Density dependence is not very prevalent in a heterogeneous subtropical forest

There is a growing body of evidence demonstrating that tree survival is influenced by negative density-dependence, but it is still controversial how the effect may vary with life-stage, and to what extent it plays a role in regulating tree survival in heterogeneous subtropical forests. In this study, we investigated density-dependent effects on tree survival of six tree species in a 5-ha subtropical forest in eastern China. The roughly 45 000 individuals in the forest were fully censused in 2003 and 2008. For each of these species, we used an inhomogeneous pair-correlation function to quantify the change in spatial distribution for different size classes, and a case-control design to study seedling—adult associations in 2003. Autologistic regression was used to determine the influence of neighborhood factors on individual survival from 2003 to 2008. We found that seedlings of five species were repulsed by distance to nearest conspecific adults in terms of their survival, consistent with predictions of the Janzen—Connell mechanism. By contrast, only the least shade-tolerant Schima superba had a negative relationship with individual survival and conspecific distance-weighted basal area. This suggests that the Janzen—Connell effect is only prevalent at the early seedling stage or seed-to-seedling phase. The strength of clustering significantly declined at sapling—pole and pole—adult transitions for Sycopsis sinensis and at seedling—sapling transition for Cleyera pachyphylla. Correlations between individual survival and conspecific abundance for these species were consistent with trends in the strength of clustering. These results suggest that density dependence plays a limited role in individual survival and species spatial structure beyond the early seedling stage (i.e. after true leaves growing) in this forest. In addition, this study indicates that including individuals from early life-stages and factoring out potential confounding factors such as habitat preference are important in studies that seek evidence for density dependence in forest trees.