Ecological drivers of tree assemblage in tropical, subtropical and subalpine forests

Aims: It is widely accepted that deterministic and neutral dispersal processes are two of the principal mechanisms driving community assembly. However, the relative importance of these ecological drivers between distinct life stages both at community and individual species levels remains poorly understood in different vegetation types. Here, we address the following questions: (a) is there a ubiquitous process shaping tree assemblage; (b) how does the relative importance of environmental and dispersal processes vary through ontogeny in different vegetation types; and (c) will the spatial patterns of trees at community and individual species levels reflect similar contribution between ecological processes?

Location: Yunnan province, southwest China

Methods: We used the Homogeneous Poisson, Homogeneous Thomas, Inhomogeneous Poisson, and Inhomogeneous Thomas point process models to predict the effect of stochastic, dispersal and/or environmental processes, respectively, on the distribution of trees across ontogeny in tropical, subtropical and subalpine forests. To evaluate the relative importance of models, we compared the observed and simulated patterns of the species–area relationship and the pair correlation function g(r) at community and species level, respectively.

Results: The Homogeneous Thomas model was the model with the lowest Akaike information criterion (AIC) scores across ontogeny and forest types at community level. At the species level, however, Homogeneous Thomas, and Inhomogeneous Thomas predicted the distribution of large and small trees, respectively, with lower AIC values in two of our three plots. In the third plot, the Homogeneous Thomas model is dominant across ontogeny at the species level.

Conclusions: Tree communities are assembled mainly by the dispersal process at community level. The relative importance of ecological processes for species distribution varies across life stages at the species level, suggesting that there is an ontogenetic shift of ecological processes in shaping tree distribution. Using different summary statistics at both community and species levels helps to discern the different spatial structures of species and increases our appreciation of the processes underpinning community assembly.