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

The effects of elevated atmospheric CO₂ and increased wet N deposition on leaf quality and insect herbivory were evaluated in nine model ecosystems composed of 7-year-old spruce trees (Picea abies) and three understorey species established on natural forest soil. Each model ecosystem was grown in a simulated montane climate, and was exposed to one of three CO₂ concentrations (280, 420, and 560 μl 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 experiment second to third instars of the nun moth (Lymantria monacha) were allowed to feed directly on current-year needles of top canopy branches of each tree for 12 days. Specific leaf area (SLA), water content, and N concentration decreased in needles exposed to elevated CO₂, whereas the concentrations 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 consumption 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 μl l^-1-needles, but they maintained similar N accumulation rates due to increased N utilization efficiencies (NUE). However, over the 12-day experimental period larvae gained less N overall and reached a 35% lower biomass in the two high-CO₂ treatments compared to those at 280 μl l^-1. The effects of increased N deposition on needle quality and insect performance were generally opposite to those of CO₂ enrichment, but were lower in magnitude. We conclude that altered needle quality in response to elevated CO₂ will impair the growth and development of L. monacha larvae. Increasing N deposition may mitigate these effects, which could lead to altered insect herbivore distributions depending on regional patterns of N deposition.