Structure and Species Diversity of Subtropical Evergreen Broad-leaved Forest in Northern Okinawa Island, Japan

The structure and tree species diversity of a subtropical evergreen broad-leaved forest in northern Okinawa Island, Japan, were studied. Enumeration of the six sampling plots revealed an average density of 5,580 individuals with DBH≥3.0 cm/ha, having an average basal area of 55 m2. The large-size trees of DBH ≥ 20 cm contributed 10% of the total individuals, and 49% of the total basal area. The forest showed a high diversity of tree species, which is comparable to some tropical rain forests. A total of 54 over-story species of 24 families and a total of 63 understory species of 26 families were identified in the six sampling plots. Fagaceae and Theaceae were the most important families; Castanopsis sieboldii, Schima wallichii and Distylium racemosum were the most important species. The diversity index and equitability index of species were 4.15 and 0.72 for the overstory plots, and 4.72 and 0.79 for the understory subplots, respectively. The diversity index for the overstory was significantly correlated to the total basal area of trees over 20 cm DBH (p<0.05) and the importance value of C. sieboldii (p<0.001), while for understory, the diversity index was not correlated to the structural parameters (all p>0.16). The size distribution pattern and age structure indicated differences in regeneration strategies for canopy dominants. In population dynamics of the succession process, C. sieboldii and D. racemosum were self-maintaining types, and S. wallichii was a gap- or opening-dependent type.