Vegetation changes after the late period of the Last Glacial Age based on pollen analysis of the northern area of Aso Caldera in central Kyushu, Southwest Japan

摘要

Within the Aso Caldera in central Kyushu, Japan, the Asodani valley is located in the northern part of the inner area, encircled by a wide caldera rim plain consisting largely of grassland. Pollen analyses and radiocarbon dating of samples from the Senchomuta core of the upper part of the basal sediments in the inner area revealed a change in vegetation after around 14,000 cal BP. According to the analytic results from the Hosenbashi core, the forest was mainly formed under cool-temperate to sub-arctic conditions at about 24,000-17,000 cal BP, followed by a change to temperate conditions with predominance of deciduous trees, and subsequently followed by warm-temperate conditions to the present consisting largely of evergreen trees.Herb pollen abundance indicated that the grassland on the broad caldera rim plain 300 to 500 m above Asodani developed at the Last Glacial Age. During the climatic warming of the Post Glacial Age, the grassland basically continued with trees until just after the K-Ah ash layer (7280 cal BP) was deposited. At about 6500 to 5000 cal BP, the herb pollen rate decreased, and the grassland changed to some scattered low-density forest vegetation composed of pine, deciduous and/or evergreen Quercus, and willow. After that period, it is suspected that the grassland was restored by human activity.