Vegetation Distributions along Lichen-Dominated Slopes of Opposing Aspect in the Eastern Canadian Subarctic

Abstract

Detailed vegetation surveys along two opposing slopes in the boreal forest-tundra ecotone of Quebec-Labrador reveal distinct distributions and habitat preferences of prevalent lichen and shrub species. The presence or absence of a protective shrub layer was the main determinant of lichen distributions. Generally, there was an inverse relationship between the proportional surface coverages of vascular plants and lichens. At the most exposed site, nearest the ridge top, Cetraria nivalis (L.) Ach., Alectoria ochroleuca (Hoffm.) Massal. and Cetraria nigricans Nyl. were integrated well into the lichen mat and surrounded stoney earth circles. Increased shrub growth and protection quickly eliminated these lichens at lower elevations but allowed for more widespread growth of Cladina rangiferina (L.) Nyl. Cladina stellaris (Opiz) Brodo was the most pervasive lichen and appeared to be a generalist in habitat selection. Slope aspect influenced the distributions of three lichens of limited habitat range. The northerly facing slope provides preferable conditions for their growth farther downslope, thus extending their suitable habitat range. These results suggest the possibility of defining distinct lichen habitats based on exposure or, conversely, on protection provided by the presence of a shrub-tree canopy.

Key words: lichens, vegetation gradient, subarctic, topography, tundra, eastern Canadian Subarctic