Reduced postfire tree regeneration along a boreal forest-forest tundra transect in northern Quebec

The large 1950s fires that burned >5500 km^2 of land across a south-to-north climatic gradient in northern Québec provide an opportunity to evaluate the role of fire in forest-tundra development on a demographic basis. The tree population density before and ≈ 30 yr after fire was estimated by censusing trees in plots of 400 m^2 located in upland and lowland within four representative ecoregions of northern Québec. The analysis of tree recruitment before and after fire, in 410 randomly selected sites along a transect crossing the upper boreal forest and forest-tundra zones, indicated that wild fires induced substantial depletion of tree populations. Taken as a whole, fires have significantly reduced the density of black spruce populations in forest-tundra uplands, but not in the lowlands. A reduction in tree population density of ≥ 75% was observed in 22% of upper boreal forest sites, and 45% and 93% of sites located in the forest and shrub subzones, respectively, of the forest-tundra zone. Complete exclusion of tree populations by fire was observed in 43% of upland sites in the northern part of the transect, while complete removal was a rare event in the southern part. Sustained reduction of tree population density after several destructive fires appears as one of the main deforestation processes in the subarctic zone. This leads to the patchy distribution of forest stands and scattered tree populations typical of the forest-tundra biome. Comparisons with paleoecological data suggest that the impact of the 1950s fires contribution to the expansion of the forest tundra into the upper boreal forest. The ecological impact of these fires was probably similar to those fires responsible for development of the forest tundra during the Holocene. It is suggested that the fire-climate interaction should be considered in order to predict the ecological impact of warming climate on high-altitude forest ecosystems.