Neighbourhood diversity effects on insect herbivory: Plant leaf traits mediate associational resistance

Abstract The vulnerability of trees to insect herbivory can be influenced by forest structure and diversity. Associational resistance theory posits that trees surrounded by diverse neighbours are likely to suffer reduced herbivory. However, the underlying mechanisms of this effect are debated, with accumulating evidence suggesting that leaf traits could mediate the strength and direction of the diversity‐herbivory relationships. To determine the role of tree trait variation in mediating this relationship, we measured leaf herbivory and nine morphological and nutritional leaf traits known to influence herbivory on 394 focal trees of twelve species cultivated in monocultures and mixtures of four, eight, and sixteen species in a 4‐year‐old large‐scale manipulated tree diversity experiment. A reduction in the proportion of focal trees in species‐rich neighbourhoods resulted in increased leaf carbon: nitrogen ratio of focal trees, which mediated a reduction in insect herbivory. Moreover, an increase in plant height apparency, defined as the disparity in total height between a focal tree and its closest neighbours, indirectly amplified herbivory by reducing leaf phosphorus concentration. Synthesis : The study suggests that neighbourhood diversity and physical structure can indirectly affect herbivory on a focal plant by modifying its leaf trait. Accounting for the functional differences between forests could enhance our understanding of diversity–herbivory relationships.