Abiotic factors promote plant heterogeneity and influence herbivore performance and mortality in Gambel's oak (Quercus gambelii)

Environmental gradients are expected to alter the relative effects of host‐plants and natural enemies on phytophagous insects. Moreover, studies of gradients may assist in an identification of the factors important to the outcomes of trophic interactions. We investigated the role of elevation‐based variation in environmental conditions (temperature and relative humidity) and foliar nitrogen on tri‐trophic interactions in Quercus gambelii Nutt. (Fagaceae) during 2001 and 2002. Quercus gambelii displayed significant elevational and seasonal fluctuation in foliar nitrogen content, and sites with similar environmental conditions produced similar foliar quality. However, leaf‐miners, Phyllonorycter spec. (Lepidoptera: Gracillariidae) and Cameraria spec. (Lepidoptera: Gracillariidae), did not perform better on trees with a greater nitrogen content. Overall densities of both species declined significantly in 2002, most likely due to severe drought conditions in the south‐western USA. Both species exhibited significant, but distinct, patterns in emergence rate with elevation. While environmentally based fluctuation in foliar nitrogen failed to predict the result of trophic interactions, site environmental conditions, as measured by temperature and relative humidity, were strongly related to differences in leaf‐miner performance and mortality. The ordination of sites by variation in environmental conditions accurately predicts the relative effect of unexplained vs. natural‐enemy sources of mortality for leaf‐miners.