Individual and combined effects of simulated drought stress and nitrogen deposition on red maple (Acer rubrum) genotypes and their insect herbivores

Trees improve human health and support biodiversity in cities. Cities are hotspots of atmospheric reactive nitrogen (Nr) deposition, but the influence of Nr deposition on urban trees and their insect herbivores is not well understood. Previous work has shown that excess Nr deposition increases tree foliar N content and exacerbates drought stress. Urban trees experience drought stress caused by soil compaction, impervious surfaces, and the urban heat island effect. Drought stress also increases foliar N, and foliar nutritional quality for N-limited herbivores may be further enhanced where drought stress and Nr deposition co-occur. Nr deposition reduces insect diversity and increases pest outbreaks in natural ecosystems, and we hypothesize that similar patterns may occur in urban ecosystems.

The purpose of this study was to disentangle the effects of simulated drought stress and Nr deposition on popular red maple (Acer rubrum) genotypes and their herbivores in a common garden experiment. We present two years of data demonstrating treatment effectiveness, tree physiological response, and piercing/sucking insect abundance. While genotypes significantly differed in their response to drought stress, we do not find evidence that differences were mediated by N treatments, and we do not find significant differences in piercing/sucking insect herbivore abundance. Future analyses will incorporate measurements of tree growth, photosynthesis, respiration, and foliar N content, as well as evidence of pest damage. Results of this study will improve understanding of how urbanization and Nr deposition influence urban trees and their associated herbivores, and will inform efforts to meet ecosystem service goals in cities.