1. Vantaa Research Center; Finnish Forest Research Institute; P.O. Box 18 FIN-01301 Vantaa Finland
2. Rovaniemi Research Station; Finnish Forest Research Institute; P.O. Box 16 FIN-96301 Rovaniemi Finland
3. Dep. of Limnology and Environmental Protection, P.O. Box 62, FIN-00014; Univ. of Helsinki; Finland