1. Hokkaido Research Center, Forestry and Forest Products Research Institute, Hitsujigaoka 7, Sapporo 062‐8516, Japan; Fax +81 11 851 4167; E‐mail
2. Center for Ecological Research, Kyoto University, Shimosakamoto 4‐1‐23, Otsu 520‐0105, Japan
3. Forestry and Forest Products Research Institute, P.O. Box 16, Tsukuba Norin Kenkyu Danchi‐nai, Ibaraki 305‐8687, Japan