Species-specific responses of leaf herbivory and defenses along a latitudinal gradient

Latitudinal variation in biotic interactions has long been hypothesized to shape plant trait evolution, with stronger herbivory at lower latitudes predicted to drive increased plant defenses. Yet, recent studies reveal inconsistent patterns, suggesting that context dependence and methodological limitations constrain broad inference. Two key issues may underlie these discrepancies: most research examines single defensive traits in isolation rather than integrated defense syndromes, and studies often focus on single species within restricted geographic ranges. To address these gaps, we tested for latitudinal variation in insect herbivory and multiple leaf traits across four tree species distributed along a 21.4° gradient in China, spanning tropical to temperate regions. We sampled 7-10 populations per species across nearly their entire climatic ranges. Our results show that latitudinal patterns in herbivory and defenses are highly species-specific. In Prunus persica and Prunus salicina, herbivory followed a unimodal gradient, peaking at mid-latitudes, while Malus halliana and Malus micromalus showed no significant latitudinal trends. Leaf physical traits varied with latitude: leaf area increased with latitude in P. salicina and M. halliana but decreased in M. × micromalus, and leaf thickness declined at higher latitudes only in M. × micromalus. Chemical traits, including C:N ratio, tannins, and flavonoids, generally declined toward higher latitudes, though patterns were species-dependent. Importantly, herbivory was mediated by interactions between latitude and traits such as flavonoid content and specific leaf area. Our findings highlight that latitudinal effects on herbivory emerge through species-specific trait variation, underscoring the need for multispecies, trait-based approaches to understand plant-herbivore dynamics at broad scales.