In cool temperate forest, it is usual that the smaller trees and shrubs flush earlier than taller canopy trees. In Japan’s snowy region, however, a canopy tree species Fagus crenata flushes much earlier than other trees. In this paper, we assumed: 1) The trunk receives sunlight and snow reflection, 2) The bark gets warmer, 3) The heat is conveyed to the sap in the trunk, 4) The warmed sap promotes flushing.

The study was conducted in Daigonji highland (720m asl) in Niigata prefecture from end of April to June. Leaf-opening phenology, patterns of heat absorption of bark, thermal changing of the sap and sap velocity were compared between beech and oak (Quercus crispula).

At the end of April (> 3m snow depth), the bark’s surface temperature of the two species showed higher value than air temperature, but no significant difference was found between the two species. On the other hand, beech’s sap temperature tended to be higher than for oak. This suggests that beech can more easily convey heat into the trunk because of its thinner bark compared to oak with thick cork layer. We found some amount of sap flowing in both species even before flushing started. This suggests the flow of warmed sap enables beech to start flushing, though oak cannot flush because the sap is not so warmed and probably regulated flushing in case it is damaged by frost.