1. Wood Anatomy and Quality Laboratory, Forestry and Forest Products Research Institute, Tsukuba Norin PO Box 16, Ibaraki 305‐8687, Japan;
2. Section of Earth System Science, Faculty of Environmental Earth Science, Hokkaido University, Sapporo 060‐0810, Japan;
3. Institute for Biological Problems of the Cryolithozone, Yakutsk, Russia