Plant neighbours have well known effects on insect herbivores. One dominant body of theory predicts that herbivore attack on host plant species may decrease in heterospecific relative to conspecific neighborhoods (i.e., associational resistance) as a result of reduced host accessibility. However, recent studies have reported that plant diversity can indirectly affect herbivory on focal plants by modifying its nutritional quality and defenses, independently of host accessibility. Despite the well known effect of abiotic factors on herbivores, the potential interacting effects of plant diversity and abiotic factors (e.g., drought) on herbivores has been neglected. Yet, both neighboring plants and abiotic conditions may independently and interactively alter plant accessibility, nutritional quality and defenses. Develop a predictive framework of plant diversity effects on higher trophic levels requires integrating these combined effects.
By using a tree diversity experiment in SW France, we measured leaf insect herbivory, nutritional quality (water content, C/N ratio, sugar and starch content) and chemical defenses (total polyphenolics and condensed tannins) on birch (Betula pendula) trees growing in monocultures and mixtures with oak, pine or both species. We alleviated water stress by irrigating trees in half of the plots while trees remained unwatered (i.e., drought-stressed) in the other half.
Results/Conclusions
Contrary to associational resistance predictions, insect leaf herbivory was higher among heterospecific neighbors. Consistent with this finding, leaves had lower amount of anti-herbivore defenses in heterospecific neighbors. Herbivores caused more damage in drought-stressed conditions, but such effect was independent of leaf chemistry. Importantly, we also found that the effect of tree species diversity on herbivory was contingent upon drought conditions as increased herbivory among heterospecific neighbors only occurred in drought-stressed trees. Finally, the independent and interactive effects of neighbor diversity and irrigation on leaf herbivory remained significant after accounting for plant traits, suggesting that unmeasured plant traits or some other mechanisms not associated with plant trait variation might be playing a pivotal role in the observed herbivory patterns.
In conclusion, our results suggest that although leaf traits did correlate with insect leaf herbivory, their contribution to associational effects may be weaker than density-dependent processes controlling for herbivore recruitment. Importantly, by demonstrating that associational effects are contingent upon abiotic constraints, we bring new insights into our understanding of the mechanisms driving diversity - resistance relationships across climatic gradients.