Cross‐Scale Effects of Tree Diversity on Ecosystem Multifunctionality via Diversity Across Functional Groups in a Subtropical Mountain Forest

ABSTRACT Aim Climate change and anthropogenic disturbances are driving biodiversity loss across multiple functional groups and spatial scales, potentially weakening the ability of ecosystems to sustain multiple functions simultaneously. However, it remains unclear how diversity across functional groups regulates ecosystem multifunctionality (EMF) across spatial scales, particularly in mountain forests with strong climatic gradients. Location Southeastern China. Taxon Woody plants. Methods Here, we assembled data on seven functional groups, including trees, shrubs, herbs, bacteria, pathogenic fungi, saprotrophic fungi and symbiotic fungi, and 10 ecosystem functions representing supporting, provisioning and regulating services. Within the Hill–Chao numbers framework, we quantified the diversity of each functional group and partitioned EMF at the larger spatial scale (γ‐EMF) into its α and β components. We then synthesized the diversity measures of all functional groups (except trees) into a single composite diversity measure across functional groups. We used a multiscale perspective to evaluate how tree diversity regulates EMF through multidiversity across functional groups at different spatial scales. Results We found that α‐ and β‐diversity increased γ‐EMF through their positive effects on α‐EMF and β‐EMF, respectively. Specifically, tree diversity can promote EMF across spatial scales both directly and indirectly by enhancing diversity across multiple functional groups. The α‐ and β‐diversity across multiple functional groups had stronger positive effects on α‐ and β‐EMF, respectively, than diversity within any single functional group. Additionally, elevation influenced EMF across spatial scale by reducing diversity across functional groups at multiple spatial scales. Main Conclusions Our results highlight that maintaining higher diversity across multiple functional groups is critical for sustaining EMF at both local and broader spatial scales in mountain forests. By integrating diversity across multiple functional groups and spatial scales, our study advances understanding of the patterns and mechanisms linking biodiversity and EMF in mountain forest ecosystems and highlights the necessity of a holistic conservation strategy that explicitly considers multiple functional groups.