Acquisitive, diverse tree stands resist vine invasion via contrasting temporal effects on crown volume and herb cover

A longstanding perspective suggests that high biodiversity reduces the success of invasive species by constraining available resources, yet little is known about how functional diversity and identity affect invasion resistance of tree communities. Forest invasion should be influenced by both indirect resource competition and direct environmental filtering mediated by forest structural attributes. We hypothesized that tree communities characterized by high functional diversity and dominated by resource-acquisitive species increase the resistance to vine invasion by reducing understory herbaceous coverage due to strong competition for the limited resources. We tested this hypothesis in a tree diversity experiment composed of eight woody species differing widely in plant economic strategies. The coverage of both understory herbs and invasive vine Pueraria montana (kudzu) were measured over six years (2018–2023). We found that tree functional diversity (FD) improved community invasion resistance by increasing tree crown volume (CVol), which enhanced light limitation and consequently reduced herbaceous cover. Moreover, stands characterized by acquisitive traits, such as high leaf nitrogen content, enhanced resistance through resource competition, but their apparent effect was largely attributable to their association with functional diversity. However, herbaceous cover actively promotes P. montana invasion as stands develop. These findings highlight that stands with low functional diversity and limited representation of functionally acquisitive species are at high risk for vine invasion, and show that understory herbs mediate the diversity effects by linking tree community composition to invasion resistance.