We studied the latitudinal and altitudinal variations in composition and species richness of trees in 24 plots across temperate to subtropical regions from southern Kyushu to the northern Ryukyu Archipelago, Japan. Cluster analysis based on presence/absence of species in 0.25-ha segments of the plots identified three forest zones reflecting climate and geohistory: cool-temperate mixed conifer-broadleaf forest (MF) north of the Tokara Gap (Kyushu and Yakushima), warm-temperate evergreen broadleaf forest (EBF) north of the Tokara Gap, and subtropical EBF south of the Tokara Gap (Amami-Oshima, Tokunoshima, and Okinawajima). The MF zone corresponded to Kira's Warmth Index (WI) < 100°C. The two EBF zones overlapped in the range of WI, and thus their boundary was determined by geohistory rather than by climate. Plot-level species richness per 400 stems increased with increasing WI, with decreased values in Yakushima, suggesting that geohistory (isolations from Kyushu as well as by the Tokara Gap) depressed the species richness of Yakushima. Greater species richness in the subtropical EBF was explained by the presence of species whose distributions were restricted to the south of the Tokara Gap, indicating that the size of tree flora influences the plot-level species richness. By contrast, lower species richness of MF in Yakushima than in Kyushu was due to the absence of species whose distributions were restricted to Kyushu northward, indicating the impoverishment of the cool-temperate flora of Yakushima. In conclusion, climate and geohistory shape the tree-community composition and diversity of old-growth forests in this transitional zone from temperate to subtropical regions.

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