In the context of global changes, understanding the role of climate and wildfires on the composition of boreal forest is crucial to predict vegetation trajectories. Our work investigates the incidence of fire regimes on multi-millennia vegetation trajectories in two coniferous boreal forest areas from Quebec-Labrador, Canada which currently display different compositions (black spruce forests in the west and white spruce - balsam fir forests in the east). We believe that this difference results from dissimilar Holocene fire regimes modulated by climate and relief. Charcoal and pollen analyses were carried out on nine lacustrine cores to reconstruct the history of fire (frequency and size) and vegetation during the last 8,000 years.
Between 7000 and 2000 cal. years BP, the high fire frequency recorded in both regions, as compared to Early and Late Holocene periods, may have been promoted by dry and warm climatic conditions. Western forests, submitted to larger fires than those in the eastern region, were mainly composed by species adapted to fire: Picea mariana, Pinus banksiana and Alnus viridis ssp. crispa. On the other hand, eastern forests were dominated by Picea sp., Abies balsamea and Betula sp., poorly adapted to fire. Between 2200 and 1500 cal. years BP, fires became larger in the eastern region, allowing the development of Pinus banksiana and Alnus viridis ssp. crispa. In the coniferous boreal forests of eastern North America, the long-term vegetation trajectories were influenced more by the size of fires rather than their frequency. The impact of current climate change on the rate of ignition of large fire episodes must be considered as a key point in future investigations, so that accurate models to predict vegetation dynamics can be proposed. This effort is essential to choose adequate forest management policies and strategies to attenuate the negative ecological and socio-economic consequences of increased boreal wildfire activities.