大雪山中央部高山帯における秋季の地温分布とその支配要因

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  • Distribution of the Autumn Ground Temperature and Its Controlling Factors in the Alpine Zone of the Central Daisetsuzan Mountains
  • ダイセツザン チュウオウブ コウザンタイ ニ オケル シュウキ ノ チオン ブ

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In addition to air temperature, snow cover, vegetation cover and superficial deposits are very impor-tant factors affecting the ground thermal regime in the alpine zone.<br> Ground temperatures to a depth of 100cm were measured in the alpine zone of the Daisetsuzan Moun-tains in the autumn of 1993. The alpine zone is a periglacial environment and is occupied by the Pinus pumila community, Sasa kurilensis community, alpine tall herbaceous community, wind-blown plant community and peat land. <br> Higher ground temperatures were recorded in the alpine tall herbaceous community and Sasa kuril-ensis community, where the ground surfaces are covered with more than 100 cm of snow in winter. The snow cover protects the ground from heat loss because of its very low thermal conductivity. In summer, vegetation cover does not play an equivalent role as an insulator. Consequently the ground temperatures in autumn are comparatively high.<br> The snow depth in the Pinus pumila community exceeds 100 cm as well. However, the autumn ground temperature in the community is the lowest except for that in the permafrost area. The low ground temperature is probably caused by the vegetational condition of the Pinus pumila communi-ty, which is distinctive in the summer alpine zone. The dense vegetation canopy of the Pinus pumila community considerably reduces the solar radiation reaching the ground surface. Additionally, the evap-oration from the leaves has a cooling effect in the Pinus pumila community. Since Pinus pumila communi-ty, characterizing the alpine zone of Japan, creates a peculiar environment for the ground thermal regime, the significant effect of the community has to be well taken into consideration when the periglacial environment is studied there.<br> The surface thermal regime in wind-blown bare ground is directly influenced by the fluctuation of air temperature and solar radiation. In the study area, the ground surface temperature falls below -20°C in winter and rises higher in summer than in the ground under other surface conditions. As a result, the temperature of the wind-blown bare ground in the Daisetsuzan Mountains is comparative-ly high in autumn. Permafrost, however, occurs in the bare ground in areas such as Hokkaidaira where the surface layer has a loose structure composed of uncompressed deposits of pumice and scoria. The ground thermal conditions in winter are not very different from that of other sandy and grav-elly bare ground. But in summer, the heat conductivity of the surface layer is lower, since the layer?under dry conditions contains a great deal of air.<br> Water content is a very significant controlling factor for the ground temperature in peat land. Higher ground temperature was recorded in autumn in the low peat land, where the peat layer was sat-urated with water and had high thermal conductivity. In contrast, in a palsa bog, whose surface is 80 cm above the ground water level, permafrost is protected by a dry peat layer more than 60cm thick with very low thermal conductivity.<br> Organic layers, such as humus layer and lichen mat, have relatively low thermal conductivity under dry conditions as well. In the wind-blown ground of the study area, when the organic layer is more than 30 cm thick and, moreover, is underlain by a silty layer with plant remains, the ground ther-mal condition is similar to that of palsa. Consequently, permafrost may occur there.

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