1. Department of Ecology and Environmental Science, University of Kuopio, PO Box 1627, FIN–70211 Kuopio, Finland;
2. Department of Environmental Sciences, Bioteknia 2, University of Kuopio, PO Box 1627, FIN–70211 Kuopio, Finland;
3. Department of Biology, University of Joensuu, PO Box 111, FIN–80101 Joensuu, Finland;
4. Dating Laboratory, University of Helsinki, PO Box 64, FIN–00014 Helsinki, Finland