1. Department of Biological and Environmental Science; University of Jyväskylä; P.O. Box 35 Jyväskylä 40014 Finland
2. Joensuu Unit; Finnish Forest Research Institute; P.O. Box 68 Joensuu FI-80101 Finland
3. School of Forest Sciences; University of Eastern Finland; P.O. Box 111 Joensuu FI-80101 Finland