Although boreal forests are currently sinks for atmospheric C, there is some concern that they may not remain so under hypothesized warming of the boreal climate. The ecosystem model ecosys was used to evaluate possible changes in ecosystem C exchange and accumulation under changes in atmospheric CO2 concentration (Ca) proposed in emissions scenario IS92a, and accompanying changes in air temperature and precipitation proposed by general circulation models running under IS92a. Ecosys was first tested under current climate by comparing modelled rates of C exchange and accumulation with those measured in a mixed aspen–hazelnut stand in central Saskatchewan. The model was then run with daily increments of Ca, temperature and precipitation, and differences in C exchange and accumulation between current and changing climates were evaluated. Model results indicated that over a 120-y period, a mixed aspen–hazelnut stand currently accumulates about 14 kg C m−2. Under the hypothesized changes in climate this stand would accumulate an additional 8.5 kg C m−2, largely through higher rates of CO2 fixation and longer growing seasons under higher Ca and temperature. This additional accumulation would be entirely as aspen wood, while soil organic matter would change little. This accumulation would therefore be vulnerable to losses from fire and insects.