Floristic composition, woody species diversity, and spatial distribution of trees based on architectural stratification in a subtropical evergreen broadleaf forest on Ishigaki Island in the Ryukyu Archipelago, Japan

The floristic composition, woody species diversity, and spatial distribution of trees in a subtropical evergreen broadleaf forest on Ishigaki Island, Japan, were investigated based on architectural stratification. Five architectural layers made up the forest stand. The floristic composition was almost similar between the third and the bottom layers, whereas it was almost exclusive between the top and the lower four layers. The fourth layer contained the highest potential number of species (90). Castanopsis sieboldii, Daphniphyllum teijmannii, Neolitsea aciculate, and Distylium racemosum were typical facultative shade species that appeared in all layers. Ardisia quinquegona was the most dominant species, as it had the highest importance value at the stand level; it was also considered a small climax species because of its disappearance in the top layer but higher importance value in the lower four layers. The values of Shannon's index H' and Pielou's index were 4.36 bit and 0.69, respectively, for the entire stand; these values tended to increase from the bottom layer upward, except for the H'-value of the top layer. Furthermore, woody species diversity was higher in the upper strata than the lower strata. The spatial distribution of trees was random in each layer, except in the bottom layer where trees were aggregately distributed. The degree of overlap in tree habitats differed among layers, and the results suggested that trees in the second layer could capture moderate light, while light could not easily penetrate the lower three layers. Therefore, most species in the lower layers must be shade-tolerant. The mean tree weight of each layer decreased from the top layer to the bottom layer, whereas the corresponding tree density increased. We concluded that this trend might be a general phenomenon in subtropical evergreen broadleaf forests of the Ryukyu Archipelago, and we designated this trend as the quasi mean tree weight density trajectory of forest stratification. The relationship of mean tree height to tree density for the upper two layers supported Yamakura's quasi-3/2 power law system.