Effects of topographic heterogeneity on tree species richness and stand dynamics in a subtropical forest in Okinawa Island, southern Japan

Abstract

1 We used a structural equation model (SEM) to clarify the relationship between productivity and species richness across a topographic gradient in a subtropical forest in southern Japan and to quantify the direct and indirect effects of topographic position and soil properties on community attributes. 2 Higher soil nitrogen on ridges was associated with low soil pH and high soil organic matter, but differences in soil chemistry were small compared with those observed in tropical forests. 3 Species richness increased from the valley, via slopes, to ridges. Understorey and microphyllous species, both of which are typically found in gaps or in windy sites, were more abundant on slopes and ridges than in the valley. Habitat preferences of the species, reflecting shade/drought tolerance rather than soil fertility, may be a primary cause of differences in species richness between the topographic types. 4 The total effect of species richness on biomass increment was greater than that of tree density, and biomass increment increased linearly with species richness. This, together with the fact that combined effects of topography, soil properties and ground flora on biomass increment were comparable with that of tree density, indicates that the relationship was caused by habitat heterogeneity rather than a density effect. 5 Frequent disturbances and chronic wind stress resulted in higher mortality and a greater loss of biomass, especially on the ridges, and probably prevented species with different functional traits (e.g. maximum height) from competing for light. 6 Topographic variation therefore appears to lead to structural/compositional heterogeneity within this subtropical forest, enabling the occurrence of a wider range of species. The growth and survival of a variety of species with different functional traits are diversified by environmental conditions along the topographic position. This causes variation in productivity between the topographic types, and suggests that functional resilience associated with species richness contributes to high productivity.