The response of net ecosystem productivity (NEP) and evaporation in a boreal aspen (Populus tremuloides Michx.) forest and a black spruce (Picea mariana (Mill.) BSP) forest in Canada was compared using a newly developed realistic model of surface-atmosphere exchanges of carbon dioxide (CO2), water vapor, and energy as well as eddy covariance flux measurements made over a 6-year period (1994-1999). The model was developed by incorporating a process-based two-leaf (sunlit and shaded) canopy conductance and photosynthesis submodel in the Canadian Land Surface Scheme (CLASS). A simple submodel of autotrophic and heterotrophic respiration was combined with the photosynthesis model to simulate NEP. The model performed well in simulating half-hourly, daily, and monthly mean CO2 exchange and evaporation values in both deciduous and coniferous forests. Modeled and measured results showed a linear relationship between CO2 uptake and evaporation, and for each kilogram of water transpired, approximately 3 g of carbon (C) were photosynthesized by both ecosystems. The model results confirmed that the aspen forest was a weak to moderate C sink with considerable interannual variability in C uptake. In the growing season, the C uptake capacity of the aspen forest was over twice that of the black spruce forest. Warm springs enhanced NEP in both forests; however, high mid-summer temperatures appear to have significantly reduced NEP at the black spruce forest as a result of increased respiration. The model suggests that the black spruce forest is a weak C sink in cool years and a weak C source in warm years. These results show that the C balance of these two forests is sensitive to seasonal and interannual climatic variability and stresses the importance of continuous long-term flux measurement to confirm modeling results.