Impact of atmospheric change on insect-mediated ecosystem processes in a northern temperate forest

Atmospheric change is arguably one of the most pressing ecological and environmental issues the world currently faces. Two gases that have emerged as playing an important role in the sustainability of forest ecosystems are atmospheric carbon dioxide (CO₂) and tropospheric ozone (O ₃). Anthropogenic activities are increasing levels of these gases to the extent that major ecosystem processes are being altered. Little is known, however, about how elevated CO₂ and O₃ will influence the major consumers in forest ecosystems: herbivorous insects. To address this need, we followed canopy damage, organic substrate deposition, nitrogen flux, and the impact of insect damage on forest net primary productivity (NPP) over three years (2006-2008) in two forest community types at the Aspen FACE research site in northern Wisconsin, USA. Elevated CO₂ increased canopy damage rates and biomass removed by 87% and 120%, respectively, while elevated O₃ decreased canopy damage rates and biomass removed by 15% and 22%, respectively. Elevated CO₂ increased organic deposition and nitrogen flux from the canopy by 37% and 44%, respectively, while these processes decreased by 14% and 23%, respectively, under elevated O₃. Insect-mediated reductions in NPP were greater in elevated CO₂, compared with ambient CO₂, while the effect of insects on NPP under elevated O₃ was less than that of ambient O₃. Our findings demonstrate that elevated CO₂ and O₃ will alter multiple insect-mediated ecosystem processes in a northern temperate forest. Additionally, our findings suggest that the influence of insects on forest productivity will be altered under future atmospheres.