The long held prediction that herbivore-plant interactions are more intense and defense traits are greater at low latitudes has increasingly been called into question. While a few studies find support for such gradients, the majority contradict this predicted biogeographical pattern, illustrating the need to further investigate the generality and contexts with which it may hold true. In this study, we tested for latitudinal clines in both herbivore pressure and the leaf defense traits of Quercus garryana. The Garry oak ecosystem in which Q. garryana is endemic, represents a unique system to test for biogeographic patterns in herbivory, given that populations are highly fragmented and the climate factors, such as precipitation, vary non-linearly across latitude. We quantified insect herbivore damage and multiple leaf traits for trees from 18 populations across the entire latitudinal range (850 km) of Quercus garryana var. garryana. We then explored the potential for latitude or alternate environmental factors, such as climate and population isolation, to explain observed trends in herbivore pressure and leaf traits.
Results/Conclusions
Although herbivory and leaf traits varied significantly among populations, we found latitude alone to be insufficient in explaining the observed variation. Only in conjunction with other climate variables were we able to characterize the variation in herbivory across the species range of Q. garryana. Specially, we found that herbivory was higher in populations with lower spring and summer precipitation, but showed no relations to temperature. Of our measured traits, specific leaf area showed no association with herbivory, while the density of trichomes and paramaters of tree size did. Our results contribute to our understanding of the biogeography of plant-herbivore interactions and the importance of incorporating local conditions, such as climate and population dynamics, and not simply latitude, to our understanding of such complex interactions.