Aim The objectives of this study were to: (1) identify episodes of establishment and mortality of young and mature trees at several sites at the alpine tree line in the western Northwest Territories and the latitudinal tree line in northern Manitoba; (2) infer changes in the structure and location of the tree line from patterns of establishment; (3) evaluate any relationship between these changes and climate; and (4) investigate sources of variability between sampling sites and study areas.
Location Taiga Cordillera of the western Mackenzie Mountains in the Northwest Territories, and the western Hudson Bay Lowlands in northern Manitoba, Canada.
Methods Recent tree line dynamics were examined at six climatically similar sites: three in the western Mackenzie Mountains and three around Churchill, Manitoba. Dendroecological techniques were employed to construct static age distributions of species present at each site. Static age structures, residuals from modelled age distributions, and reconstructions of dynamic stand density were used to identify patterns of establishment and mortality and to compare these to changes in climate.
Results Tree line locations advanced and stand density increased during the early-to-mid-20th century around Churchill, although responses were not uniform across sites or species. Results were less conclusive in the Mackenzie Mountains, although the tree line probably advanced during the late 18th century, and stand infilling occurred during the mid-20th century. Correlation analyses with temperature suggest that conditions during establishment and particularly during recruitment are crucial for controlling tree line dynamics.
Main conclusions Tree line advance and stand infilling have continued to the present at Churchill, while the tree line has stagnated in the western Mackenzie Mountains. The results of this study indicate that site- and species-specific responses play a large role in determining the tree line response at multiple scales, illustrating the complexity of tree line dynamics in the context of a changing climate.