Consequences of elevated carbons 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 eco systems. We assessed the independent and interactive effects of CO2 and O-3 fumigation on foliar chemistry of quaking aspen (Populus tremuloides) and paper birch (Betula papyrifera) at a Free-Air CO2 Enrichment (FACE) facilit y in northern Wisconsin. Leaf samples were collected at five time, periods during a single growing season, and analyzed for nitrogen, starch and conde nsed tannin concentrations, nitrogen resorption efficiencies (NREs), and C: N ratios. Enriched CO2 reduced foliar nitrogen concentrations in aspen and birch; O-3 only marginally reduced nitrogen concentrations. NREs were unaff ected by pollution treatment in aspen, declined with O-3 exposure in birch, and this decline was ameliorated by enriched CO2- C:N ratios of abscised l eaves increased in response to enriched CO2 in both tree species. O-3 did n ot significantly alter C:N ratios in aspen, although values tended to be hi gher in + CO2 + O-3 leaves. For birch, O-3 decreased C:N ratios under ambie nt CO2 and increased C:N ratios under elevated CO2. Thus, under the combine d pollutants. the C:N ratios of both aspen and birch leaves were elevated a bove the averaged responses to the individual and independent trace gas tre atments. Starch concentrations were largely unresponsive to CO2 and O-3 tre atments in aspen, but increased in response to elevated CO2 in birch. Level s of condensed tannins were negligibly affected by CO2 and O-3 treatments i n aspen, but increased in response to enriched CO, in birch. Results from t his work suggest that changes in foliar chemical composition elicited by en riched CO2 are likely to impact herbivory and decomposition, whereas the ef fects Of O-3 are likely to be minor, except in cases where they influence p lant response to CO2. (C) 2001 Elsevier Science Ltd. All rights reserved.