1. Max‐Planck‐Institute for Biogeochemistry, Jena, Germany; Present address: Complex Systems Research Center, Morse Hall, University of New Hampshire, Durham, New Hampshire 03824‐3525, USA;
2. Present address: Institut für Pflanzenwissenschaften ETH Zentrum, LFW C48.1, Universitätsstrasse Z, 8092 Zürich, Switzerland