Induced Volatile Chemistry and Arthropod Predation Across a Tree Diversity Gradient

Plant volatile organic compounds (VOCs) play a key role in defence against herbivores and in mediating multitrophic interactions. Yet, how surrounding plant diversity modulates these responses in forest trees remains poorly understood. We examined the effects of tree species richness on induced VOC emissions and arthropod predation in Quercus petraea in a biodiversity-ecosystem functioning experiment. Individual branches were repeatedly induced with methyl jasmonate and paired with non-induced controls within the same tree. VOCs were collected using PDMS tubes after the first and last induction rounds and analyzed by TD-GC-MS. Predation was quantified using plasticine caterpillars. Jasmonate induction strongly altered VOC profiles and effects increased after repeated treatments. Tree species richness modulated VOC composition during the first sampling period, particularly affecting sesquiterpenes. Compound-specific analyses revealed that β-Ocimene was negatively associated with predation probability during the second sampling period. However, absolute VOC class emissions did not explain variation in arthropod predation probability, while compound class-level composition showed only weak and inconsistent effects across sampling periods. These results suggest that indirect defence outcomes in terms of predator responses are unlikely to depend on VOC class-level emission strength or broad VOC class composition but may instead be mediated by fine-scale properties of volatile blends.