The Upper Cenozoic Yamato Group in Aizu Basin, Northeast Honshu, ranges from the Upper Miocene to the Middle Pleistocene and is divided into the Fujitoge, Izumi, Nanaorezaka, and Todera Formations, in ascending order. Each formation is subdivided into three, two, five, and six plant fossil assemblage zones, respectively. For the Upper Pleistocene, four plant fossil assemblage zones are recognized in terrace deposits. The Shirako and Sudani plant fossil assemblage zones in the Late Miocene are characterized by temperate elements accompanied by a few subtropical elements. After the Natsui zone, the Koyanaizu zone in the Early Pliocene is mainly characterized by temperate elements. Around the boundary of the Koyanaizu and the Fukurohara zones, close to the Gauss- Matsuyama boundary, many taxa disappeared and several taxa including exotic ones appeared. Since the end of the Olduvai subchron, Metasequoia became rare except for its pollen. The most significant vegetation change characterized by the extinction of Tertiary-type taxa and exotic taxa and the appearance of subarctic taxa occurred between the Jaramillo subchron and the Matsuyama-Brunhes boundary. After the Jaramillo subchron, periodical changes of vegetation were associated with more distinct climatic cycles of warm and cold periods. The temperate deciduous broad-leaved forest alternated with the boreal coniferous forest, and the vegetation gradually became similar to the modern one.

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We reconstructed the formation process and vegetation history of the Bibai Peatland in the middle of the Ishikari Peatland, central Hokkaido, during the past 8000 years, based on geologic and topographic studies, physical and chemical analyses of sediments, and paleobotanical studies of diatom, pollen, and fossil wood assemblages. The Bibai Peatland was created on a terrace surface between 8000 to 7500 yr B.P. with moistening of the surface caused by the Jomon transgression. From 7500 to 5000 yr B.P., peaty mud deposits with marsh forests of Alnus and Fraxinus first settled and were succeeded by low moor peat with high to transitional moor vegetation of Cyperaceae and Myrica. From 5000 to 3500 yr B.P., muddy deposits with small scale Alnus forests settled with high fluvial activities. From 3500 to 2200 yr B.P., peaty mud deposits with marsh forests of Alnus and Fraxinus again formed with stabilization of fluvial activity. In buried forests, Fraxinus grew close to the Bibai River and Alnus far from it. Since 2200 yr B.P., this area became a stable back marsh, and low moor, transitional, and high moor peat was deposited successively at the central part of the Bibai Peatland. The main vegetation of this stage consisted first of Alnus, Fraxinus, and Lysichiton, and of Cyperaceae and Sphagnum afterwards.

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