1. University of Tübingen, Faculty of Biology, Institute of Botany, Physiological Ecology of Plants, Auf der Morgenstelle 1, D‐72076 Tübingen, Germany;
2. Present address: UFZ, Department of Soil Ecology, Helmholtz‐Centre for Environmental Research, Theodor‐Lieser‐Str. 4, D‐06120 Halle, Germany