A model of the daily carbon balance of a black spruce/feathermoss boreal forest ecosystem was developed and results compared to preliminary data from the 1994 BOREAS field campaign in northern Manitoba, Canada. The model, driven by daily weather conditions, simulated daily soil climate status (temperature and moisture profiles), spruce photosynthesis and respiration, moss photosynthesis and respiration, and litter decomposition. Model agreement with preliminary field data was good for net ecosystem exchange (NEE), capturing both the asymmetrical seasonality and short-term variability. During the growing season simulated daily NEE ranged from -4 g C m-2 d-1 (carbon uptake by ecosystem) to + 2 g C m-2 d-1 (carbon flux to atmosphere), with fluctuations from day to day. In the early winter simulated NEE values were + 0.5 g C m-2 d-1, dropping to + 0.2 g C m-2 d-1 in mid-winter. Simulated soil respiration during the growing season (+ 1 to + 5 g C m-2 d-1) was dominated by metabolic respiration of the live moss, with litter decomposition usually contributing less than 30% and live spruce root respiration less than 10% of the total. Both spruce and moss net primary productivity (NPP) rates were higher in early summer than late summer. Simulated annual NEE for 1994 was -51 g C m-2 y-1, with 83% going into tree growth and 17% into the soil carbon accumulation. Moss NPP (58 g C m-2 y-1) was considered to be litter (i.e. soil carbon input; no net increase in live moss biomass). Ecosystem respiration during the snow-covered season (84 g C m-2) was 58% of the growing season net carbon uptake. A simulation of the same site for 1968-1989 showed ≈ 10-20% year-to-year variability in heterotrophic respiration (mean of + 113 g C m-2 y-1). Moss NPP ranged from 19 to 114 g C m-2 y-1; spruce NPP from 81 to 150 g C m-2 y-1; spruce growth (NPP minus litterfall) from 34 to 103 g C m-2 y-1; NEE ranged from +37 to -142 g C m-2 y-1. Values for these carbon balance terms in 1994 were slightly smaller than the 1969-89 means. Higher ecosystem productivity years (more negative NEE) generally had early springs and relatively wet summers; lower productivity years had late springs and relatively dry summers. © 1996 Blackwell Science Ltd.