K Steppe1, F Zeugin2, R Zweifel3. 1. Ghent University, Faculty of Bioscience Engineering, Laboratory of Plant Ecology, Coupure links 653, B-9000 Ghent, Belgium. 2. ETH Zurich, Institute of Plant Sciences, Universitätsstrasse 2, CH-8092 Zurich, Switzerland. 3. Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland.