The net ecosystem productivity (NEP) of boreal aspen is strongly affected by comparative rates of annual potential evapotranspiration (E a ) and precipitation (P a ). Changes in E a versus P a during future climate change will likely determine changes in aspen NEP and consequently the magnitude of the carbon sink/source of a significant part of the boreal forest. We hypothesize that the effects of E a versus P a on aspen NEP can be modelled with a soil–root–canopy hydraulic resistance scheme coupled to a canopy energy balance closure scheme that determines canopy water status and thereby CO 2 uptake. As part of the ecosystem model ecosys, these schemes were used to model diurnal declines in CO 2 and latent heat (LE) exchange during a 3-year drought (2001–2003) at the Fluxnet-Canada Research Network (FCRN) southern old aspen site (SOA). These declines were consistent with those measured by eddy covariance (EC) at SOA, except that ecosystem CO 2 effluxes modelled during most nights were larger that those measured by EC or gap-filled from other EC measurements. Soil CO 2 effluxes in the model were close to, but sometimes smaller than, those measured by automated surface chambers at SOA. Diurnal declines in CO 2 exchange during the drought caused declines in annual NEP in the model, and in gap-filled EC measurements (model versus EC in gCm −2 : 275 versus 367±110 in 2001, 82 versus 144±43 in 2002 and 23 versus 104±31 in 2003). Lower modelled NEP was attributed to the larger modelled CO 2 effluxes. Ecosys was then used to predict changes in aspen net biome productivity (NBP=NEP−C lost from disturbance) caused by 6-year versus 3-year recurring droughts during 100-year fire cycles under current climate versus climate change projected under the IPCC SRES A1B scenario. Although NBP was adversely affected during recurring 6-year droughts under current climate, it recovered quickly during non-drought years so that long-term NBP was maintained at 4gCm −2 year −1 . NBP rose by 10, 108 and 126gCm −2 year −1 during the first, second and third centuries under climate change with recurring 3-year droughts, indicating a gradual rise in sink activity by boreal aspen. However recurring 6-year droughts during climate change caused recurring negative NBP (C losses), gradually depleting aspen C reserves and eventually causing dieback of the aspen overstory during the third century of climate change. This dieback was followed by a large decline in NBP.We conclude that NBP of boreal aspen will rise gradually under current projections of climate change, except under prolonged (e.g. 6 years) recurring droughts, which would eventually cause aspen to die back and substantial amounts of C to be lost.