Effects of elevated carbon dioxide and ozone on the phytochemistry of aspen and performance of an herbivore

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.