Lowland rainforests in southern Taiwan and Lanyu, at the northern border of Paleotropics and under the influence of monsoon wind

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In order to reveal the characteristics of the vegetation affected by monsoons at the northern border of Paleotropics, a tree-by-tree census was conducted in the lowland forests in the southernmost Taiwan (Nanjenshan) and an adjacent islet (Lanyu). The census recorded a total of 109,060 individuals (≥1-cm diameter at breast height) belonging to 255 vascular tree species in 1330 quadrats (10 × 10 m). Two-way Indicator Species Analysis first classified forest types into two groups, Lanyu and Nanjenshan, reflecting biogeographical differences. Five subgroups were further classified, showing correlations with topographic position indices. Forests located on wind-exposed slopes, regardless of elevations, were characterised by low canopy height, high stem density, high proportion of small stems, and high proportion of warm-temperate-related species, compared with the wind-sheltered communities. However, there were no significant differences in basal area and species diversity. In comparison with other tropical forests, our forests are characterised by high stem density, low diversity and a lack of the pan-Paleotropical dominant Dipterocarpaceae. In conclusion, vegetation in the studied regions not only showed a transition characteristic between Paleotropics and Holarctic Kingdoms in terms of composition, but also showed differentiations caused by their biogeographical history and the interaction between topographic positions and wind stress from monsoons.