Tree diversity improves forest resistance to insect defoliators

Whether increasing forest diversity should result in less insect damage (Associational Resistance, AR) or more damage (Associational Susceptibility, AS) is still debated. Moreover little is known about diversity - resistance relationships in mature forests due to methodological constraints. To address this question we used the FundivEurope exploratory platform. In this network of 209 mature forest plots, sampled along two orthogonal gradients of increasing tree species richness (from one to five species mixtures) and latitude (spanning from the Mediterranean to the boreal forest biomes), we assessed insect defoliation in tree crown. Focusing on eleven broadleaved species, we could for the first time demonstrate a global pattern of reduced defoliation (AR) in mature forests across Europe. We replicated the comparison of insect herbivory in pure vs. mixed plots of oak trees, at the edge or within forest patches. Here we found again significantly less damage on oaks surrounded by heterospecific neighbors, and showed that the magnitude of AR was larger at forest edge than in forest interiors. Last we tested the AR hypothesis with an invasive alien species, the Asian chestnut gall wasp, in Italy. Based on the same methodological approach, our study showed a significant decrease in gall damage with increasing tree species richness in mixed chestnut stands. Our work therefore provides new experimental evidence supporting the Associational Resistance hypothesis with three original contributions: i) AR does apply to mature forests, ii) irrespective of the identity of insect herbivores, iii) including alien pest species. These findings may have important implications for pest management in forest with the maintenance or improvement of tree species diversity being a promising tool for preventing insect damage