Distribution of Evergreen and Summer-green Broad-leaved Secondary Forests and Causal Factors in the Southern Part of the Boso Peninsula, Japan

Abstract

In this paper, the author attempts to clarify the regional difference in the distribution of evergreen and summer-green broad-leaved secondary forests and the causal factors involved, from the viewpoint of “mesoscale” which corresponds to the same scale as mesoclimate, in the southern part of the Boso Peninsula in eastern Japan. The study area is located at the northern limit of the dominant area of evergreen broad-leaved secondary forests (Castanopsis, Quercus coppice forests) along the coast of the Pacific Ocean (Fig. 1). Summer-green broad-leaved secondary forests (Quercus coppice forests) are also dominant in the area.
A distinct areal difference was found in the distribution of evergreen and summer-green broad-leaved secondary forests in the study area at a mesoscale extension. The evergreen broad-leaved secondary forests were dominant along the Pacific coast while the summer-green broad-leaved secondary forests in the inland area (Fig. 2).
The principal causal factors forming such areal difference of these forests were not artificial factors such as the intensity of cutting (Figs. 3-5), nor past land-use as grassland (Fig. 6) but apparently one or more natural ones, in particular, the temperature during winter half year (Figs. 7-9). From our ecological knowledge of evergreen and summer-green broad-leaved trees, the higher the temperature is during the dormant season (From November to April), the greater the surplus production of evergreen trees is. Evergreen trees can store the production for shoot elongation in spring. Thus the competitive ability of evergreen trees against summer-green trees in the secondary forest canopy is strengthened as the cumulative temperature condition during the winter bud season becomes warmer along an areal cline.
Therefore, I can conclude that the mesoscale areal defference of the secondary forests is determined principally by the cumulative temperature during the winter bud season. In fact, the distributional pattern of evergreen and summer-green broad-leaved secondary forests corresponds to that of the monthly cumulative temperature during the winter bud season. The boundary between the dominant areas of each forest type coincides with the 52 [°C·month] isobar (Figs. 2 and 9). Furthermore, summer-green broad-leaved forests are now more frequent in areas that were used as grasslands in the 1890's rather than in areas that were used as woodlands at that time, especially in the transitional zone between the dominant areas of each forest type (Fig. 10). This fact also suggests that the areal difference discussed above is determined not by artificial factors but by one or more natural ones.