Settlement of eastern North America has generated significant modifications in forest composition. In regions highly influenced by human activity, historical ecology can be used to reconstruct pre-settlement forest composition. In this study, we reconstruct the composition of the pre-settlement (1909-1937) forest of a 4,134 km 2 sector of the boreal mixedwood forest using early land survey archives. The presettlement composition was compared with modern composition using recent ecoforest inventories (1980-2008), and the influence of surficial deposits on compositional changes assessed. During the pre-settlement period, the landscape was primarily dominated by spruce, which was evenly distributed across surficial deposit types. Trembling aspen, although widespread, rarely dominated stands. In contrast, the present-day landscape is dominated by trembling aspen, notably on clay and till deposits. In general, conifers have undergone a severe reduction in frequency. Spruce and pine forests are today mainly restricted to organic and sandy surficial deposits, respectively, compared to their historical frequencies. Composition changes observed in the boreal mixedwood forest of western Quebec are essentially the results of fires and forest harvesting, but surficial deposits have affected the current abundance and spatial distribution of the different taxa. In the context of sustainable forest management, considerable effort should be deployed to restore conifer dominance in the region, notably on the fertile deposits that appear particularly susceptible to composition changes.

In the context of global changes, the future dynamics of trembling aspen ( Populus tremuloides Michx.) are uncertain in the middle of its range. An increase in climate-related mortality could occur, but the modification of disturbance regimes could also favor its expansion. In this study, we document trembling aspen dynamics over 40 years at the scale of a boreal forest landscape (10 930 km²), as well as the role of disturbances in these dynamics. The results indicate that trembling aspen has experienced a substantial expansion over the last four decades (+102% occurrence), particularly between 1987 and 1997 (+70.9% occurrence). Nevertheless, these dynamics vary both spatially and temporally, with for example a phase of weak decline since 1997 (−5.9% occurrence). Anthropogenic disturbances, particularly clear-cutting, have played a major role in the expansion of trembling aspen. This expansion could influence the response of ecosystems to climate change, by modifying both fire and insect outbreak activities.