We examined the effects of elevated CO2 and O3 and their interaction on leaf litter chemistry and decomposition in pure stands of aspen ( Populus tremuloides) and mixed stands of birch ( Betula papyrifera) and aspen at the Aspen Free Air CO2 Enrichment (FACE) experiment. A 935-day in situ incubation study was performed using litterbags filled with naturally senesced leaf litter. We found that elevated CO2 had no overall effects on litter decomposition rates, whereas elevated O3 reduced litter mass loss (−13%) in the first year. The effect of O3 on mass loss disappeared in the second year. For aspen litter but not mixed birch-aspen litter, decomposition rates were negatively correlated with initial concentrations of condensed tannins and phenolics. Most soluble components (94% of soluble sugars, 99% of condensed tannins, and 91% of soluble phenolics) and any treatment effects on their initial concentrations disappeared rapidly. However, the mean residence time (MRT) of birch-aspen litter (3.1 years) was significantly lower than that of aspen litter (4.8 years). Further, because of variation in total litterfall, total litter mass, C, lignin and N remaining in the ecosystem was highest under elevated CO2 and lowest under elevated O3 during the incubation period. Our results indicate that elevated CO2 and O3 can alter short-term litter decomposition dynamics, but longer-term effects will depend more on indirect effects mediated through changes in forest community composition. Treatment effects on soluble components are likely to influence cyclical microbial processes and carbon pulses in the ecosystem only when coupled with increased (CO2) or decreased (O3) litter inputs. [ABSTRACT FROM AUTHOR]