1. National Research Institute of Science and Technology for Environment and Agriculture (Irstea), Research Unit “Forest ecosystems”, Domaine des Barres, F-45290 Nogent-sur-Vernisson, France.
2. French Agency for Environment and Energy Management (ADEME), 2 square La Fayette, BP 90406, 49004 Angers CEDEX 01, France.
3. Department of Ecology, Swedish University of Agricultural Sciences, Box 7044, SE-750 07 Uppsala, Sweden.