Regional and global changes in the composition of Earth’s atmosphere are impacting the productivity of forest ecosystems. Elevated concentrations of CO₂ and O₃ are of particular interest because of their opposing effects on tree productivity. Tree productivity will also be influenced by the effects of CO₂ and O₃ on insects, the most abundant herbivores in forest ecosystems. We investigated how elevated CO₂ and O₃ alter the abundance and diversity of insect communities, as well as their ecosystem-level impacts (canopy defoliation and organic substrate deposition) in aspen and birch forests at the Aspen FACE (Free Air CO₂ Enrichment) site in northern Wisconsin, USA. We examined the effects of elevated CO₂ and O₃ on tree chemical composition (nutrients and defensive chemistry), foliar damage rates, and substrate (frass and greenfall) deposition from 2006 to 2008. The insect community was simultaneously sampled using a combination of visual surveys for herbivores and pan traps placed on the forest floor to capture natural enemies. 

Results/Conclusions

Elevated CO₂ and O₃ had tree species-specific, negligible to modest impacts on foliar chemistry. Examination of more than 42,000 leaves revealed that insect damage rates (% leaf area) increased 77% in elevated CO₂ stands and decreased 15% in elevated O₃ stands, averaged across years and tree species. The amount (mass) of foliar biomass removed by insects showed similar results. Frass and greenfall deposition (g*m^-2*yr^-1) increased 37% in elevated CO₂ stands and decreased 21% in elevated O₃ stands, averaged across years and tree species. Transfer of nitrogen (g*m^-2*yr^-1) from the canopy to the forest floor increased 39% in elevated CO₂ and decreased 19% in elevated O₃. Analysis of more than 85,000 insects revealed species-specific, minor to moderate CO₂ and O₃ effects on chewing herbivore abundance and community composition but stronger effects on natural enemy abundance and community composition. Clearly, the dynamics of tree-insect interactions will shift in forests of the future. These changes will be reflected in modified forest productivity, insect community composition, and ecosystem structure and function.