The Fagus forest zone dominated by Fagus crenata streches over the Japanese Archipelago from the southwestern part of Hokkaido to Kyushu. The thermal conditions of its upper and lower limits of distribution are examined by a new method inferred from multiple regression analysis of mountainous meteorological data. The vertical distribution of the Fagus forest zone is strictly affected by the temperature during the summer, rather than by the warmth index and annual mean temperature. The summer temperature, which is represented by the mean of monthly mean temperatures for July and August, shows 16.8°C for the upper limit and 21.0°C for the lower limit of the Fagus forest zone. Using both the thermal conditions of the Fagus forest zone described above and the history of migration of the Fagus forest zone indicated by pollen stratigraphies of many sites in Japan, the changes in summer temperature from the latest Pleistocene to the middle Holocene (from 12, 000y.B.P. to 3, 000y.B.P.) are discussed, as follows (Fig. 7); The summer temperature during the latest Pleistocene from 12, 000y.B.P. to 10, 000y.B.P. was estimated to be between 3°C and 7°C lower than at present. Through the period from 9, 500y.B.P. to 6, 000y.B.P., the summer temperature at each site was higher than at present, showing the early phase of the Hypsithermal. Every site was warmer than at present during the period from 6, 000y.B.P. to 4, 500y.B.P.. Although the exact age of the warmest phase of each site could not be specified, the summer temperature during the climax phase of the Hypsithermal rose to 1° and/or 2.5°C higher than at present. The period between 4, 500y.B.P. and 3, 000y.B.P. was the end of the Hypsithermal, and a decrease in summer temperature was shown at every site, though some sites were still warmer than at present.
Alluvial fans and alluvial terraces are well developed along the rivers around the Matsumoto Basin, Central Japan. Stratigraphy and morphological features of fluvial surfaces and mountain slopes were examined to clarify the landform development of the river basins through the Last Glacial Age and the Post-Glacial Age. The results are as follows; 1) Fluvial surfaces formed during the Last Glacial Age and the Post-Glacial Age are divided into four: the Hata surface (formed in the earlier half of the Last Glacial Age), the Moriguchi surface group (in the latter half of the Last Glacial Age), the Kamikaido surface group (in the Late Glacial and Post-Glacial Age) and the Oshide surface (at present). The Hata surface is a depositional one, while the Moriguchi surface is an erosional one. The Kamikaido and Oshide surfaces are generally depositional in the alluvial fan but erosional in the mountainous area. 2) Surveys on the deposits on mountain slopes indicate that periglacial slow mass movement on the slopes had been active in the latter half of the Last Glacial Age, yet it diminished in the Late Glacial and the Post-Glacial Age. Active transportation of materials by running water occurred along the tributaries and hollows on mountain slopes in the Late Glacial and the Post-Glacial Age, resulting in the dissection of mountain slopes. The changes in the processes on mountain slopes can be attributed to the climatic change from the cold climate in the Last Glacial Age to the wet and warm one in the Post-Glacial Age. 3) The increase in the effects of running water on mountain slopes in the Late Glacial and Post-Glacial Age suggests that river discharge increased in these ages. On the other hand, the changes in the activity of material movement on mountain slopes indicates that the debris supply from the slopes increased or was unchanged in these ages. Thus, the degradation of rivers in the mountainous areas during these ages is ascribable to the increase in river discharge. It is considered that the active transportation of gravels to the lower reach occurred during the Late Glacial and Post-Glacial Age due to the discharge large enough to remove the debris supplied from mountain slopes, resulting in aggradation in the alluvial fan areas. 4) The characteristics of terrace deposits and the previous studies of paleoclimate in Central Japan suggest that river discharge in the latter half of the Last Glacial Age was not larger than that in the earlier half. Thus, the degradation during the latter half of the Last Glacial Age is ascribable to the decrease in debris supply. The decrease is probably due to the diminution of material movement along tributaries and hollows on mountain slopes which was induced by the decrease in precipitation in the latter half of the Last Glacial Age.