1. Department of Ecology; Swedish University of Agricultural Sciences; Box 7044 Uppsala SE-750 07 Sweden
2. Department of Agricultural Sciences; University of Helsinki; PO Box 27, Latokartanonkaari 5 Helsinki FI-00014 Finland
3. Haapastensyrjä Unit; Natural Resources Institute Finland; Haapastensyrjäntie 34 Läyliäinen FI-12600 Finland
4. Department of Ecology, Environment and Plant Sciences; Stockholm University; Svante Arrhenius väg 20A Stockholm Sweden