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

Abstract.

The purpose of this study was to assess the independent and interactive effects of CO₂, O₃, 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 O₃ 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 µl/l), elevated ozone (+O₃; ambient ×1.5), and elevated CO₂+O₃. We measured the effects of CO₂ and O₃ on aspen phytochemistry and on performance of forest tent caterpillar (Malacosoma disstria Hübner) larvae. CO₂ and O₃ treatments influenced foliar quality for both genotypes, with the most notable effects being that elevated CO₂ reduced nitrogen and increased tremulacin levels, whereas elevated O₃ 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 +O₃ treatment, but this response was negated by the addition of elevated CO₂ (i.e., +CO₂+O₃ treatment). We conclude that tent caterpillars will have the greatest impact on aspen under current CO₂ and high O₃ 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 O₃ levels, due to moderate changes in insect performance and increases in tree growth.