COS 35-4 - Elevated ozone alters poplar phytochemistry and performance traits for an outbreak forest insect

Background/Question/Methods

Insect-mediated biomass loss is a major factor impacting the carbon sequestration potential of forests. Insect herbivory is likely to be affected by environmental changes, such as increased ground-level ozone (O₃) which has become an important environmental problem in Asia. Ozone is toxic to insects and can alter the chemical composition of host plants. However, little is known about how elevated O₃may alter insect feeding through induced phytochemical changes. Using open-top chambers (OTCs) and a free air O₃concentration enrichment (O₃-FACE) facility, we evaluated the effects of elevated O₃ on the phytochemical composition of hybrid poplar (Populus euramericanacv. ‘74/76’ and P. deltoidescv. ‘55/56’ × P. deltoidescv. ‘Imperial’) foliage, and the relationship between O₃-altered foliar phytochemistry and consumption by fall webworm (Hyphantria cunea). We measured foliar chemistry at five different O₃exposure levels (exposure duration ≥90 days), and performed insect feeding assays on foliage with a history (2 years) of ambient and 1.5 × ambient O₃exposure. Effects of O₃on phytochemistry were evaluated using analysis of variance (ANOVA) and principal component analysis in conjunction with multiple analysis of variance; relationships between phytochemical variations and insect performance were evaluated using ANOVA and partial least squares regression.

Results/Conclusions

O₃exposures at 40 ppb above ambient altered foliar nutritional quality, increasing nitrogen, sugar, and lignin and decreasing starch. A gradient of increasing O₃exposure caused nonlinear changes in foliar phenolics, with concentrations initially increasing and then decreasing in accordance with a biphasic exposure-response profile. This response pattern was driven by changes in phenylpropanoids with high antioxidative capacity (e.g. condensed tannins) but not in phenolics with low antioxidative capacity (e.g. salicinoids). Results of a bioassay involving H. cunea showed that exposure of hybrid Populusto elevated O₃shortened larval development time, decreased food conversion efficiency, and increased total leaf consumption. These results suggest compensatory insect feeding in response to reduced host quality under elevated O₃. Within the range of observed phytochemical variation, concentrations of nitrogen, starch, and condensed tannins were strongly associated with differences in the performance of larvae. Collectively, our study revealed that increasing ground-level O₃will alter insect feeding and performance, likely mediated through concentration-dependent effects of O₃on phytochemistry. Our results further suggest that ignoring O₃dose-response relationships may lead to inaccurate prediction of the ecological effects of O₃pollution, thereby limiting the utility of some approaches to forecasting the future impacts of increasing air pollution.