèces.<br />
Il convient tout <strong>de</strong> même <strong>de</strong> rester pru<strong>de</strong>nt quand la présence <strong>de</strong> facilitation dans notre<br />
milieu d’étu<strong>de</strong>. Le choix <strong>de</strong>s méthodologies utilisées pour étudier la compétition est en effet<br />
susceptible <strong>de</strong> modifier les observations (Goldberg & Barton, 1992).<br />
230
5.4. Une étu<strong>de</strong> plus précise <strong>de</strong> l’effet <strong>de</strong>s patrons <strong>de</strong> pluviosité dans les milieux<br />
méditerranéens<br />
Les milieux méditerranéens étudiés sont caractérisés par une hétérogénéité et une<br />
faible disponibilité <strong>de</strong> la ressource en eau (variabilité <strong>de</strong>s patrons <strong>de</strong> pluviosité). Avec les<br />
relevés floristiques réalisés sur la pério<strong>de</strong> 2001-2008, il est difficile d’étudier précisément<br />
l’effet <strong>de</strong> la variabilité <strong>de</strong>s patrons <strong>de</strong> pluviosité sur la structure <strong>de</strong>s communautés végétales,<br />
car nous n’avons tout simplement pas <strong>de</strong> point <strong>de</strong> comparaison où la disponibilité <strong>de</strong> la<br />
ressource en eau serait plus gran<strong>de</strong>. L’expérimentation, qui est actuellement toujours en cours<br />
(cf. Partie Modèles Biologiques, section 2.2), nous permettra <strong>de</strong> connaître plus précisément<br />
l’effet <strong>de</strong> l’hétérogénéité <strong>de</strong> la ressource en eau sur la structure <strong>de</strong>s communautés végétales.<br />
Elle nous permettra également d’étudier le succès <strong>de</strong>s espèces annuelles et pérennes, et leurs<br />
interactions compétitives, face à l’augmentation <strong>de</strong> la disponibilité <strong>de</strong> l’eau. Grâce à cette<br />
expérimentation et aux méthodologies <strong>de</strong> mesure <strong>de</strong> la compétition in natura, il serait possible<br />
d’étudier les interactions compétitives entre pérennes et annuelles, et ainsi déterminer si la<br />
compétition a un rôle structurant <strong>de</strong>s communautés végétales et si ce rôle structurant varie<br />
entre années.<br />
231
232
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244
ANNEXE 1<br />
Liste <strong>de</strong>s abréviations <strong>de</strong>s espèces présentes dans les prairies humi<strong>de</strong>s du Marais Poitevin<br />
AGRSTO Agrostis stolonifera<br />
ALOBUB Alopecurus bulbosus<br />
ALOGEN Alopecuris geniculatus<br />
BALRAN Bal<strong>de</strong>llia ranunculoï<strong>de</strong>s<br />
BELPER Bellis perennes<br />
BROCOM Bromus commutatus<br />
CALLIT Callitriche<br />
CARDIV Carex divisa<br />
CARPRA Cardamine pratense<br />
CERGLO Cerastium glomeratum<br />
CIRARV Cirsium arvense<br />
CIRVUL Cirsium vulgare<br />
CYNCRI Cynosurus cristatus<br />
ELEPAL Eleocharis palustris<br />
ELYREP Elymus repens<br />
EPITET Eîlobium tetragonum<br />
FESARU Festuca arundinacea<br />
FESRUB Festuca rubra<br />
GALDEB Galium <strong>de</strong>bile<br />
GAUFRA Gaudinia fragilis<br />
GERDIS Geranium dissectum<br />
GLYFLU Glyceria fluitans<br />
HORMAR Hor<strong>de</strong>um marinum<br />
HORSEC Hor<strong>de</strong>um secalinum<br />
JUNART Juncus articulatus<br />
JUNGER Juncus gerardii<br />
LEOAUT Leontodon autumnalis<br />
LEOTAR Leontodon taraxacoï<strong>de</strong>s<br />
LOLPER Lolium perenne<br />
LOTTEN Lotus tenui<br />
MENPUL Mentha pulegium<br />
MOUSSE Mousse<br />
MYOLAX Myosotis laxa<br />
OENFIS Oenanthe fistulosa<br />
PARSTR Parapholis strigosa<br />
PLACOR Plantago coronopus<br />
PLAMAJ Plantage major<br />
POAANN Poa annua<br />
POATRI Poa trivialis<br />
POTANS Potentilla anserina<br />
PUCMAR Pucinnellia maritima<br />
RANAQU Ranunuculus aquaticus<br />
RANOPH Ranunculus ophioglossus<br />
RANREP Ranunculus repens<br />
RANSAR Ranunculus sardou<br />
RANSP Ranunculus sp.<br />
RANTRI Ranunculus<br />
RUMCON Rumex conglomerata<br />
SONASP Sonchus asper<br />
SPEMAR Spergularia maritima<br />
TAROFF Taraxacum officinale<br />
TRIFRA Trifolium fragiferum<br />
TRIMIC Trifolium michaelinum<br />
TRIORN Trifolium ornithopoï<strong>de</strong>s<br />
TRIRES Trifolium resupinatum<br />
TRISQU Trifolium squamosum<br />
VULBRO Vulpia bromoï<strong>de</strong>s<br />
245
ANNEXE 2<br />
Liste <strong>de</strong>s abréviations <strong>de</strong>s espèces présentes dans pelouses xéro-halophiles <strong>de</strong> Camargue<br />
Aetbul Aetheorhiza bulbosa<br />
Allvin Allium vineale<br />
Anaarv Anagallis arvensis<br />
Arelep Arenaria leptoclados<br />
Astste Asterolinum stellatum<br />
Avebar Avena barbata<br />
Belann Bellis annua<br />
Belper Bellis perennis<br />
Blasp. Blackstonia sp.<br />
Bradis Brachypodium distachyon<br />
Brapho Brachypodium phoenicoi<strong>de</strong>s<br />
Brohor Bromus hor<strong>de</strong>aceus<br />
Bromad Bromus madritensis<br />
Bupsem Bupleurum semicompositum<br />
Carsp Carduus sp.<br />
Carten Cardus tenuiflorus<br />
Catrig Catapodium rigidum<br />
Cenmel Centaurea melitensis<br />
Censp. Centaurium sp.<br />
Cerglo Cerastium glomeratum<br />
Cerpum Cerastium pumilum<br />
Cersemi Cerastium semi<strong>de</strong>candrum<br />
Cersic Cerastium siculum<br />
Cersp. Cerastium sp.<br />
Cirvul Cirsium vulgare<br />
Comsp. Composée sp.<br />
Crecap Crepis capillaris<br />
Cresan Crepis sancta<br />
Cresp. Crepis sp.<br />
Creves Crepis vesicaria<br />
Cyndac Cynodon dactylon<br />
Cynech Cynosurus echinatus<br />
Dachis Dactylis hispanica<br />
Elysp. Elytrigia sp.<br />
Erocic Erodium cicutarium<br />
Eupexi Euphorbia exigua<br />
Euppep Euhorbia peplus<br />
Eupsul Euphorbia sulcata<br />
Evapyg Evax pygmaea<br />
Filsp. Filago sp.<br />
Filvul Filago vulgaris<br />
Galmur Galium murale<br />
Galpar Galium parisiense<br />
Gerdis Geranium dissectum<br />
Germol Geranium molle<br />
Haicyl Hainardia cylindrica<br />
Halpor Halimione portulacoi<strong>de</strong>s<br />
Hedcre Hedypnois cretica<br />
Hipbif Hippocrepis biflora<br />
Hormar Hor<strong>de</strong>um marinum<br />
Hympro Hymenolobus procumbens<br />
Hypgla Hypochaeris glabra<br />
Hypsp. Hypochaeris sp.<br />
Leotub Leontodon tuberosus<br />
Limnar Limonium narbonense<br />
Limvir Limonium virgatum<br />
Linbie Linum bienne<br />
Linstr Linum strictum<br />
Lolper Lolium perenne<br />
Lolrig Lolium rigidum<br />
Lolsp. Lolium sp.<br />
Medlup Medicago lupulina<br />
Medmin Medicago minima<br />
Medpol Medicago polymorpha<br />
Medrig Medicago rigidula<br />
Medsp. Medicago sp.<br />
Medtru Medicago truncatula<br />
Minhyb Minuartia hybrida<br />
Minten Minuartia tenuifolia<br />
Myoarv Myosotis arvensis<br />
Myodis Myosotis discolor<br />
Myosp. Myosotis sp.<br />
Narmar Nardurus maritima<br />
Ophsph Ophrys sphego<strong>de</strong>s<br />
Parfil Parapholis filiformis<br />
Parinc Parapholis incurva<br />
Parlat Parentucellia latifolia<br />
Phiang Phillyrea angustifolia<br />
Placor Plantago coronopus<br />
Plalag Plantago lagopus<br />
246
Poaann Poa annua<br />
Poabul Poa bulbosa<br />
Polmar Polypogon maritimus<br />
Poltet Polycarpon tetraphyllum<br />
Psiari Psilurus aristatus<br />
Psiinc Psilurus aristatus<br />
Romram Romulea ramiflora<br />
Romsp. Romulea sp.<br />
Roscri Rostraria cristata<br />
Sagsp. Sagina sp.<br />
Salver Salvia verbenaca<br />
Scolac Scorzonera laciniata<br />
Scomur Scorpiurus muricatus<br />
Senvul Senecio vulgaris<br />
Shearv Sherardia arvensis<br />
Sidrom Si<strong>de</strong>ritis romana<br />
Sonasp Sonchus asper<br />
Sonole Sonchus oleraceus<br />
Sonsp. Sonchus sp.<br />
Tarery Taraxacum erythrospermum<br />
Tarobo Taraxacum obovatum<br />
Taroff Taraxacum officinale<br />
Tornod Torilis nodosa<br />
Triang Trifolium angustifolium<br />
Tricam Trifolium campestre<br />
Trilap Trifolium lappaceum<br />
Trimon Trigonella monspeliaca<br />
Trinig Trifolium nigrescens<br />
Trires Trifolium resupinatum<br />
Trisca Trifolium scabrum<br />
Trisp. Trifolium sp.<br />
Trisqu Trifolium squamosum<br />
Triste Trifolium stellatum<br />
Trisuf Trifolium suffocatum<br />
Tritom Trifolium tomentosum<br />
Valmic Valerianella microcarpa<br />
Valmur Valantia muralis<br />
Valmuri Valerianella muricata<br />
Verarv Veronica arvensis<br />
Vulsp. Vulpia sp.<br />
247
Protocole <strong>de</strong> l’étu<strong>de</strong> <strong>de</strong> la compétition en conditions contrôlées<br />
ANNEXE 3<br />
Une expérimentation en conditions contrôlées a été mise en place au printemps 2008<br />
pour étudier la variation <strong>de</strong> l’intensité <strong>de</strong> la compétition le long d’un gradient expérimental<br />
d’inondation variant en hauteur et en durée. Ci-<strong>de</strong>ssous la <strong>de</strong>scription <strong>de</strong> cette<br />
expérimentation, qui a duré <strong>de</strong>ux saisons <strong>de</strong> croissance.<br />
1) Les traitements expérimentaux :<br />
Les traitements expérimentaux consistaient en un couplage du facteur hauteur d’eau<br />
avec le facteur durée d’inondations, c’est-à-dire avec une durée d’inondation augmentant avec<br />
la hauteur, <strong>de</strong>ux variables corrélées (Casanova & Brock 2000). Ainsi différentes espèces ont<br />
été soumises à 4 traitements : 10 cm-courte durée ; 20cm-courte durée ; 20cm-longue durée et<br />
40cm-longue durée. La différence entre les traitements courts et les traitements longs étaient<br />
<strong>de</strong> un mois.<br />
Les espèces soumises à ces traitements étaient <strong>de</strong>s espèces représentatives <strong>de</strong>s<br />
différentes communautés végétales : Lolium perenne, Juncus gerardii, Leontodon autumnalis,<br />
Juncus articulatus et Mentha pulegium. Quatre transplants <strong>de</strong> chaque espèce ont été<br />
transplantés en monocultures et en mixture face à une espèce : Agrostis stolonifera, sous <strong>de</strong>ux<br />
<strong>de</strong>nsités <strong>de</strong> compétiteurs (1 faible, 1 forte).<br />
L’expérimentation a été conduite dans le jardin expérimental <strong>de</strong> l’université <strong>de</strong><br />
<strong>Rennes</strong> : les réplicas ont été assignés aléatoirement dans <strong>de</strong>s 4 bacs pour chaque traitement<br />
d’inondation.<br />
A la fin <strong>de</strong> l’expérimentation, les biomasses aériennes et racinaires ont été mesurées,<br />
après avoir été nettoyées et séchées à 70°C pendant 72h.<br />
2) La quantification <strong>de</strong> la contrainte :<br />
La fluorescence permet d’apprécier les capacités photosynthétiques <strong>de</strong>s plantes et<br />
représente un proxy <strong>de</strong> l’intensité <strong>de</strong> la contrainte subie par les espèces car l’appareil<br />
photosynthétique peut être affecté par le stress (Saltmarsh et al. 2006). Les mesures <strong>de</strong><br />
ren<strong>de</strong>ment photosynthétique ont été effectuées à l’ai<strong>de</strong> d’un diving-PAM (Pulse Amplitu<strong>de</strong><br />
Modulation). Ce fluorimètre permet <strong>de</strong> calculer la part du ren<strong>de</strong>ment associé aux processus<br />
photochimiques (e.g. photosynthèse principalement). Les plantes sont préalablement laissées<br />
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30 min à l’obscurité, <strong>de</strong> sorte à initier les mesures sur <strong>de</strong>s échantillons en « dormance<br />
photosynthétique » (pas <strong>de</strong> transport d’électron). Puis, trois mesures vont être effectuées par<br />
plante cible (sur 3 tiges ou feuilles différentes) ainsi que trois mesures par feuille.<br />
L’équipement photosynthétique <strong>de</strong> l’échantillon est alors excité par 2 types <strong>de</strong> sources<br />
lumineuses :<br />
- un faible éclairement : MB <strong>de</strong> 0,15 µmole.photons.m -² .s -1 : cette première source<br />
induit une émission <strong>de</strong> fluorescence sans mise en œuvre <strong>de</strong> la photosynthèse et permet <strong>de</strong><br />
déterminer le paramètre F0 correspondant à la fluorescence minimum <strong>de</strong> l’échantillon.<br />
- un flash <strong>de</strong> lumière saturante (rouge vif, 655 nm) court et intense : SatPULSE <strong>de</strong><br />
8000 à 10.000 µmole.photons.m-2.s-1, <strong>de</strong> 0,1 à 1s : ce flash aboutit à la fermeture <strong>de</strong>s centres<br />
<strong>de</strong> réactions du PSII ce qui provoque une augmentation substantielle <strong>de</strong> la fluorescence émise<br />
et <strong>de</strong>s pertes thermiques, ce qui permet <strong>de</strong> déterminer le paramètre Fm correspondant à la<br />
fluorescence maximum équivalente à l’activité photosynthétique.<br />
Le ren<strong>de</strong>ment maximal <strong>de</strong> fluorescence du PSII est défini par :<br />
Yield = Fv/Fm= (Fm-F0)/Fm<br />
Ce ren<strong>de</strong>ment mesure la fraction maximale <strong>de</strong> photons absorbés qui peut être utilisée pour la<br />
mise en place <strong>de</strong>s processus d’échanges chimiques et <strong>de</strong> transport d’électrons pour <strong>de</strong>s<br />
échantillons préalablement adaptés au noir.<br />
3) Mesure <strong>de</strong>s traits fonctionnels:<br />
Afin <strong>de</strong> connaître la réponse <strong>de</strong>s espèces à l’augmentation <strong>de</strong>s conditions d’inondation,<br />
<strong>de</strong>ux traits foliaires ont été mesurés en fin d’expérimentation : le Specific Leaf Area (SLA) et<br />
la longueur <strong>de</strong> la tige, suivant les protocoles standardisés (Weiher et al. 1999; Cornelissen et<br />
al. 2001).<br />
Afin <strong>de</strong> déterminer si l’allocation <strong>de</strong>s ressources peut varier suivant les conditions<br />
d’inondations, pouvant sur plus long terme d’affecter la survie et la capacité compétitive <strong>de</strong>s<br />
espèces, le ratio racine/tige a été mesuré. En effet, une altération du métabolisme racinaire<br />
peut être observée en raison <strong>de</strong> l’anoxie <strong>de</strong>s sols, provoquant une réallocation <strong>de</strong>s ressources<br />
<strong>de</strong>s organes souterrains vers les aériens.<br />
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ANNEXE 4<br />
Clonal growth strategies along flooding and grazing gradients in Atlantic coastal<br />
meadows.<br />
Marie-Lise Benot, Cendrine Mony, <strong>Amandine</strong> <strong>Merlin</strong>, Benoit Marion, Jan-Bernard Bouzillé<br />
& Anne Bonis<br />
Folia Geobotanica (2011) 46: 219-235<br />
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