大雪山北部東斜面の森林限界高度における気温状況 Air Temperature Conditions at the Forest Line Level on the Eastern Side of the Northern Daisetsuzan Mountains, Central Hokkaido, Northern Japan
In Japanese mountains, the altitude of the forest line is often controlled by factors, such as thermal conditions, prevailing wind, and superficial deposits. The vegetation above the forest line of high Japa-nese mountains is characterized by the <i>Pinus pumila</i> community which essentially belongs to the upper part of the forest zone. The vegetation is different from that of high European mountains which are characterized by conifer krummholz. From this vegetational point of view, the existence of the alpine zone in vertical distribution such as in high European mountains is sometimes not the same as in Japan. The area above the forest line in high Japanese mountains, however, nearly corre-sponds to the periglacial area. Therefore, when natural phenomena in the upper part of the forest line of high Japanese mountains are studied from the viewpoint of climatic geomorphology, it is impor-tant to clarify the controlling factors and the current climatic conditions of the forest line.<br> In this study, air temperature observations were carried out at the forest line of 1, 700 m a.s.l. (Ginsen-dai observation site) on the eastern side of the northern Daisetsuzan Mountains, and the air tempera-ture at the wind-blown site of 1, 710m a.s.l. (Takanegahara observation site) on the main ridge run-ning north to south, about 7km SSW of the Ginsendai observation site, was also observed for the pur-pose of comparing the air temperature with that of the former site. The results of air temperature observations in 1993, 1994, and 1995 at the forest line, Ginsendai, are shown in Table 1. The annual mean air temperatures were -1.8°C in 1994 and -1.9°C in 1995. The mean air temperatures in the coldest month in 1993, 1994, and 1995 were -14.9°C, -17.9°C, and -16.0°C, respectively, and those in the warmest month were 12.5°C, 14.8°C, and 12.6°C, respec-tively. The monthly mean air temperatures from June to September in 1995, which were higher than the standard temperature (5°C) and were almost normal, gave a warmth index (WI) of 18.3°C •months at the forest line. This WI value exceeds 15°C •months of Kira's WI for the forest line in Japan.. The al-titude of WI 15°C •months around the observation site is estimated at 1, 823m a.s.l. This result sup-ports the proposal that the forest line of the Daisetsuzan Mountains is controlled not by thermal condi-tions but by other factors such as the prevailing westerly wind in winter or superficial deposits.<br> The results of air temperature observations from October 1992 to September 1993 at Ginsendai and Takanegahara are shown in Table 3. The mean annual air temperature at Takanegahara, a wind-blown site, was -3.0°C, lower than that at Ginsendai at the forest line by 1°C, which suggests that the thermal conditions of Takanegahara correspond to those of the discontinuous permafrost zone. The WI at Takanegahara is estimated 13.8°C •months, and the altitude of WI 15°C •months is 1, 658 m a.s.l. These thermal conditions indicate that Takanegahara clearly belongs to the alpine zone. The present forest line is situated at about 1, 500m a.s.l. around Takanegahara. Although both observa-tion sites are located at almost the same altitude, there is a conspicuous difference in thermal condi-tions between the two. This is considered to be caused by the difference in the controlling factors of tem-perature, such as snow cover and vegetation cover, on the ground surface which are influenced by the landform and prevailing wind conditions.<br> Around the main ridges such as Takanegahara strong prevailing winds in winter caused the alti-tude of the forest line to be lowered. Consequently climatic conditions equivalent to that of the alpine zone in terms of thermal condition are produced on the main ridge, where periglacial phenom-ena are distributed widely.
- 地理学評論. Ser. A
地理学評論. Ser. A 71(8), 588-599, 1998-08