Thermal Conditions of the Lower Limit of Pinus pumila Zone and of the Upper and Lower Limits of Betula ermanii Zone in Hokkaido, Japan

Abstract

To estimate palaeotemperature from the altitude of past vegetation zone compared with the present one requires the assumption that a vegetation zone is controlled by thermal condition. Since the lower limit of Pinus pumila zone occupying the highest zone of vegetation in the Japanese mountains is a marked boundary separating vegetation types and geomorphological processes, its physical conditions such as temperature, snow, wind and landforms have been examined. Recently, some researches have pointed that the altitude of the lower limit of Pinus pumila zone in Hokkaido, Japan, is dependent not on thermal condition, but on the factors reflecting the “top phenomenon” such as the winter season's north-westerly wind increasing in velocity with altitude and the surfacial condition of landform peculiar to the around summit of mountain. This suggests that the assumption for the estimation of palaeotemperature is not valid and the method using the lower limit of Pinus pumila zone as an indicator of temperature is not adequate.
The present authors re-calculate the distributions of the relative altitude from summit level of mountain, the altitude above sea level, the warmth index, the mean temperature, annual, winter (Jan. Feb.) and summer (Jul. Aug.), for the lower limit of Pinus pumila zone and the upper and lower limits of Betula ermanii zone occupying the altitudinal zone just below the Pinus pumila zone in Hokkaido, compare their degree of dispersion of distribution, and re-examine the thermal conditions of the vegetation zones.
Concerning the lower limit of Pinus pumila zone, the ratio of the range of the distribution of relative altitude from summit level to the range of the distribution of surface altitude of the whole area of Hokkaido is calculated to be 0.44 (1019m/2290m). In the same manner, for the range of the distribution of altitude above sea level, the ratio is 0.41 (950m/2290m). On the other hand, the ratio of range of the distribution of warmth index of the lower limit of Pinus pumila zone to the range of the distribution of warmth index for the whole area of Hokkaido shows 0.28 (16.8month·°C/59.2month·°C).In the same manner, for the range of the distribution of mean annual temperature, the ratio is 0.32 (4.7°C/14.7°C). Then, the degree of dispersion of the distribution of relative altitude from summit level is statistically evaluated to be larger than those of the distributions of thermal conditions. The same results are obtained for the upper and lower limits of Betula ermanii zone. This means that boundaries of vertical vegetation zones examined depend on thermal condition, showing the validity of the method estimating the palaeotemperature based on the changes in altitude of vegetation zone.
The thermal conditions of the boundaries of vertical vegetation zones in Hokkaido can be expressed by any indices of warmth index, mean annual temperature, mean winter temperature and mean summer temperature with almost the same errors. But the mean summer temperature is superior to the others, since its standard deviation of distribution is the smallest among them. The mean summer temperature shows 14.7°C for both the lower limit of Pinus pumila zone and the upper limit of Betula ermanii zone, and 16.3°C for the lower limit of Betula ermanii zone. Their standard deviations are 1.1-1.2°C, showing that the boundaries of vertical vgetation zones examined are restricted to narrow thermal zone.