Composition and paleoenvironment of wetland forests dominated by Glyptostrobus and Metasequoia in the latest Pliocene (2.6 Ma) in central Japan

Abstract

A latest Pliocene (2.6Ma) wetland forest vegetation that was dominated by Glyptostrobus and Metasequoia was reconstructed from in situ woods, fruits and seeds, and pollen assemblages in deposits of the Kobiwako Group exposed in the Yasu River, Shiga Prefecture, central Japan. Based on species composition of stumps and autochthonous plant macrofossil assemblages in the fossil forest beds and their sedimentological assessments, habitat partitioning of Glyptostrobus and Metasequoia was clarified. The forest floor of Glyptostrobus dominant stands were covered by wetland plants, including Cyperaceae and submersed herbs. Size class distribution of stumps indicates that Glyptostrobus regenerated continuously and grew over a long period of time in a stable environment in a water-logged backmarsh. Metasequoia dominant stands were established in environments drier than those dominated by Glyptostrobus, and were accompanied by forest floor herbs and shrubs with mesic habit preferences. The upland forest vegetation reconstructed from allochthonous assemblages in sandy channel-fill deposits was mixed coniferous and deciduous broadleaved tree forest that was dominated by Picea, and included trees now extinct in Japan, such as Sequoia, Cunninghamia, Cyclocarya, and Fagus microcarpa. Sciadopitys and Cathaya, represented only by pollen, grew on mountain slopes and ridges away from the river. Their habitats did not extend to backmarshes in the lowlands during the Neogene and the Early Pleistocene in East Asia. Paleotemperature reconstructed from plant macrofossil assemblages indicates the temperature conditions typical of the lower boundary of the cool temperate deciduous broadleaved forest zone. The occurrence of Glyptostrobus and Metasequoia in another fossil forest in the Early Pleistocene (1.8–1.9Ma) in the same sedimentary basin demonstrates their tolerance to cool-temperate climates.