An ecophysiological model of photosynthesis and respiration by forest ecosystems was used to examine CO2 fluxes in 23 mature boreal forests near Fairbanks, Alaska. Simulated soil respiration, photosynthesis, decomposition, and moss and tree productivity were consistent with observed data. Monthly ecosystem CO2 flux and net photosynthesis, averaged over the 23 sites, were correlated with atmospheric CO2 concentrations and Δ13 ratios, respectively, at Barrow, Alaska, suggesting the boreal forests of Alaska play an active role in the seasonal dynamics of atmospheric CO2 at Barrow. Only one of the 23 stands was a source of CO2, and the 23 stands absorbed (mean¿SE) 1173¿211 g CO2 m-2 yr-1. Observed productivity in these forests spans the range of productivity in the circumpolar boreal forest, suggesting the simulated CO2 fluxes are representative of the circumpolar boreal forest. If so, metabolic activity in the circumpolar boreal forest results in a significant annual uptake of CO2.