A decrease in temperature during the late Middle Pleistocene interglacial stage (MIS 7.3) altered montane zone floral diversity: Plant macrofossil evidence from central Japan

Abstract

Plant fossils from the Early and Middle Pleistocene have mainly been obtained from lowland sediments, and consequently, there is currently limited information on changes in montane zone vegetation prior to the last glacial stage. However, fossil plant assemblages occurring at high altitudes can be used to assess the influence of climate change on the vertical distribution of plants and vegetation. In this study, we analysed plant macrofossil assemblages from fluvial deposits in the upper Middle Pleistocene strata outcropping in a riverbed at 1160–1180 m a.s.l. in the southeastern foothills of Mt. Yatsugatake, central Japan. Ages of the assemblages were constrained by two widespread tephras that were detected above the interglacial peak of marine isotope stage (MIS) 7.3 (~212 ka). The annual mean temperature in the lower and middle sections of the profile was estimated to be warmer than that at present, based on the coldest temperatures limiting the distribution of Phytolacca japonica and Selaginella remotifolia, and the temperature decreased in the upper section. Since MIS 7.4, the fluctuation in plant altitudinal distribution has been exemplified by changes in the limits of S. remotifolia and Larix kaempferi distribution, and their coexistence at the same altitude varied depending on climate settings. Species diversity, estimated by rarefaction analysis of plant macrofossil assemblage composition, declined as temperatures decreased after the peak of MIS 7.3. Decreasing floral diversity during the subsequent cooling phase possibly coincided with a downward migration of the vegetation zone and extirpation of plants from the inland basin in central Japan.