CALCULATED THERMAL INDEX MAPS FOR THE RECONSTRUCTION OF PAST VEGETATION SINCE THE LAST GLACIAL AGE IN JAPAN

Abstract

Two kinds of indices, the warmth index (WI) and the coldness index (CI) were proposed by Lira (1945, 1948) to correlate the geographical distribution of vegetation zones with the thermal condition. This Lira's concept is considered to be useful for the reconstruction of past vegetation during the last 20, 000 years. The distribution maps of WI and CI are made on the basis of the mean monthly temperature for the period from 1921 A. D, to 1950 at 860 weather stations distributed over Japan (Figs. 1 and 2). In this paper, five distribution maps are presented as the representative temperature conditions during the past 20, 000 years. These maps show the thermal limitation of the vegetation zones for five periods when the mean annual tempetature are -7°C, -5°C, -3°C, -1°C and +2°C at different from the present temperature, respectively. The distribution map of WI and CI at the time when the mean annual temperature is 7°C lower than the present (Fig. 3) roughly represents the thermal limitation of the forest types at the maximum glacial period (about 20, 000_??_18, 000 years B. P.). Based on this map, Hokkaido should be covered with the tundra and subalpine conifer forests. The subalpine conifer forest also covers the Tohoku and Chubu districts. While, most of southwest Japan should be covered with the cool-temperate deciduous forest. Evergreen oak-laurel forests are only found in the lowland in southern Kyushu, Tanegashima and Yakushima islands. The second map for the case of 5°C lower than the present (Fig. 4) approximately shows the thermal limitation at the late glacial period (about 13, 000_??_10, 000 years B. P.). This map indicates that subalpine conifer forests spread in Hokkaido. While mountainous regions in the Tohoku and Chubu districts are still under the subalpine conifer forest, the forest in the lowlands should be changed into the cool-temperate deciduous one. Evergreen oak-laurel forests invade into the southern coastal area of the Kii Peninsula. It is notable that warm-temperate deciduous forests should appear in western Kyushu, Setouchi and Osaka Bay areas. The third map for the case of 3°C lower than the present (Fig. 5) is roughly correlated with the condition at the early Holocene period. The thermal limitation of the cool-temperate deciduous forest appears in the Ishikari Plain, Hokkaido. The the main part of Tohoku and Chubu districts are covered with cool-temperate deciduous forests. Warm-temperate deciduous forests extend a wide area in northern Kanto Plain, marginal part of Nobi Plain, and coastal lowlands along the western part of the Japan Sea. The northern limit of the evergreen oak-laurel forest should attain to the southern Kanto Plain in the Pacific Ocean side, while it is located at Matsue City in Shimane Prefecture in the Japan Sea side. The fourth map for the case of 2°C higher than the present (Fig. 6) approximately shows the thermal limitation at the Hypsithermal time (about 7, 000_??_6, 500 years B. P.). Forests in Hokkaido should be changed into the cool-temperate deciduous one. Inland basins of the Tohoku and Chubu district are covered by the warm-temperate deciduous forest. Evergreen oak-laurel forests spread over the central and southern part of Japan. The northern limit of this forest is located at Kuji City, Iwate Prefecture in the Pacific Ocean side and at the Lake of Jusan, Aomori Prefecture in the Japan Sea side. The fifth map for the case of 1°C lower than the present (Fig. 7) roughly represents the condition at the latest Jomon and early Yayoi periods (about 3, 000_??_2, 000 years B. P.) . This map is similar to the present potential natural vegetation maps which were made by Honda (1912), Horikawa (1968), Yoshioka (1973) and, Miyawaki et at. (1975), although there are some differences between two types of the maps.