Opposing effects of elevated CO2 and N deposition on Lymantria monacha larvae feeding on spruce trees

The effects of elevated atmospheric CO and increased wet N deposition on le af quality and insect herbivory were evaluated in nine model ecosystems com posed of 7-year-old spruce trees (Picea abies) and three understorey specie s established on natural forest soil. Each model ecosystem was grown in a s imulated montane climate, and was exposed to one of three CO2 concentration s (280, 420, and 560 mu l l(-1)) and to one of three levels of N deposition (0, 30, and 90 kg ha(-1) year(-1)) for 3 years. In the 3rd year of the exp eriment second to third instars of the nun moth (Lymantria monacha) were al lowed to feed directly on current-year needles of top canopy branches of ea ch tree for 13 days. Specific leaf area (SLA), water content, and N concent ration decreased in needles exposed to elevated CO2, whereas the concentrat ions of starch, condensed tannins, and total phenolics increased. Increased N deposition had no significant effect on SLA, and water content, but the concentrations of starch, condensed tannins, and total phenolics decreased, and sugar and N concentrations increased. Despite higher relative consumpt ion rates (RCRs) larvae consumed 33% less N per unit larval biomass and per day at the two high CO2 treatments, compared to those feeding on 280 mu l l(-1)-needles, but they maintained similar N accumulation rates due to incr eased N utilization efficiencies (NUE). However, over the 12-day experiment al period larvae gained less N overall and reached a 35% lower biomass in t he two high-CO2 treatments compared to those at 280 mu l l(-1). The effects of increased N deposition on needle quality and insect performance were ge nerally opposite to those of CO enrichment, but were lower in magnitude. We conclude that altered needle quality in response to elevated CO2 will impa ir the growth and development of L. monacha larvae. Increasing N deposition may mitigate these effects, which could lead to altered insect herbivore d istributions depending on regional patterns of N deposition.