Summary A process-based, general ecosystem model (BI-

OME--BGC) was used to simulate daily gross primary produc-tion, maintenance and heterotrophic respiration, net primary production and net ecosystem carbon exchange of boreal as-pen, jack pine and black spruce stands. Model simulations of daily net carbon exchange of the ecosystem (NEE) explained 51.7 % (SE = 1.32 g C m-2 day-1) of the variance in daily NEE derived from stand eddy flux measurements of CO2 during 1994. Differences between measured and simulated results were attributed to several factors including difficulties associ-ated with measuring nighttime CO2 fluxes and model assump-tions of site homogeneity. However, comparisons between simulations and field data improved markedly at coarser time-scales. Model simulations explained 66.1 % (SE = 0.97 g C m-2 day-1) of the variance in measured NEE when 5-day means of daily results were compared. Annual simulations of above-ground net primary production ranged from 0.6--2.4 Mg C ha-1 year-1 and were concurrent with results derived from tree increment core measurements and allometric equations. Model simulations showed that all of the sites were net sinks (0.1--4.1 Mg C ha-1 year-1) of atmospheric carbon for 1994. Older conifer stands showed narrow margins between uptake of carbon by net photosynthesis and carbon release through respiration. Younger stands were more productive than older stands, primarily because of lower maintenance respiration costs. However, all sites appeared to be less productive than temperate forests. Productivity simulations were strongly linked to stand morphology and site conditions. Old jack pine and aspen stands showed decreased productivity in response to simulated low soil water contents near the end of the 1994 growing season. Compared with the aspen stand, the jack pine stand appeared better adapted to conserve soil water through lower daily evapotranspiration losses but also exhibited a nar-rower margin between daily net photosynthesis and respira-tion. Stands subjected to water stress during the growing season may exist on the edge between being annual sources or sinks for atmospheric carbon