The purpose of this study was to assess the independent and interactive effects Of CO2, O-3, and plant genotype on the foliar quality of a deciduous tree and the performance of a herbivorous insect. Two trembling aspen (Populus tremuloides Michaux) genotypes differing in response to CO2 and O-3 were grown at the Aspen FACE (Free Air CO2 Enrichment) site located in northern Wisconsin, USA. Trees were exposed to one of four atmospheric treatments: ambient air (control), elevated carbon dioxide (+CO2; 560 mul/l), elevated ozone (+O-3; ambient x 1.5), and elevated CO2+O-3. We measured the effects Of CO2 and O-3 on aspen phytochemistry and on performance of forest tent caterpillar (Malacosoma disstria Hubner) larvae. CO2 and O-3 treatments influenced foliar quality for both genotypes, with the most notable effects being that elevated CO2 reduced nitrogen and increased tremulacin levels, whereas elevated O-3 increased early season nitrogen and reduced tremulacin levels, relative to controls. With respect to insects, the +CO2 treatment had little or no effect on larval performance. Larval performance improved in the +O-3 treatment, but this response was negated by the addition of elevated CO2 (i.e., +CO2+O-3 treatment). We conclude that tent caterpillars will have the greatest impact on aspen under current CO2 and high O-3 levels, due to increases in insect performance and decreases in tree growth, whereas tent caterpillars will have the least impact on aspen under high CO2 and low O-3 levels, due to moderate changes in insect performance and increases in tree growth.
