Trait shifts and trophic rifts: Biodiversity-driven leaf trait plasticity modulates interactions with tree consumers

In response to neighboring tree diversity, trees modify leaf traits related to growth or defense, ultimately resulting in phenotypic plasticity. Phenotypic plasticity refers to individual trees of the same species displaying distinct phenotypes under contrasting environmental conditions, such as growing nearby conspecifics compared to heterospecifics. Although diversity-driven leaf plasticity is often observed, impacts on tree consumers remain unclear. Here, we characterized leaf trait phenotypes of temperate tree species growing in single- and mixed-species environments using a long-term biodiversity ecosystem functioning experiment. We tracked more than 300 individual trees over time, measuring leaf traits and quantifying damage from tree consumers (e.g., insect herbivores, pathogens). In addition to consumer damage estimation, we also observed the composition of insect herbivore communities on tree leaves and assessed fungal pathogen richness. We found phenotypic expression of individual traits changed from single- to mixed-species environments, indicating trees generally favored fast-growth strategies when growing in species mixtures. Additionally, trees growing in species mixtures suffered greater insect herbivory but lower pathogen damage relative to trees in single-species environments. When evaluating multivariate leaf trait phenotypes, we found neighboring tree diversity altered phenotypes in a species-specific manner and correlated with consumer damage. Further, we determined diversity-driven leaf plasticity impacted insect herbivore and pathogen community composition. Taken together, our findings indicate trees exhibit phenotypic plasticity in response to neighbor diversity, contributing to trade-offs in resistance against insect herbivory and pathogen infection. This work shows diversity-driven leaf plasticity strongly impacts consumers, highlighting a previously unknown component of biodiversity-ecosystem functioning in temperate forests.