Tree functional diversity mediates landscape fragmentation effects on insect herbivory in a natural riparian forest

Forest fragmentation poses a serious threat to biodiversity and ecosystem functioning of riparian forests, but there is little empirical evidence of how landscape fragmentation and local tree diversity drive insect herbivory—a critical knowledge gap for adaptive forest management. We investigated the effects of landscape fragmentation and tree diversity (particularly functional diversity) on foliar insect damage on Populus laurifolia in 30 natural forest plots along the Irtysh River in Xinjiang, China. In addition to insect damage, tree species richness and key functional traits (e.g., specific leaf area, wood density, maximum height) were quantified to calculate functional richness (FRic) and functional evenness (FEve). Landscape metrics, such as edge density (ED) and mean patch size (MPS), were derived within 300 m radius circular buffers. Structural equation modeling revealed that landscape fragmentation (i.e., increased ED and decreased MPS) directly increased foliar insect herbivory, whereas FEve reduced it. Crucially, FEve significantly mediated fragmentation–insect herbivory relationship, i.e., higher ED reduced FEve, and lower FEve in turn increased herbivory, while larger MPS increased FEve and thereby reduced herbivory. These findings demonstrate that functional diversity is indeed a key mechanism linking landscape structure to forest health. From a management perspective, the results indicate that maintaining functionally even tree communities can substantially mitigate the adverse effects of landscape fragmentation on insect herbivory on P. laurifolia . We recommend that forest managers prioritize not only the conservation of large, connected forest patches at the landscape scale, but also the promotion of functionally diverse and evenly structured tree communities at the stand scale to enhance forest resilience to biotic disturbances.