Seasonal cycles of atmospheric CO2 and δ18O-CO2 at high northern latitudes have the potential to serve as indicators of ecological change in response to climate changes. Effective interpretation of these observations requires an understanding of how different species and ecosystems contribute to biosphere-atmosphere exchange. Here we examined the effect of postfire stand age in boreal forest ecosystems on the seasonal distribution of CO2 and δ18O-CO2 fluxes. We measured net CO2 fluxes in a 3-year burn scar, a 15-year trembling aspen stand, and an 80-year black spruce stand in interior Alaska using eddy covariance. By combining measurements of the oxygen isotopic composition of ecosystem water pools at each stand with measured CO2 fluxes, we predicted half-hourly δ18O-CO2 fluxes and used a one-box atmosphere model to make relative comparisons of the effect of stand age on the shape and amplitude of the seasonal cycle of CO2 and δ18O-CO2. A shorter growing season and higher rates of net ecosystem uptake during midsummer at the 15-year stand resulted in a larger seasonal CO2 amplitude and a delay in the drawdown of atmospheric CO2 as compared with the 80-year stand. Reduced levels of gross primary production isoforcing from the 15-year stand during spring and early summer caused atmospheric δ18O-CO2 to increase more gradually between April and June as compared with fluxes from the 80-year stand. Our analysis suggests that increased boreal forest disturbance would delay the phase of CO2 drawdown at high northern latitudes, but would advance the phase of δ18O-CO2 drawdown.