There is some uncertainty whether net ecosystem productivity (NEP) of boreal black spruce forests is positive or negative under current climates and how NEP would change under hypothesized changes in future climates. The mathematical model ecosys was used to examine NEP of a boreal black spruce forest by testing rates of C transfer simulated from an integrated set of scientific hypotheses against rates measured with chamber, micrometeorological, isotopic and allometric techniques at the northern old black spruce site of the Boreal Ecosystem-Atmosphere Study (BOREAS). Daily aggregations of modeled and measured hourly C fluxes indicated that NEP rose with solar radiation if air temperatures remained below 15 ¿C, but declined if air temperatures rose above 20 ¿C. Daily NEP was thus largest under the higher radiation and lower temperatures of June, but declined under the lower radiation and higher temperatures of July and August. Yearly aggregations of modeled hourly C fluxes indicated that under current (1994--1996) climate NEP of the 150 year old northern old black spruce site was about 40(wood)+15(soil)=55 g C m-2 yr-1. Moss was estimated to contribute about 0.25 of net primary productivity (NPP) at this site and was thus an important component of ecosystem C exchange. The model was then used to predict changes in NEP at the old northern old black spruce site under the IS92a climate change scenario. After 150 years NEP was predicted to be 120 (wood) minus 10(soil)=110 g C m-2 yr-1. Cumulative gains of C under this scenario were predicted to be 5700 (wood) minus 700(soil)=5000 g C m-2 after 150 years. Gains of wood C would be vulnerable to loss by fire and insects, both of which could also increase under warmer climates. ¿ 2001 American Geophysical Union