MEDITERRANEAN EVERGREEN SPECIES TOLERANCE THRESHOLD TO AIR TEMPERATURE INCREASING

Global change effects on Mediterranean region are likely to produce warmer and drier conditions, water deficit and more frequent and stronger drought periods. Increased drought stress may be a discriminant of species abundance and distribution, determining changes in vegetation in the long time. The high plant diversity of the Mediterranean vegetation reflects different adaptive strategies to cope with summer drought stress; when prolonged or more severe drought periods occur, the capacity of the different species to avoid damaging effects determines their survival. Search for those traits indicative of plant species adaptability to environmental constraints is crucial to the understanding of their functional ecology, also in consideration of global change. The main objectives of our researches were to define the adaptive strategies of the most important evergreen species co-occurring in the Mediterranean maquis, and to determine their tolerance threshold to increasing stress factors, in particular to air temperature increasing. The results on the whole underline that the combination of phenological, morphological, anatomical and physiological plant and leaf traits provides the placing of the considered species in different groups, which are characterized by different adaptive strategies. On an average, species characterized by leaves with a higher amount of biomass per unit of area (i.e. high LMA and LTD) might be at a competitive advantage relative to the other ones in regard to drought stress increasing. Moreover, high LMA and LTD values are associated to a longer leaf longevity, which seems to be a key traits discriminating among the typical sclerophillous species and the semi-deciduous ones. Nevertheless, a lower LTD allows a higher CO2 diffusion path from stomata to chloroplasts which determines a higher photosynthetic capacity. The results can be used to hypothesize trends in traits variation and environmental conditions, and to identify global change bioindicators.