The Yoshinogari Site is one of the nuclear sites in northern Kyushu during the Yayoi Period, and from the Yayoi Period to the Middle Ages. Pollen analyses and plant macrofossil analyses were carried out for sediments from the East and West Points of the surrounding lowland. Pollen assemblage zones and plant macrofossil assemblage zones were recognized for each site, and five stages, from A to E, were established based on them. At stage A before the Early Yayoi Period, laurel forests grew on the hills with Alnus marsh or a river in the lowland with no human activities. At stage Bat the Early to early Middle Yayoi Periods, human beings began to inhabit the hill, and Alnus trees were cut down, while Cyperaceae and Phragmites marsh developed in the lowland. The hills were deforested to some extent. At stage C before the later Middle Yayoi Period, the marsh was developed into paddy fields and continuous activities of human beings on the hills induced secondary forests of Celtis or Aphananthe. At stage D around the Ancient or Middle Ages, the paddy fields near the hill became a farm or an open field. Overuse of secondary forest trees on the hill decreased their number. At stage E around the Recent Age, paddy fields got the largest expansion in the lowland, and Pinus forests expanded with Cryptomeria, which might be an indication of tree planting. Boundaries of these stages reflect changes of human activities in relation to vegetation. The land development or employment of natural resources changed according to the cultural needs, from the Yayoi Period to Recent Ages.
To clarify sedimentary process of plant macrofossil assemblages, we observed cross sections in a meandering channel buried between latest Yayoi and earliest Kofun Periods, Late Holocene, at the Kouseki Site, Chiba Prefecture, central Japan. We compared species compositions and shape and size of fossils with sedimentary structures and grain size distribution for 12 plant macrofossil assemblages and their sediments. Plant macrofossil assemblages were included in several beds composed mainly of fine to very fine sands accreted laterally to the concave bank of the meandering channel. Each bed in the channel fill deposits had washed out the lower beds and composed an upward fining sequence. With the decrease in water current force from the lower to the middle part of each bed, the shape and size of deposited plants changed, as from larger fruits and seeds, and woods to leaves and smaller fruits and seeds, which resulted in the change in species composition of fossil assemblages. Mean and standard deviation of fruit and seed size correlated with mean and standard deviation of sediment particles respectively. Fruits and seeds seemed to have been transported and deposited by surface creeping and/or saltation together with medium sands of ca. 1.5-2.0ø in diameter. On the other hand, leaves were assumed to have been transported and deposited by suspension together with fine to medium sands of ca. 3.1ø in diameter. The species composition of leaves and that of reproductive organs in the whole fossil assemblages were quite different: leaves mainly consisted of evergreen broadleaved trees including Cyclobalanopsis and some conifers, but fruits and seeds included abundant deciduous broadleaved trees and shrubs. This bias may have resulted from the difference in the production of organs between evergreen broadleaved trees and deciduous broadleaved trees or shrubs.
We described and reconstructed paleovegetation from plant macrofossil assemblages in a latest Early Pleistocene cold stage from the upper member of the Shobudani Formation in Hashimoto and Gojyo, southern Kinki District, southwestern Japan. Trees distributed in the cool temperate and subarctic zones such as Pinus subgen. Haploxylon including P. koraiensis, Picea sect. Picea, Abies including A. veitchii, and Betula platyphylla var. japonica dominated in the fossil assemblages with wetland herbs. The occurrence of A. veitchii, B. platyphyl/a var. japonica, and Pterocarya rhoifolia represent the oldest records in the Quaternary in southwestern Japan. The results show that paleovegetations dominated by subarctic conifers similar to those in the early Middle Pleistocene cold stages were distributed not only in northeastern Japan, but also in southwestern Japan.