1. Swiss Federal Institute of Forest, Snow, and Landscape Research, Zürcherstr. 111, 8903 Birmensdorf, Switzerland, Institute of Plant Sciences, Eschikon Experimental Station, P.O. Box 185, 8315 Lindau, Switzerland, Institut de Radiophysique appliquée, Centre universitaire, Grand Pré 1, 1007 Lausanne, Switzerland, and Institute of Terrestrial Ecology, Grabenstr. 11a, 8952 Schlieren, Switzerland