Between-tree variations in leaf delta C-13 of Quercus pubescens and Quercus ilex among Mediterranean habitats with different water availability

In this study, sun leaf carbon isotope composition (delta(13)C) of two co-occurring woody Mediterranean species (Quercus pubescens Willd., a deciduous oak, and Q. ilex L., an evergreen one) was investigated on four sites with different water availability. The total range of delta(13)C values was 4.4 and 3.1 parts per thousand for Q. pubescens and Q. ilex respectively. The intra-site variability was about 3 parts per thousand. Total mean per species was equal. There were significant differences among sites, but at each site means of delta(13)C were not significantly different between species. A simple physiological model predicts no difference in intrinsic water-use efficiency (WUEi) between evergreen and deciduous oaks. The relationship between site means of delta(13)C and water parameters suggests that there is a leaf functional adjustment with respect to available water resource. No correlation was found between delta(13)C and the contents of any mass-based biochemical constituent. Nevertheless there was a significant correlation between delta(13)C and leaf mass per area of Q. ilex. For both species, there is also a positive correlation between leaf delta(13)C and individual crown area, i.e. a structural characteristic at tree level. Causal relations between delta(13)C and plant-environment interactions are discussed.