Ecologists have limited understanding of how geographic variation in forest carbon arises from differences in growth and mortality at continental to global scales. Using forest inventory data from across North America, we partitioned continental-scale variation in biomass growth and mortality rates of 49 tree species groups into (1) species-independent spatial effects and (2) inherent differences in demographic performance among species groups. 82% and 51% of the respective variation in growth and mortality was explained by spatial factors that were independent of species-group composition. Moderate additional variation in mortality (26%) was related to species-group turnover across the continent. Biomass accumulation models showed that variation in forest biomass can be explained primarily by spatial gradients in growth that were unrelated to species-group composition. Species-dependent patterns of mortality explained additional variation in biomass, with forests supporting less biomass when dominated by species groups that are highly susceptible to competition or to biotic disturbances.