Elevated CO 2 resulted in changes in chemical foliar composition that are likely to impact herbivory and decomposition Atmospheric chemical composition affects foliar chemical composition, which in turn influences the dynamics of both herbivory and decomposition in ecosystems. We assessed the independent and interactive effects of CO 2 and O 3 fumigation on foliar chemistry of quaking aspen ( Populus tremuloides) and paper birch ( Betula papyrifera) at a Free-Air CO 2 Enrichment (FACE) facility in northern Wisconsin. Leaf samples were collected at five time periods during a single growing season, and analyzed for nitrogen, starch and condensed tannin concentrations, nitrogen resorption efficiencies (NREs), and C:N ratios. Enriched CO 2 reduced foliar nitrogen concentrations in aspen and birch; O 3 only marginally reduced nitrogen concentrations. NREs were unaffected by pollution treatment in aspen, declined with O 3 exposure in birch, and this decline was ameliorated by enriched CO 2. C:N ratios of abscised leaves increased in response to enriched CO 2 in both tree species. O 3 did not significantly alter C:N ratios in aspen, although values tended to be higher in +CO 2+O 3 leaves. For birch, O 3 decreased C:N ratios under ambient CO 2 and increased C:N ratios under elevated CO 2. Thus, under the combined pollutants, the C:N ratios of both aspen and birch leaves were elevated above the averaged responses to the individual and independent trace gas treatments. Starch concentrations were largely unresponsive to CO 2 and O 3 treatments in aspen, but increased in response to elevated CO 2 in birch. Levels of condensed tannins were negligibly affected by CO 2 and O 3 treatments in aspen, but increased in response to enriched CO 2 in birch. Results from this work suggest that changes in foliar chemical composition elicited by enriched CO 2 are likely to impact herbivory and decomposition, whereas the effects of O 3 are likely to be minor, except in cases where they influence plant response to CO 2.