Elevated concentrations of atmospheric carbon dioxide and ozone are anticipated to significantly impact the health and sustainability of forest ecosystems. Although considerable research has explored how those gases may alter tree-insect interactions at the level of individual insects, little work has investigated the effects of CO2 and O3 on forest insect communities. Fewer still have assessed the composite effects of changes in individual insect performance and community composition on stand-level processes such as primary production, defoliation and organic substrate deposition. We explored those relationships at the Aspen Free Air CO2 Enrichment site (Wisconsin, U.S.A.), where the independent and interactive effects of elevated CO2 and O3 on aspen-birch forests were evaluated. We assessed insect numbers and diversity, and measured canopy defoliation rates and organic substrate deposition, over three summers. We estimated the influence of changes in herbivory under elevated CO2 and O3 on stand productivity using a light-use efficiency (ε) model adjusted to account for herbivory.
Enriched CO2 and O3 environments elicited changes in insect feeding behavior and population densities such that canopy damage increased 88% under elevated CO2 and decreased 16% under elevated O3. Insect-mediated substrate (frass and greenfall) deposition and nitrogen flux increased 36 and 38%, respectively, under CO2. While transfer of substrate from the canopy to the forest floor was not affected by elevated levels of O3, nitrogen deposition decreased 19% in enriched O3 environments. Enriched CO2 and O3 environments also changed the chemical quality of insect frass. The loss of actual and potential productivity due to herbivory increased by167% under elevated CO2, while the negative effect of herbivory decreased by 22% under elevated O3. Our results demonstrate that levels of atmospheric CO2 and O3 predicted for 40 years in the future will affect even nominal levels of herbivory to an extent that forest primary production and nutrient transfers are altered, and the C sequestration capacity of forests may be compromised.