北海道中央高地, 大雪山における高地湿原の起源とその植生変遷 (I)
1985 年 24 巻 2 号 p. 99-109
詳細
1985 年 24 巻 2 号 p. 99-109
Two different types of upland bogs in the Daisetsu Mountains, central Hokkaido, were studied in order to examine their origin and vegetational succession.
The Numano-hara bog (1, 424m, a.s.l.) which is located on a lava plateau in the southeastern part of the Mountains, formed around 4, 000y.B.P. in a cool and moist climate. The result of pollen analysis suggests the following 3 pollen assemblage zones. During the period of Zone Nu-III (ca. 4, 000-2, 000y.B.P.), Betula ermanii was most dominant. Zone Nu-II (ca. 2, 000-400y.B.P.) is characterized by the decrease of Betula ermanii and the dominance of Picea glehnii as a rerult of the expansion of the bog in a cooler and moister climate. Also, a needle-leaved forest composed of Picea glehnii and/or Picea jezoensis and Abies sacharinensis spread to the lower areas. Finally, the bog decreased in size under a slightly drier condition, and Betula ermanii invaded again at the period of Zone Nu-I (ca. 400y.B.P.-present).
The bogs around the Midorino-numa and the Basho-numa (1, 365m, a.s.l.) located near the eastern margin of Takanega-hara in the central part of the Mountains were observed in some places around the depressions formed by landslide. According to tephrochronology of 5 thin volcanic ash layers, the beginning of the Midorino-numa bog is a little prior to 1, 300y.B.P., whereas the Basho-numa bog is considered to have originated between 1, 300 and 5, 000y.B.P. Although Betula ermanii invaded into the landslide area in an early stage, Picea glehnii increased after the bog formed (Zone Mi-II). At about 1, 300y.B.P. the forest around the bog reached the climax Picea glehnii forest just like the present one (Zone Mi-I).
Four other upland bogs developed in Hokkaido are classified into 2 types in the same way as the bogs in the Daisetsu Mountains. The Pankenai bog in the Teshio Mountains and the Ukishima bog in the Takinoue Plateau on flat plains like Numanohara formed in a cool and moist climate (Zone Nu-II). On the other hand, the bogs developed around the depression by landslide such as the Midorino-numa and the Basho-numa belong to different ages. The age is Orochiga-hara bog on Mt. Muine is estimated to be approximately 6, 000y.B.P. and the age of a small bog located on the southeastern slope of Mt. Onnebetsu-dake is approximately 300y.B.P.. Therefore, their difference in age probably depends on the formation of the depressions.
