1. Centre d’Ecologie Fonctionnelle et Evolutive, Equipe BIOFLUX, CNRS, 1919, route de Mende, F–34293 Montpellier Cedex 5, France
2. Present address: Forest Ecology, Institute of Terrestrial Ecosystems, Department of Environmental Sciences, Swiss Federal Institute of Technology ETH, Universitätstrasse 22, CH–8092 Zürich, Switzerland