We examined the long-term dynamics of upland boreal forest floors after disturbance by fire. We selected two important and contrasting upland tree species, Pinus banksiana (jack fine) and Populus tremuloides (trembling aspen), in three distinct climatic zones across the boreal forest of western Canada, and sampled 80 fire-originated stands divided into six chronosequences with ages ranging from 14 to 149 years. The forest floor was a large component of carbon storage. Averaged across ages and zones, it was 1.31 and 2.78 kg C m-2 for P. banksiana and P. tremuloides, respectively, compared with 4.03 and 5.56 kg C m-2 in aboveground trees. These data exclude decomposing coarse woody debris, which was a significant component of the forest floor (0.18/0.13 kg C m-2) and requires further study. The contributions from shrubs (0.035/0.151 kg C m-2), ground vegetation (0.019/0.026 kg C m-2), and moss-plus-lichen (0.179/0.004 kg C m-2) were relatively small. An analysis of covariance (ANCOVA) model showed that forest floor carbon was positively related to stand age, as well as being affected by species and climate zone. Much of the variability was explained by species, and species-specific regression models showed that for P. tremuloides forest floor carbon was strongly related to stand age, mean annual temperature, and mean annual precipitation, and for P. banksiana, forest floor carbon was strongly related to an index of moss dominance. The regression models suggest that the forest floor carbon pool in upland forests of the western Canadian boreal will be sensitive to climate change, but this sensitivity would need to be tested with process-based models. © 1999 by the American Geophysical Union.