Temporal variations of net primary production (NPP) and net ecosystem production (NEP) in west central Canadian forests over the period of 19201995 and their responses to natural and anthropogenic disturbances were simulated using the Carbon Budget Model of the Canadian Forest Sector (CBM-CFS2). The results show that forest NPP in the region was 215 g C·year1·m2 in 1920, varied between 105 and 317 g·C year1·m2 depending on ecoclimatic province, but gradually increased to 330 (158 to 395) g C·year1·m2 in the early 1980s before declining to 290 (148 to 395) g C·year1·m2 by 1995. Forest NEP was estimated to be 53 (13 to 88) g C·year1·m2 in 19201924, increased to 75 (5 to 98) g C·year1·m2 in 1960, and then declined to 26 (14 to 53) g C·year1·m2 in 19911995. Natural disturbances played a greater role than harvest in determining the temporal pattern of forest NPP and NEP during the period because of the larger area affected by natural disturbances. This study also indicated that ignoring disturbances would lead to an overestimation of forest NPP and NEP in ecosystem modeling.