The range limit of boreal tree species in Québec-Labrador: an ecological and palaeoecological interpretation

Abstract

Most boreal tree species in northern Québec-Labrador spread towards their northern range limit from different source areas during the Holocene. Two major pathways were utilized by the dispersing species, one on either side of the slowly decaying Laurentide ice. The Labradorean pathway was deglaciated early during the Holocene, thus favoring the northward expansion of boreal trees adapted to the humid environment. The Hudsonian pathway was formed later and it included a less diversified flora with black spruce [ Picea mariana (Miller) Britton, Sterns and Poggenburg] as the only tree species to have reached the northern tree-line. In eastern Québec-Labrador, tree species were able to occupy their potential sites throughout the region. In western Québec-Labrador, many tree species have not yet colonized all the potential sites. The northern range limit of black spruce throughout northern Québec-Labrador appears out-of-phase with present climate. White spruce [ Picea glauca (Moench) Voss] is in equilibrium with present climate in the eastern part of the region, whereas it is in the process of expansion along the Hudson Bay coast. Balsam fir [ Abies balsamea (L.) Miller] is slowly expanding in the boreal James Bay area. The spread of jack pine ( Pinus banksiana Lambert) followed a SW-NE direction towards central Québec-Labrador, a pattern probably determined by climate-mediated fire regimes. This species is at equilibrium with present climate and it has not gone beyond the southern edge of the forest-tundra because of late arrival and possibly unfavorable conditions. Populus probably arrived early after deglaciation; it is not known to what extent both species ( P. tremuloides Michaux and P. balsamifera L.) are at equilibrium with present climatic conditions at their northern range limit. The geographical pattern of final deglaciation most likely controlled the spread of tamarack [ Larix laricina (Du Roi) K. Koch]. This species showed contrasting abundance east and west of 74°W, i.e. along a line where the residual ice lasted until ca. 5500-5000 BP. Tamarack is still spreading towards the tree-line along the Hudson Bay coast. Instead of important latitudinal movements, boreal tree species reacted to postglacial environmental change by changes in their regional abundance.