1. Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan
2. Forest Research Department, Hokkaido Research Organization, Higashiyama, Koshunai, Bibai 079-0166, Japan
3. Northern Station of the Experimental Forest, Field Science Center for Northern Biosphere, Hokkaido University, Tokuda, Nayoro 096-0071, Japan