Leaf longevity and drought: avoidance of the costs and risks of early leaf abscission as inferred from the leaf carbon isotopic composition

Plant species with longer leaf longevity tend to maintain lower photosynthetic rates. Among other factors, differences in stomatal limitation have been proposed to explain the negative effects of leaf longevity on photosynthesis, although it is not yet clear why stomatal limitations should be stronger in species with longer leaf longevity. We measured carbon isotopic composition (delta C-13) in the fresh leaf litter of several Mediterranean woody species to estimate the mean stomatal limitations during the photosynthetically active part of the leaf life. Interspecific differences in delta C-13 were best explained by a multiple regression including, as independent variables, the maximum leaf longevity and the annual water deficit. For a similar level of water availability, stomatal limitations were higher in species with longer leaf longevity. We hypothesise that stronger stomatal control of transpiration in longer-living leaves arose as a mechanism to reduce the risk of leaf desiccation and to avoid the high costs for the future C assimilation of anticipated leaf mortality in species with a long leaf life expectancy. This stronger sensitivity to drought should be added to the suite of traits accompanying long leaf longevity and contributes decisively to the overall limitations to C assimilation in long-lived leaves.