COOCCURRENCE OF TREES WITH DIFFERENT LEAF HABIT - A FUNCTIONAL-APPROACH ON MEDITERRANEAN OAKS

Tree species can be split into two groups in terms of their leaf life- spans: evergreens and deciduous. Their distinct geographical distribut ion suggests that these two groups have functional characteristics ada pted to specific environments. However, deciduous and evergreen trees co-exist in some regions, such as those with a Mediterranean climate. They provide good models for comparing the properties of both trees an d obtaining an understanding of how diversity is maintained. This is t he case in southern France, where the evergreen helm oak (Quercus ilex ) and the deciduous downy oak (Quercus pubescens) co-exist. A research programme has been conducted which compares the functioning of these two species at various scales, with the aim of anticipating their dist ribution in the event of climatic change. The 'cost-benefit' model of Mooney and Dunn has been tested at leaf scale, Q. pubescens has a lowe r area-based construction cost than Q. ilex, but does not have a highe r photosynthetic capacity. Despite differences in biochemical composit ion, size and mass per unit area, the leaves of the two species respon d similarly to limited water conditions. Furthermore, the carbon isoto pe composition suggests that they have similar intrinsic water-use eff iciencies. AL the ecosystem scale, preliminary data are available on w ater, carbon and nitrogen use: i) measurements of leaf water potential s show that drought constraint starts at the same time and with the sa me rare and intensity in both species; ii) leaf area index was higher in Q. ilex woodlands; and iii) the release rate of nitrogen from the l itter was faster in Q. ilex ecosystems. Together, these results indica te that the key factors distinguishing functions of deciduous and ever green Quercus are more apparent at the ecosystem level than at the lea f level. (C) Elsevier, Paris.