1 - Ecologia Mediterranea

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ecologia mediterranea 23 (314),11-19 (1997) Phenology and leaf adaptive strategies of Mediterranean maquis plants Loretta GRATANI & Maria Fiore CRESCENTE Department ofPlant Biology, University of Rome « La Sapienza », P.le A. Moro 5, 00185 Rome, Italy. ABSTRACT The dynamics of vegetative and reproductive activities were compared among various Mediterranean maquis plants, and their adaptive strategies were studied. Evergreen sclerophyllous species and semi-deciduous species exhibited differences in their phenological trend and in seasonal shoot and leaf growth patterns. The evergreen sclerophyllous species showed a high relative growth rate (RGR) for short vegetative activity period during the year. Leaf initiation occurred before maximum leaf fall and the maximum RGR at the tirst stage of vegetative activity caused the leaf to reach 80-95% of its definitive size at the end of July. Average leaf life-span was 2-3 years. Mature leaves were characterized by a high leaf consistency. The seasonal rhythm of shoot elongation seemed to be closely dependent on air temperature, while good rainfall distribution seemed to increase shoot length. Cistus incanus, which dropped a substantial percentage of ils canopy before drought stress, exhibited a lower RGR for longer vegetative activity periods during the year. Mature leaves were characterized by a lower leaf consistency; leaf drop occurred before leaf initiation. Arbutus unedo exhibited an intermediate adaptative strategy: leaf RGR was similar to that of the typical evergreen sclerophyllous species, but its leaves had a reduced consistency and a longevity of less than one year. Key-words : maquis species, phenology, shoot growth, leaf development, RGR, leaf fall RESUME La dynamique des activités végétatives et reproductives a été comparée entre différentes espèces du maquis méditerranéen et leurs stratégies adaptatives ont été étudiées. Les espèces sclérophylles sempervirentes et serni-caduques montrent des différences dans le comportement phénologique et dans le développement des bourgeons et des feuilles. Les espèces sclérophylles possèdent un taux de croissance relatif (RGR) élevé pour une période végétative courte dans l'année. Les nouvelles feuilles apparaissent avant même la tombée des anciennes. La croissance rapide (max. RGR) en début d'activité végétative permet à la feuille d'atteindre 80-95% de sa taille en juillet. La vie moyenne des feuilles est de 2-3 ans. Les feuilles matures se caractérisent par une forte consistance foliaire. Le rythme saisonnier d'élongation des bourgeons semble être dépendant de la température de l'air, tandis que la bonne distribution des pluies semble influencer la longueur des bourgeons. Cistus incanus, qui perd une partie majeure de sa canopée avant l'arrivée du stress hydrique, présente un faible RGR, pour une période végétative plus longue dans l'année. Les feuilles matures sont caractérisées par une faible consistance foliaire et la chute des feuilles se produit avant l'apparition des nouvelles. Arbutus unedo présente une stratégie d'adaptation intermédiaire: le RGR des feuilles est similaire à celui des espèces sclérophylles, mais ses feuilles ont une consistance réduite et une longévité inférieure à l'année. Mots-clés : espèces du maquis, phénologie, croissance des bourgeons, développement foliaire, RGR des feuilles, chute des feuilles INTRODUCTION The relationship between c1imate and vegetation forms the very foundation of ecological plant geography (Mooney & Dunn, 1970). There has been a long history of attempts to utilize various non-floristic physiognomic characterizations of vegetation and to relate these to regional, local c1imate and microclimate (Du Rietz, 1931 ; Cooper, 1961; Knight & Loucks, 1969; Mooney & Dunn, 1970). Plant species have adapted to their local c1imate through the evo1ution of certain phenological trends (Chabot & Hicks, 1982; Kikuzawa, 1989), which allow them to control shoot elongation, leaf fall. tlowering and fruiting. ec%gia mediterranea 23 (3/4) - 1997 Phenological events retlect in fact the complex interrelationships between plants and their environment and highlight the adaptiveness of plant species. It is wide1y accepted that the timing of phenological events is determined by long-term characteristics of the environment (Ratheke & Lacey, 1985), and the main Iimiting factors in a given environment are considered to be the principal selective forces operating on organism living there (Aronson et al., 1992). The presence of phenological adaptations or their absence aides in explaining the ecological and geographical distribution of plant species. Loveless (1962) emphasized the relevance of phenological events in characterizing vegetation. Indeed, the different 11
Gratani & Crescente Phenology and leafadaptative strategies ofMediterranean maquis plant reactions of species to environmental stimuli create the broad mosaic of resource utilization patterns, which characterizes a multispecies plant community (Kummerow et al., 1981). Phenology which describes the functional rhythm of a plant community is not simply the sum of the rhythms of individual species, but is the result of inter-relationships among ail the individuals of the community. It represents the lifehistory strategies of a plant community and its resource use. At the end of the nineteenth century and during the early part of the twentieth century, there was great interest in determining the role of morphology in the phenological adaptation of plants to specific environments (Grime & Hunt, 1975; Carpenter & Smith, 1981; Kikuzawa, 1989). These studies did not have an experimental basis and their potential functional significance has been investigated more thoroughly only in recent times (Ehleringer & Werk, 1990). The present paper deals with sorne phenological pattern exhibited by Mediterranean maquis species and their shoot and leaf growth dynarnics. The results have been interpreted in terrns of adaptive strategies of resouree use. MATERIALS AND METHODS The sample area, 1200 m 2 , is located within the Presidential Estate of Castelporziano (S-SW of Rome, Latium, ltaly). The experimental plot consists of Mediterranean low maquis dominated by Quercus ilex L., Pistacia lentiscus L., Phillyrea latifolia L., Smilax aspera L., Arbutus unedo L., Cistus incanus L., Erica arborea L., Erica multiflora L., Daphne gnidium L., Lonicera implexa Ait. and Rosmarinus officinalis L. (Gratani & Marinucci, 1985; Gratani & Amadori, 1991). It is characterized by shrubs 0.5 - 1.5 m tall and by 2.5 leaf area index (LAI) (Gratani, 1995). Individuals were chosen randomly, and detailed phenological observations were carried out at weekly intervals from January 1992 to Deeember 1993. For each species, 150 branches formed from the previous year's growth were marked and labelled with nylontape at about 1 mm from the end of the growing point. The following phenological events were observed: (l) presence of buds (when the length was less than 2mm); (2) vegetative activity (period of shoot elongation, and unfolding of leaves until their final size corresponding to mature leaves); (3) flowering 12 period (from bud flower opening until the end of anthesis); (4) fruiting period (presence of fruits until the seeds begin to fall); (5) leaf fall (six litter traps consisting of 1 m 2 boxes with nylon mesh, were randomly located in the study area, under shrubs). Climatic data were provided by the Meteorological Station located within the Estate. During the period January 1992 - December 1993, soi! moisture was monitored monthly by three soi! samples taken at 30 cm depth. Soi! dry weight was recorded after drying at 90° C until constant weight. The seasonal course of shoot elongation and leaf area development was monitored: each week fifty leaves were collected for each species along the full length of new shoots in order to coyer the whole range of leaf size. Leaf surface area was measured by the Image Analysis System (Delta-T Deviees, LID, England). Leaf dry weight (drying at 90°C to a constant weight) was determined. Relative growth rate (RGR) was defined as the increase at any instant in time of leaf dry weight and leaf surfaee area (BIackman & Wilson 1951; Beadle et al., 1985). Specifie leaf weight (SLW) was calculated as leaf dry weight per unit of leaf surface area (Cappelletti, 1954). The cycle of phenological phenomena, such as bud break, flowering, fruiting and leaf fall, is traditionally linked with climate (Wells, 1971; Larcher, 1995). It is possible to monitor seasonal growth pattern and to assess the contribution of a species to the community (Rorison, 1981). RESULTS The clirnate of the area was of the Mediterranean type. The average total annual rainfall was 765 mm, the mean minimum air temperature of the coldest months (January and February) was 3.6 oC and the mean maximum air temperature of the hottest month (August) was 30 oC (data from 1981-1993) (Table 1). Most of the total annual rainfall occured in autumn and winter; during the summer drought soil moisture was characteristically low (Figure 1). Phenologieal periodicity of the Mediterranean maquis species Figure 2 summarizes the phenological periodicity of the studied Mediterranean maquis species (mean of the years 1992-1993). ecologia mediterranea 23 (3/4) - 1997
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