1. Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland
2. Department of Biological and Environmental Sciences and Konnevesi Research Station, PO Box 35, University of Jyväskylä, FIN-40351 Jyväskylä, Finland
3. Finnish Forest Research Institute, Rovaniemi Research Station, PO Box 16, FIN-96301 Rovaniemi, Finland