大雪山のハイマツ群落内における夏季の地温環境 Summer Ground Temperature Conditions in the Pinus pumila Community in the Alpine Zone of the Daisetsuzan Mountains
The alpine zone of Japanese mountains is characterized by the existence of the <i>Pinus pumila</i> community. Ground temperature observations in the alpine zone of the Daisetsuzan Mountains in the autumn of 1993 showed that the <i>Pinus pumila</i> community experienced the lowest value of ground temperature among all the plant communities that were covered by equal snow thickness in winter (Takahashi, 1995b). This indicates that the <i>Pinus pumila</i> community creates a peculiar environment for the ground thermal regime in summer.<br> To clarify the summer thermal conditions in the <i>Pinus pumila</i> community, observations of air and ground temperatures were carried out in the alpine zone of the Daisetsuzan Mountains from July to September 1994.<br> The monthly mean air temperatures above the canopy of the <i>Pinus pumila</i> community in July, August, and September were 14.9°C, 14.6°C, and 9.3°C, respectively. Since the shrub layer below the canopy of the <i>Pinus pumila</i> community has 100% coverage, the solar radiation reaching the ground surface is reduced to less than one-tenth of that above the canopy of the community. The monthly mean value of the daily maximum air temperature below the canopy of the community is about 1°C lower than that outside the community in summer.<br> Even in the community, the ground surface temperature exceeds the air temperature when the solar radiation passes through an opening in the canopy and reaches the ground surface. As a result, the diurnal range of the surface temperature widens. However, the thermal buffering effect of the 15cm thick litter layer significantly restricts the fluctuation of ground temperature beneath the layer. The mean diurnal range of ground temperature just beneath the litter layer was 1.3°C during the observation period. At a depth of 65 cm, the diurnal variation in ground temperature was insignificant.<br> The thermal buffering effect of the litter layer is intensified in dry conditions. On the other hand, the effect decreases with an increase in the water content in the litter layer, and the movement of soil water plays an important role in heat transfer in superficial layers. Therefore, infiltration of water causes a rapid rise in ground temperature when the surface temperature is much higher than the ground temperature.<br> Observations of summer ground temperature in 1994 suggested that frozen soil was maintained at a depth of approximately 65cm in the Pinus pumila community until early July. In an abnormally cool summer such as in 1993, frozen soil may remain at a depth of approximately 65 cm in the <i>Pinus pumila</i> community even in autumn.<br> Based on the above findings, the <i>Pinus pumila</i> community must be taken into consideration as a significant controlling factor of the ground thermal regime when the periglacial environment is studied in the alpine zone of Japan.
- 地理学評論. Ser. A
地理学評論. Ser. A 69(8), 693-705, 1996-08-01
The Association of Japanese Geographers