1. Department of Physical Sciences, University of Helsinki, P.O. Box 64, 00014, Finland
2. Department of Forest Ecology, University of Helsinki, P.O. Box 27, 00014, Finland
3. Finnish Forest Research Institute, Vantaa Research Unit, P.O. Box 18, 01301 Vantaa, Finland
4. Department of Applied Chemistry and Microbiology, University of Helsinki, P.O. Box 27, 00014, Finland