Neighbouring tree effects on leaf herbivory: Insect specialisation matters more than host plant leaf traits

Insect herbivory plays a critical role in the regulation of plant populations and community structures. Although the strength of insect herbivory varies with neighbouring tree diversity and identity, it is not clear to what extent insect guilds and leaf traits of host plants mediate the relationships, particularly in diverse natural forests. In this study, we measured leaf herbivory by chewing and mining insect herbivores in 442 saplings of nine species of deciduous trees in a warm-temperate montane forest. We censused neighbouring trees and measured five leaf traits known to influence herbivory (i.e. specific leaf area-SLA, thickness, carbon: nitrogen ratio-C:N ratio, sodium-Na and silicon-Si content) on a subset of saplings. Only herbivory by leaf-mining insects was influenced by the identity and diversity of neighbouring trees. Specifically, the incidence of leaf-miners decreased with the phylogenetic diversity of neighbouring trees and increased with conspecific adult density in the vicinity of focal trees. Furthermore, leaf mining herbivory was positively correlated with the amount of Si in the leaves of focal saplings, and leaf chewing herbivory decreased with increasing C:N ratio in the leaves. In contrast to our hypothesis, the leaf traits of focal saplings did not serve as a functional link between tree diversity and insect herbivory. Synthesis: Overall, this study highlights the importance of considering the specialisation of insect herbivores in understanding the impacts of the diversity and identity of neighbouring trees on the strength of insect herbivory.