Nitrogen deposition effects on Oregon white oak foliar chemistry and insect herbivory in the Columbia River Gorge, USA

Background/Question/Methods

Anthropogenic nitrogen (N) deposition is a main driver of plant biodiversity loss in many regions but effects on plant-insect herbivore interactions are not well understood. Nitrogen deposition often increases foliar N content which can increase herbivore abundance, but these effects have not been well documented outside of manipulative experimental studies in natural habitats experiencing chronic, low-level N deposition. We investigated the effects of N deposition on Oregon white oak (Quercus garryana) foliar chemistry and insect herbivory along a known N deposition gradient in the Columbia River Gorge National Scenic Area (Oregon and Washington State, USA). We measured N content of oak moss lichen (Evernia prunastri) at 42 sites as an index of N deposition across our study area, and collected Q. garryana foliar samples at each site to measure carbon and nitrogen content, specific leaf area, trichomes, and herbivore damage from chewing, skeletonizing, mining, and galling insects.

Results/Conclusions

Preliminary results show N deposition as indicated by lichen N content is correlated with a 5% decrease in Q. garryana foliar C:N ratio across our study area, suggesting Q. garryana could be more palatable and nutritious to insect herbivores under elevated N deposition. We will present results of the relationship between physical and chemical foliar traits of Q. garryana with abundance and composition of herbivore damage. Our results thus far support previous findings that chronic, low-level N deposition can lead to shifts in foliar chemistry associated with increased herbivory.