Tree competition and species coexistence in a warm-temperate old-growth evergreen broad-leaved forest in Japan

摘要

The growth dynamics and mode of competition between adult trees greater than or equal to 5.0 cm in diameter at breast height (DBH) of nine abundant tree species occupying ca. 85% of the total basal area were investigated in a 4 ha study plot (200 m x 200 m) of a warm-temperate old-growth evergreen broad-leaved forest in the Tatera Forest Reserve of Tsushima Island, southwestern Japan. In the plot, adult trees greater than or equal to 5.0 cm DBH co-occurred with 35 woody plant species (except for woody vine species). The most dominant and largest species, Castanopsis cuspidata var. sieboldii exhibited a bimodal DBH distribution; it was found in both the upper and lower vertical layers. Other tree species had unimodal DBH distributions corresponding mostly to the lower vertical layer. We developed a model for individual growth incorporating both intra- and interspecific competition and degree of competitive asymmetry. One-sided interspecific competition was detected in 17 cases out of the 66 possible combinations on the scale of the 4 ha study plot. The direction of interspecific competition was generally one-sided from layer-I species to layer-II and III ones. The effects of two-sided competition were detected only in layer-II and III species. Only Distylium racemosum exhibited one-sided intraspecific competition. We also found 11 cases of positive interspecific relationships. Generally, competitive relationships prevailed over positive relationships between adult trees in this warm-temperate evergreen broad-leaved forest. Competition between adult trees greater than or equal to 5.0 cm in DBH did not occur in the same vertical layer, but occurred only between trees in different vertical layers. This suggests that competition between adult trees greater than or equal to 5.0 cm in DBH plays a key role in the variation in species coexistence between different vertical layers on the 4 ha scale of the warm-temperate evergreen broad-leaved forests. Moreover, it was found by comparing with three different forest types that interspecific competition is more intense in warm-temperate forests than in cool-temperate or sub-boreal forests. We conclude that, compared to cool-temperate or sub-boreal forests (which have little interspecific competition), warm-temperate forests support more complex interspecific relationships and species-specific habitat preferences that result in higher species diversity.