Impacts of elevated carbon dioxide and ozone on community herbivory in a northern temperate forest

The purpose of my dissertation was to evaluate the effects of elevated levels of atmospheric carbon dioxide (CO2) and tropospheric ozone (O3) on plant-herbivore interactions in a maturing northern temperate forest. This research was conducted at Aspen Free Air Carbon dioxide and ozone Enrichment (FACE), a 32 ha research facility located near Rhinelander, in northern Wisconsin, USA. Field and laboratory assays were conducted to assess the impact of elevated CO2 and O3 on trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) phytochemistry and the attendant effects on herbivorous insects.;My results showed that both elevated CO2 and O3 independently and interactively altered aspen and birch phytochemistry; responses under elevated CO2 were generally less pronounced than those under elevated O3 and greater in aspen than in birch. Elevated CO2 and O3 also independently and interactively altered the performance of major defoliators of northern temperate forests. Elevated CO2 generally increased, while elevated O3 decreased, herbivore performance; the magnitudes of increase and decrease were larger on aspen than on birch. Canopy damage rates at the community level generally increased under elevated CO2 and decreased under elevated O 3. The magnitudes of increase and decrease in canopy damage under elevated CO2 and O3 were larger and exhibited greater across-year consistency in aspen than in birch. The aspects of phytochemistry that were most responsive to elevated CO2 and O3 also exhibited the strongest relationships with performance of individual insects and rates of community-level damage. Phytochemical variation under elevated CO 2 and O3 explained variation in rates of community-level damage in aspen, but not in birch. This finding suggests that combinations of factors (i.e., biotic and abiotic) will likely drive patterns of canopy damage in northern temperate forests in a manner specific to tree species.;This body of research dissertation demonstrates that atmospheric change will alter plant-insect interactions in future environments. Additionally, this research suggests that responses by herbivorous insects to elevated CO 2 and O3 will have the potential to influence the functioning of forest ecosystems.