Fire-induced decrease in forest cover on a small rock outcrop in the Abitibi region of Québec, Canada

Rocky outcrops represent about 30% of the boreal forest of Abitibi, Québec, Canada. Although these outcrops have similar edaphic and climatic conditions, their vegetation can vary. Some are composed of a closed forest of black spruce (Picea mariana) and eastern white cedar (Thuja occidentalis), while others support open Pinus-dominated stands. A long-term study using palynology and charcoal analysis was used to determine the processes involved in creating the open vegetation observed on one outcrop located at Roquemaure. Changes in the fire regime through time seem to be the explanatory factor. Vegetation on this outcrop started 3,775 calibrated years ago as a closed Picea mariana-dominated stand. Increase in fire frequency ca 1,465 calibrated years ago led to the replacement of the Picea mariana-dominated stand by the more open Pinus woodland observed today. Changes in climate from wet to dry might explain this increase in fire frequency. Dry soils and exposed bedrock created by climate change and increased fire frequency limited the regeneration of Picea mariana. For future landscape management, long-term studies should be taken into consideration to determine the natural variability of ecosystems.