Atmospheric change alters frass quality of forest canopy herbivores

Abstract

This study examined the independent and interactive effects of elevated atmospheric carbon dioxide (CO2) and tropospheric ozone (O3) on the foliar and litter chemistry of two deciduous tree species and the frass chemistry of four lepidopteran folivores. Trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera) were grown under elevated levels of CO2 and/or O3 at the Aspen FACE research site in northern WI, USA. We measured the effects of CO2 and O3 on nitrogen, carbon to nitrogen (C:N), and condensed tannin levels in aspen and birch leaves and senescent litter and also in the frass of folivores fed aspen or birch green leaves. Overall, the effects of elevated CO2 on foliar chemistry were less pronounced than those of elevated O3, and aspen responded more strongly than birch. While the effects of elevated CO2 and O3 on foliar chemistry were generally reflected in frass chemistry, the magnitude of the response varied among insect species. Insect frass had higher nitrogen and condensed tannin levels and lower C:N ratios than did litter, although the magnitude of this response varied among fumigation treatments and insect species. Our findings demonstrate that the quality of insect-mediated organic deposition can be indirectly affected by atmospheric change, through altered foliar quality. Our findings also suggest that the quality of frass deposited on the forest floor via herbivory will be strongly affected by herbivore community composition.