The influence of specific leaf area on insect herbivory: a leaf-level study across multiple plant species

Understanding how specific leaf area (SLA) influences insect herbivory is crucial for predicting plant-herbivore interactions in forest ecosystems. However, most existing studies rely on species- or community-level trait averages, overlooking the fine-scale variations at the leaf level where herbivore feeding decisions actually occur. Here, we examined leaf-level relationships between SLA and herbivory using 3028 insect-damaged leaves from 609 juvenile individuals of 39 woody species, sampled over three seasons in a subtropical forest of southwest China. We identified a consistent U-shaped pattern at the community level, where leaves with intermediate SLA experienced the lowest herbivory, while both high- and low-SLA leaves suffered greater damage. This pattern persisted across seasons and reflects two distinct processes operating relative to an SLA threshold. Below this threshold, SLA is negatively associated with herbivory, potentially due to extended leaf lifespan in low-SLA leaves. Above the threshold, SLA is positively associated with herbivory, potentially due to enhanced nutritional quality in high-SLA leaves outweighing defensive traits. In addition, we observed a diversity of species-specific responses, including U-shaped, negative, positive, hump-shaped, and non-significant relationships. Our findings demonstrate that while community-level patterns suggest general ecological trends, species-specific responses vary considerably, highlighting the importance of multi-scale approaches in functional ecology. The study advances our understanding of plant defense strategies by showing how SLA's role in herbivory depends on both universal leaf economic principles and species-specific adaptations. • First evidence of a U-shaped SLA-herbivory relationship in subtropical forests. • Leaf-level approach analyzing 3028 insect-damaged leaves across 39 species. • Consistent U-shaped pattern persists across seasons with distinct threshold effects. • Substantial interspecific variation in SLA-herbivory relationships observed.