Ecologia Mediterranea

à grande échelle au sein du projet de réhabilitation de Cossure
: le semis d’espèces nurses, l’étrépage de sol, le transfert
de foin et le transfert de sol. Afin d’évaluer l’efficacité
des techniques de restauration, nous avons développé des
indices pour mesurer « l’intégrité » de la structure de la
communauté permettant de distinguer les abondances inférieures
des abondances supérieures par rapport à la communauté
de référence. Les meilleurs résultats ont été obtenus
avec le transfert du sol, suivi par l’étrépage de sol, puis
le semis d’espèces nurses et enfin le transfert de foin. Ces
résultats ont toutefois confirmé la difficulté de restaurer
totalement la communauté végétale steppique. Les
recherches menées au sein de cette thèse montrent que les
connaissances actuelles en matière de restauration écologique
permettent de restaurer au moins partiellement certaines
composantes de cet écosystème, mais suggèrent de
mettre un maximum de moyens pour la conservation in situ
des habitats naturels plutôt que de devoir les restaurer après
qu’ils aient été détruits.
Dynamics and restoration
of a Mediterranea steppe
after changes in land-use
(La Crau, Southern-France)
Keywords: biodiversity, disturbance, ecological engineering,
ecological indicator, former arable field, hay transfer,
Mediterranean rangelands, nurse species seeding, orchard,
Plant community, Plant succession, Rehabilitation, Resilience,
Restoration ecology, Soil transfer, Species diversity, Speciesrichness,
Topsoil removal.
Restoration ecology is a science at the interface between
theoretical community ecology and ecological restoration.
Community ecology aims at understanding, within an
ecosystem, the interactions between coexisting species and
with their environment while restoration ecology focuses
on ways to assist the recovery of an ecosystem that has
been degraded, damaged or destroyed. This thesis deals
with restoration ecology in a Mediterranean steppe, from
studying spontaneous succession after a large exogenous
disturbance (formerly cultivated fields) to experimenting
large-scale restoration of its plant community after an
intensive cultivation episode (peach orchard). The study
site is the La Crau area in Southeastern France, which has
been shaped by millennia of interactions between the
Mediterranean climate, a characteristic soil disconnected
from the water table, and extensive sheep grazing for more
than 6,000 years. This has led to a species-rich and unique
plant community that lost more than 80% of its area due
to anthropogenic disturbances, mainly cultivations (hay
meadows, arable fields, orchards). Some of these cultivations
are now abandoned and provide an appropriate framework
to determine what are the drivers of succession, and
if the plant community spontaneously recovers. These
points are tackled in the first two chapters of the thesis. The
subsequent chapters focus on restoration of one of the
abandoned cultivations. This 357 ha restoration occurs
within the context of the creation of the first French miti-
ecologia mediterranea – Vol. 38 (2) – 2012
Résumés de thèses
gation bank, which raises at least two questions tackled in
the two subsequent chapters: How to restore such an
ecosystem? How to assess this restoration?
Land-use changes and especially intensive cultivation abandonment
can be used to study vegetation recovery and
community assembly. Theoretical models of plant community
establishment usually describe a regional species
pool that is constrained by three filters: dispersion, abiotic
and biotic. The aim of chapter one was to measure in secondary
succession plant communities the part of variability
attributable to each filter. This study examined plant
communities after the abandonment of cultivation in a xeric
steppe located in the La Crau area. Forty former arable
fields were selected and characterized by their location on
geological and climatic gradients, and by taking into
account land-uses in their surroundings across time. We
recorded plant species richness and composition, and carried
out soil analyses. Former arable fields were compared
with each other and with areas where no abrupt anthropogenic
exogenous disturbance was applied. Data were
analyzed with variation partitioning based on redundancy
analyses to assess the relative effect of the filters on all
species, species more common in undisturbed reference
steppe or in former arable fields. The former arable field
vegetation is still different, even 30 years after cultivation
abandonment. The three filters are important in determining
their plant community composition. Nevertheless the
abiotic filter seems to exert the greatest effect, followed by
the dispersion filter and then the biotic filter.
The remaining difference, even after 30 years of abandonment,
allows us to question whether the ecosystem is
resilient to exogenous disturbances and if all of its components
have the same resilience. A growing number of studies
show the advantage of taking into account the interactions
between vegetation, soil and mycorrhizae to
understand the organization and dynamics of plant communities.
Indeed, these three ecosystem components interact
continuously, either positively or negatively, but little
research has focused on the resilience of these interactions.
The objective of the second chapter was therefore to measure
the resilience of these three components after a cultivation
episode in the La Crau area. We selected a gradient
of crop abandonment: 2 years – 30-40 years – 150 years
and the reference steppe. We surveyed plant communities
characteristics and soil chemical properties and we measured
the mycorrhizal infestation of four species (Brachypodium
distachyon, Bromus madritensis, Carduus pycnocephalus
and Carthamus lanatus): two Poaceae, and two
Asteraceae, and in each family, one that shows greater
abundances in undisturbed areas, and one that shows greater
abundances in recently disturbed areas. Our results
show that vegetation and soil properties are permanently
affected by the impact of the cultivation episode. Mycorrhizal
infestations are also lower after a disturbance regardless
of the family studied or of the species preference for
areas recently disturbed or not.
When spontaneous succession does not lead to the reference
community trajectories, active ecological restoration
has to be implemented. The Strategic Plan for Biodiversity
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