Tree size structure of stands and each species in primary warm-temperate rain forests of Southern Japan

The frequency distribution of trunk diameter was analyzed for forest stands of various developmental stages and for each tree species population in primary warm-temperate evergreen rain forests, dominated byDistylium racemosum, on Yakushima Island of southern Kyushu, Japan. Trunk diameter distribution in stands showed the inverse J shape. A regression model, lnf(x)=b+b ₁x+b₂ lnx, expressed the distribution well, wherex is diameter andf(x) trunk density atx. Three coefficients of the model, calculated from the data in 20 stands, clearly showed linear relations to each other and they had high correlations with the basal area per stands. The result suggests thatb andb ₁ increase andb ₂ decreases with stand age. Fourteen populations of abundant tree species also showed inverse J-shaped diameter distribution, which can be well expressed by the above model. In both open sites (gaps) and closed stands, these species were abundant and their characteristics in diameter distribution were persistent.