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.
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