Seasonal and annual changes in leaf delta C-13 in two co-occurring Mediterranean oaks: relations to leaf growth and drought progression

1. Changes of delta(13)C and its relation to leaf development, biochemical content and water stress were monitored over a 2 year period in two co-occu rring Mediterranean oak species: the deciduous Quercus pubescens and the ev ergreen Quercus ilex. 2. The time course of leaf delta(13)C showed different patterns in the two species. Young Q. pubescens leaves had a high delta(13)C and a marked decre ase occurred during leaf growth. In contrast, leaves at budburst and maturi ty did not differ significantly in the case of Q. ilex. We suggest that the difference between delta(13)C of young leaves was linked to differential u se of reserves of carbon compounds in the two species. 3. delta(13)C values of mature leaves were negatively correlated with minim um seasonal values of predawn water potential, suggesting that a functional adjustment to water resources occurred. 4. There was a significant correlation between individual delta(13)C values for two successive years. This interannual dependence showed that delta(13 )C rankings between trees were constant through time.