Spatial patterns of tree species richness in two temperate forests

1. The relative contribution of external vs. internal clustering mechanisms for determining community structure and its manifestations has been the subject of a continuous debate, but few attempts have been made to examine their single and joint effects in a compound process model. 2. In this study, we tested four a priori hypotheses on the relative importance of habitat heterogeneity (topography and soil) and internal clustering mechanisms such as dispersal limitation on the species- area relationship (SAR) in two fully mapped 25-ha plots of temperate forests in the Changbaishan (CBS) Nature Reserve, China, and the Chequamegon-Nicolet National Forest in Wisconsin, USA. 3. We used the distance decay curve to test the generality of the results obtained for the SAR. To find out if the relative importance of internal and external clustering mechanisms changed with life stage, we conducted separate analyses for small, large and all trees. 4. Model selection favoured the most complex hypothesis that assumed an influence of both habitat heterogeneity and internal clustering on SAR and the distance decay curve. For the CBS plot, which shows weak topographical structuring, models were consistent with data only if soil factors were included into assessment of habitat heterogeneity. At the Wabikon plot, we could not test soil variables, but inclusion of topographical variables substantially improved the fit of the distance decay curve. 5. In general, the results of the SAR agreed with those of the distance decay curve, but the latter was sensitive to positive habitat-mediated species associations. The SAR, but not distance decay, distinguished among competing hypotheses for the community of large trees at CBS, where species exhibited only weak clustering. 6. Contrary to our expectations, we did not find substantial differences in the relative importance of internal and external clustering mechanisms with life stage. 7. Synthesis. Our analysis of spatial community structure for two relatively diverse temperate forests revealed that the factors governing spatial community structure may not substantially differ from those in highly diverse tropical forests. This result adds to our understanding of the ecological processes underlying the spatial diversity structure in natural forest communities.