High plant diversity and characteristic plant community structure in broad-leaved evergreen forests on Amami-Oshima and Tokunoshima Islands, Japan's newest natural World Heritage Site

Amami-Oshima and Tokunoshima Islands (the Amami Islands) are part of an important area for biogeography in Japan. Although the uniqueness of the islands' flora is well recognized, their plant diversity, including non-tree life forms, has yet to be compared with that of other regions. Here, we characterize the plant community structure of the Amami Islands, and analyze the alpha and beta diversity patterns with respect to environmental factors as well as the frequency of threatened and endemic species compared with that in regions at higher and lower latitudes. We used 5 x 100 m plots at seven sites on the Amami Islands, three sites further north, and one site further south to record the abundance of all vascular plant species and tree sizes. We also used tree census data from five plots from the Monitoring Sites 1000 Project. Three principal components from five environmental variables were used as factors affecting diversity. We found that the broad-leaved evergreen forests on the Amami Islands have higher alpha diversity resulting from species richness anomalies in shared families among regions. Additionally, generalized linear mixed models indicated that temperature and altitude affect alpha diversity positively. Furthermore, multiple regression models on distance matrices showed that temperature and regional differences in weather conditions affect beta diversity. Tree species were found to affect species diversity and rareness, but herbaceous plants also affect these parameters substantially. These results indicate that the two studied islands are biodiversity hotspots and that evaluating herbaceous species is important for determining forest biodiversity.