鳥海山麓の桑ノ木台湿原周辺における晩氷期以降の植生変遷と「鳥海ムラスギ」の消長

抄録

We conducted pollen and macroscopic charcoal analyses and accelerator mass spectrometry (AMS) 14C dating of sediment from a mire at the foot of Chokai volcano in northern Japan to reconstruct the vegetation history since the late-glacial period. Between 15 and 13 ka (15,000–13,000 cal yBP), the pollen assemblage indicated vegetation dominated by Betula with subarctic conifers. However, considering the richness of herb and shrub taxa, pseudo-alpine landscape likely occurred around the mire. Between 13 and 8.5 ka, Betuladecreased and Fagus increased, becoming comparable in their occurrences. However, herb taxa remained rich, and a dense forest had not yet been established. Such scattered forest vegetation was caused by the relatively high elevation (approximately 700 m) of the study site. After 8.5 ka, the meadow around the mire became smaller, and the Fagus-dominant forest became denser. Fagus remained dominant until recently. Cryptomeria pollen occurred sparsely but continuously beginning in the late-glacial period, so it is estimated that the natural sugi species currently growing around the mire were already distributed at that time.Although the occurrence of Cryptomeria was at a low level until the middle post-glacial, it began to increase in 3.3 ka, dominating around the mire between 3.0 and 1.0 ka. In 1.0 ka, macroscopic charcoals occurred abruptly, and Cryptomeria decreased. Subsequently, several herb taxa (e.g., Gramineae and Cyperaceae) increased, and Cryptomeria declined until ca. AD 1850. It is presumed that these vegetation changes were caused by human activities such as logging and burning. After ca. AD 1850, human impacts on the forest reduced and herb taxa decreased, while Cryptomeria increased because of tree planting.