The black spruce-moss forest is the largest bioclimatic domain in Québec (Canada), extending east from the Abitibi region to the Basse-Côte-Nord region. We documented changes in climate, ecological disturbance, surface deposit, topography and vegetation cover along a 1 000-km longitudinal transect through this domain. In the western portion, where the drier continental climate favours wildfire, black spruce and mixed black spruce–jack pine forests predominate. In the central portion, black spruce–balsam fir forests increase in importance. Further east, the more humid climate of the Côte-Nord region, where wildfires are less frequent, favours pure balsam fir and balsam fir–black spruce forests, which have been heavily affected by insect outbreaks. The proportion of mature stands (>100 years) is highest in this region. Lichen woodlands and lichen-heaths are more abundant east of Sept-Îles. The extent of the changes observed shows that the longitudinal gradient is just as important as the latitudinal gradient in explaining the distribution of plant communities in the boreal forest, although the latter has been studied more often.