The architectural stratification and woody species diversity in subtropical evergreen broadleaf forests along a latitudinal thermal gradient of the Ryukyu Archipelago, Japan
In order to compare stand structure and woody species diversity of subtropical evergreen broadleaf forests along a latitudinal thermal gradient of the Ryukyu Archipelago, tree censuses in a 750 m2 plot in Okinawa Island and a 400 m2 plot in Ishigaki Island were performed. The number of layers increased along a latitudinal thermal gradient from four in the forest of Okinawa Island to five in the forest of Ishigaki Island. The values of Shannons index H' and Pielous index J' tended to increase from the top layer downward in the forest of Okinawa Island, whereas in the forest of Ishigaki Island, these values tended to increase from the bottom layer upward. High woody species diversity depended on small-sized trees in the Okinawa forest, whereas it depended on large-sized trees in the Ishigaki forest. The woody species diversity is higher in the Okinawa forest (H' = 4.83 bit) than in the Ishigaki forest (H' = 4.36 bit). According to successively decreasing height of layers from the top downward, the value of H' increased continuously from the top layer downward in the Okinawa forest. This increasing trend was different from the Ishigaki forest, where the value of H' increased up to the second layer and then decreased downward. In the Okinawa forest, the expected number of species increased continuously from the top toward the bottom layer, i.e. the bottom layer contained the highest potential number of species (65). However, in the Ishigaki forest, it increased from the top to the fourth layer and then decreased to the bottom layer, i.e. the fourth layer contained the highest potential number of species (90). The floristic composition in the Okinawa forest was different from that in the Ishigaki forest in terms of similarity index, though approximately half of the species were common between them. The highest degree of similarity in floristic composition was between the second and third layers in the Okinawa forest, whereas it was between the third and bottom layers in the Ishigaki forest. The degree of similarity in floristic composition among layers was higher in the Okinawa forest than in the Ishigaki forest. Except the top and the bottom layer respectively in the forests of Okinawa Island and Ishigaki Island, the spatial distribution of trees was random in each layer. The degree of overlapping in the spatial distribution of trees among layers in these two forests suggested that trees in the upper two layers in the Ishigaki forest can catch sufficient light, while light can not penetrate easily to the lower three layers in both of the forests. As a result, almost species in the lower layers might be shade-tolerant in both of the forests. For both of the forests, mean tree weight of each layer decreased from the top downward, whereas the corresponding tree density increased from the top downward. This trend resembled the mean weight-density trajectory of self-thinning plant populations.