Abstract
Betula ovalifolia is a shrubby birch counted as one of the relicts of the Last Glacial Age in Japan. To elucidate the history of this species in the Nishibetsu Mire, Betsukai in eastern Hokkaido, we studied the pollen morphology of !ve Betula species native to Hokkaido and the temporal distribution of B. ovalifolia type pollen in the mire sediments. Measurement of the equatorial length (E), ectopore length (EP), and pore depth (PD) of modern pollen grains revealed that pollen grains of B. ovalifolia occasionally have clearly larger E/EP or E/PD than those of the other Betula species. In the sediments collected by hand boring in the Nishibetsu Mire that dated back to ca. 6500 cal BP according to radiocarbon dating and correlation of tephra layers, Betula pollen accounted for 4–34% of the total arboreal pollen with 7–50% of them having larger E/EP that characterizes B. ovalifolia pollen. This result indicated that B. ovalifolia has probably been dominant in the mire vegetation since ca. 6500 cal BP.
