Quercus humilis increase of productivity in the Mediterranean area

Several recent studies have shown an increasing long-term growth trend for various forest tree species in western Europe. Nevertheless such studies ha ve not yet, been performed in Mediterranean Europe. The aim of this work is to analyse changes in productivity of some Mediterranean forest ecosystems compared with other medioeuropean forest ecosystems. Sixteen Quercus humil is (Miller) populations were sampled in south-east France. Tree ring widths were measured for each tree according to three radius, and annual basal area increments were calculated. Two growth indexes (IP and IC) were calculated with two different standardi zation techniques, in order to remove age and interstation productivity eff ects. From the IP and IC indexes we can see that there was a productivity i ncrease during the last century, this increase being evaluated at 100 % (IC index). These results indicate that the Mediterranean forest ecosystems have shown a high productivity increase over the last century, as have the medioeurope an forest ecosystems. The best hypothesis to explain this increasing long-term growth trend is a direct CO2 fertilization along with N deposition fertilization. ((C) Inra/E lsevier, Paris.).