Consequences of elevated carbon dioxide and ozone for foliar chemical composition and dynamics in trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera)

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 CO₂ and O₃ fumigation on foliar chemistry of quaking aspen (Populus tremuloides) and paper birch (Betula papyrifera) at a Free-Air CO₂ 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 CO₂ reduced foliar nitrogen concentrations in aspen and birch; O₃ only marginally reduced nitrogen concentrations. NREs were unaffected by pollution treatment in aspen, declined with O₃ exposure in birch, and thisdecline was ameliorated by enriched CO₂. C:N ratios of abscised leaves increased in response to enriched CO₂ in both tree species. O₃ did not significantly alter C:N ratios in aspen, although values tended to be higher in +CO₂+O₃ leaves. For birch, O₃ decreased C:N ratios underambient CO₂ and increased C:N ratios under elevated CO₂. 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 CO₂ and O₃treatments in aspen, but increased in response to elevated CO₂ in birch. Levels of condensed tannins were negligibly affected by CO₂ and O₃ treatments in aspen, but increased in response to enriched CO₂ in birch. Results from this work suggest that changes in foliar chemical composition elicited by enriched CO₂ are likely to impact herbivory and decomposition [ABSTRACT FROM AUTHOR]