Species Richness Mitigates but Does Not Eliminate Functional Vulnerability in Natural Forests

ABSTRACT Aim Ecosystem functioning is intimately tied to the functional composition of communities, particularly to the distribution of species across trait space. Although species‐rich communities are widely assumed to promote the stability of functional diversity, whether this diversity–function coupling is consistently maintained in natural forest ecosystems remains to be systematically assessed. Location China. Methods In this study, we used datasets from five 6‐ha natural forest dynamics plots spanning approximately 6000 km across northern and southern China. On the basis of functional traits of woody plant species, we constructed functional entities and their corresponding functional space, then quantified the number of species within each functional entity to characterise community functional structure. Using this framework, we calculated functional redundancy, functional over‐redundancy, and functional vulnerability to evaluate the degree of species substitutability within functional entities, thereby revealing the potential impacts of species loss on the stability of community functioning. Furthermore, we employed a null model approach to generate the expected values of these functional indices in order to test whether the observed community functional patterns under species loss result from random assembly. Results Along the climatic gradient from cold‐temperate to tropical regions, both functional redundancy and functional over‐redundancy of woody plant communities increased and were significantly positively correlated with species richness. In contrast, functional vulnerability gradually decreased and was negatively correlated with species richness. Null model testing indicated that species distribution across functional entities is highly uneven and right‐skewed; regardless of overall species richness, most species clustered in a few FEs, whereas a high percentage of FEs (on average 55%) were occupied by a single, functionally unique species. This structural imbalance underscores a relatively high functional vulnerability. Main Conclusions Functional redundancy is highly unevenly distributed across different natural forest communities. Regardless of species richness, many unique combinations of functional traits are supported by only a single species, making the functional diversity of forest communities highly vulnerable to species loss. This consistency across functional dimensions exposes the vulnerability of forest functional structure. To maintain the stability of ecosystem functioning under ongoing climate change, it is necessary to integrate functional vulnerability with the local abundance and extinction risk of functionally unique species, thereby enabling the development of more targeted conservation and management strategies.