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.
