No evidence of a northward biome shift of treeline in the Mackay Lake region, north-central Canada

Major changes in boreal and subarctic climates have occurred in recent decades, but uncertainty exists as to how biota are responding, e.g., whether the subarctic forest-tundra (‘treeline’) will advance northward. Ground-based vegetation data from the Mackay Lake region of the Northwest Territories were used to determine if there is evidence of a northward biome shift. Near the treeline, current plant communities are similar to those documented 40–67 years ago. There was little or no evidence of recent tree regeneration by seed, expansion of tree stands, or colonization by trees of adjacent tundra. Black spruce stems require ~75 years to reach 2.5 m in height and be discernible as trees on photography and satellite imagery. Branches 20–30 years old are the preferred age for initiation of an upright stem from layering. Upright stems of clonal spruce within the forest-tundra may be far younger than their near-ground or belowground stems. Whether northward biome migration will occur will depend on the interplay between climatic, landscape, and soil factors, species migration capacities, wildfires, and whether afforestation outpaces deforestation. In the forest-tundra, climatic change may not result in significant biome shifts, but rather in changes in the relative abundance of species already present.