We used eddy covariance; gas-exchange chambers; radiocarbon analysis; wood, moss, and soil inventories; and laboratory incubations to measure the carbon balance of a 120-year-old black spruce forest in Manitoba, Canada. The site lost 0.3 ± 0.5 metric ton of carbon per hectare per year (ton C ha⁻¹ year⁻¹) from 1994 to 1997, with a gain of 0.6 ± 0.2 ton C ha⁻¹year⁻¹ in moss and wood offset by a loss of 0.8 ± 0.5 ton C ha⁻¹ year⁻¹ from the soil. The soil remained frozen most of the year, and the decomposition of organic matter in the soil increased 10-fold upon thawing. The stability of the soil carbon pool (∼150 tons C ha⁻¹) appears sensitive to the depth and duration of thaw, and climatic changes that promote thaw are likely to cause a net efflux of carbon dioxide from the site.