北海道西南部高地の京極湿原における約13,000年間の植生変遷史

抄録

The Kyogoku Mire is located on a volcanic plateau in the mountains of southwestern Hokkaido. It lies in the subalpine zone, about 870m a.s.l., where Picea jezoensis-Betula ermanii mixed forests and Betula ermanii forests grow. It is the oldest highland mire in Hokkaido : the base of the peat layer was dated at 12,430 yr BP. Samples from two points in the mire and from a Picea glehnii forest floor were studied for pollen, sediment facies, and radiocarbon ages. During the last late-glacial, the vegetation changed repeatedly with short-term climatic changes. Grassland and open forest composed of Larix, Pinus, Picea, and Betula grew before 12,430 yr BP. The mire originated with climatic warming and heavy snowfall during the interstadial around 12,300 yr BP. After the establishment of the mire, Larix, Pinus, and Selaginella selaginoides shifted to higher elevations. They survived between 10,000 and 6,000 yr BP on the sediments around the mire that were derived from higher areas by mass movement. Although Larix disappeared by 8,000 yr BP in most of lowland Hokkaido, it survived at the mire 2,000 years longer. Two periods of Quercus increase were recognized. During the first (10,000-8,500 yr BP), Quercus invaded the piedmont. During the second (6,000-1,000 yr BP), Quercus forests shifted to the present subalpine zone under a warmer and moister climate than at present. Picea has been dominant in this area since the late-glacial. It became especially predominant between 8,500 and 7,500 yr BP. Picea forest spread to lower altitudes under cold climates. Picea abruptly decreased 7,500 yr BP and began to increase again 1,000 yr BP. The present Picea glehnii forests around mires were established 1,000 yr BP.