Density-dependent effects of tree species in a 15 ha seasonal rain forest plot in northern tropical karst in Nonggang, Guangxi, southern China

Density dependence is one of the most important mechanisms that has been proposed to explain species coexistence in natural forests. However, habitat heterogeneity influences density-dependent effects. A strongly heterogeneous plot usually shows weak density-dependent effects. High heterogeneity of space horizontally and multi-level heterogeneity vertically are characteristics of the seasonal rain forests in karst ecosystems. We used data from a 15-ha plot in a northern tropical seasonal rain forest in a karst landscape in southern China to address density-dependent effects. In this study, we investigated density-dependent effects and habitat associations of all individuals, juveniles, and adults using environmental variables of 96 species having at least 15 individuals in each life stage in the plot. For each of these species, we used a non-homogeneous pair-correlation function to quantify the change in spatial patterns for two life stages, and used a case-control design to study density-dependent self-thinning and thinning effects. Then we used a Berman-test to factor out the effects of habitat association. We found (1) that only 14 species showed a decline in aggregation with an increased life stage that was strongest from juveniles to adults; (2) about 23 species had juveniles that were relatively more frequent around juveniles than having adults around juveniles; (3) with three exceptions, all species were associated with at least one environmental variable during at least one of their life stages, but the frequency of significant habitat associations declined with increasing life stage. Our results suggested that density-dependent effects were not the dominant mechanism in structuring the spatial patterns of established trees in this northern tropical seasonal rain forest in a karst ecosystem. The density-dependent effects may be weakened by the influence of strong topographical heterogeneity and the special geological history of this area, or by the exclusion of the seedling life stage.