1. Department of Biology/Botany, University of Oulu, P.O. Box 333, FIN‐90571, Oulu, Finland
2. To whom correspondence should be addressed. E‐mail:
3. Department of Ecology and Systematics, University of Helsinki, P.O. Box 7, FIN‐00014, Helsinki, Finland
4. Sheffield Centre for Arctic Ecology, 26 Taptonville Road, Sheffield S10 5BR, UK
5. Abisko Scientific Research Station, Abisko, S‐98107, Sweden
6. Department of Plant Physiology, University of Lund, P.O. Box 117, S‐22100, Lund, Sweden
7. Department of Plant Ecology, Ecology Building, University of Lund, S‐22362, Lund, Sweden