Stochastic assembly in a subtropical forest chronosequence: evidence from contrasting changes of species, phylogenetic and functional dissimilarity over succession.

Xiangcheng Mi,Nathan G Swenson,Mide Rao,Daniel P Bebber,Qi Jia,Gang Feng,Haibao Ren,Keping Ma

doi:10.1038/srep32596

Abstract

Deterministic and stochastic processes jointly determine the community dynamics of forest succession. However, it has been widely held in previous studies that deterministic processes dominate forest succession. Furthermore, inference of mechanisms for community assembly may be misleading if based on a single axis of diversity alone. In this study, we evaluated the relative roles of deterministic and stochastic processes along a disturbance gradient by integrating species, functional, and phylogenetic beta diversity in a subtropical forest chronosequence in Southeastern China. We found a general pattern of increasing species turnover, but little-to-no change in phylogenetic and functional turnover over succession at two spatial scales. Meanwhile, the phylogenetic and functional beta diversity were not significantly different from random expectation. This result suggested a dominance of stochastic assembly, contrary to the general expectation that deterministic processes dominate forest succession. On the other hand, we found significant interactions of environment and disturbance and limited evidence for significant deviations of phylogenetic or functional turnover from random expectations for different size classes. This result provided weak evidence of deterministic processes over succession. Stochastic assembly of forest succession suggests that post-disturbance restoration may be largely unpredictable and difficult to control in subtropical forests.

Highlights

Τst values from the 999 random communities
We found that beta diversity (Bst) and τst values of all traits fall inside the 95%-confidence interval of random expectations (Fig. E1d–g,m,o,r) for most dbh size classes, indicating that observed Bst and τst values were not significant from random expectations
Disentangling the relative roles of deterministic and stochastic processes in shaping community dynamics over succession remains a major challenge in community ecology

Summary

Introduction

Τst values from the 999 random communities. Bst and τst values inside the interval indicate phylogenetic or functional randomness, and Bst and τst values outside the interval indicate the significant phylogenetic or functional clustering or dispersion. Christensen and Peet[11] tested these predictions by examining species beta diversity and have found that in forests of five age-classes, the mature hardwood stands had the highest beta diversity while young pine stands had the lowest beta diversity, failing to support Clement’s deterministic view[9] Despite these predictions, deterministic and stochastic processes may potentially produce similar patterns of species beta diversity over succession[4,13]. Increasing species beta diversity created by stochastic processes may arise from deterministic processes such as increasing variation in environmental conditions across locations. Severe disturbances such as forest clearing may homogenize environmental conditions like light and soil properties. If traits are not phylogenetically conserved, it is difficult to understand the community assembly processes from community phylogenetic structures

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Results

Discussion

Conclusion