1. Institute of Plant Biology; University of Zurich; Zürich CH-8008 Switzerland
2. Swiss Federal Research Station for Agronomy and Nature; Agroscope Reckenholz-Tänikon; Zürich CH-8046 Switzerland
3. Institute of Microbiology; Swiss Federal Institute of Technology; Zürich CH-8093 Switzerland
4. Snow and Landscape Research; Swiss Federal Institute for Forest; Birmensdorf CH-8903 Switzerland
5. Department of Ecology and Evolutionary Biology; University of Colorado; Boulder CO 80309-0216 USA