1. National Agricultural Research Center for Hokkaido Region; Toyohira-ku Hitsujigaoka 1 Sapporo 062-8555 Japan
2. Institute of Biological, Environmental and Rural Sciences, Aberystwyth Univ.; Plas Gogerddan Aberystwyth Ceredigion SY23 3EB UK
3. Field Science Center for Northern Biosphere; Hokkaido Univ.; Kita-ku Kita 11 Nishi 10 Sapporo 060-0811 Japan