COS 111-3 - CANCELLED - Investigating effects of nitrogen deposition on oak chemistry and insect herbivory across a deposition gradient in the Columbia River Gorge

Abstract: Anthropogenic nitrogen (N) deposition is a major driver of global change and threat to plant biodiversity. The effects of N deposition on interactions between plants and insect herbivores is not well understood, particularly outside of experimental studies at ecologically relevant levels of long-term N deposition in natural areas. Our goal was to investigate insect herbivory and foliar nutritional quality in Oregon white oak (Quercus garryana) along a known N deposition gradient in the Columbia River Gorge National Scenic Area (Oregon and Washington State, USA). We explored the spatial patterns of N deposition across Oregon white oak habitat using lichen (Evernia prunastri) N content as an index for N deposition, and quantified insect herbivore damage and foliar nutrient content from 31 sites.

Using linear mixed effects and generalized linear mixed models, we analyzed the effects of distance to major highways, surrounding developed area, and environmental variables (e.g., mean annual precipitation and elevation) on lichen and oak nutrient content, oak specific leaf area, and guild-specific herbivore damage. Our results show that lichen %N was substantially increased and carbon-to-nitrogen ratio (C:N) was decreased in sites with higher development and proximity to major highways. Oak foliar C:N, skeletonizing herbivore damage, and mining herbivore damage was lower in sites with higher surrounding development. Oak %N, chewing, and gall damage did not show any response to our indices of N deposition across our study area. The results of our study support findings that N deposition can decrease foliar C:N content and shift plant-insect herbivore interactions. Our study also furthers our understanding of guild-specific herbivore responses to N deposition and contributes a real-world example of an ecological community response to chronic, low levels of N deposition.