Forest ecosystems contain several climate-sensitive drivers that respond differentially to changes in climate and climate variability: For example, growth and regeneration processes are “stand-scale” drivers, while natural disturbances operate at “landscape-scale”. The relative contributions of these different scale drivers of change in ecosystems create great uncertainty when simulating potential responses of a forest to changes in climate. Here we assess those contributions, along with harvesting effects, on biomass (both total and of individual species) in the southern boreal forest of Canada under three climate scenarios (RCP 2.6, RCP 4.5 and RCP 8.5). Projections were performed for three future 30-year time periods, in four study regions located on an east-west transect, using a forest landscape model (LANDIS-II), parameterized using a forest patch model (PICUS). Projected future impacts were assessed for each driver of change, and found to vary greatly among regions, species, future period, and forcing scenarios. Fire, and stand-scale climate-induced impacts, had the strongest effects on forest vegetation, as well as on total and species’ biomass under most RCP scenarios, but the largest impacts occurred mostly after 2050, particularly with the RCP 8.5 scenario. The relative importance and trends in species-specific impacts varied, both spatially and according to the different RCP scenarios. Western regions were generally more sensitive to stand-scale climate-induced changes whereas eastern regions were more sensitive to changes in fire regime. Our study also highlights the importance of considering the prevalence of species-level functional traits when assessing the sensitivity of forest landscapes to a given driver of change in the context of increasing anthropogenic climate forcing.