Subarctic Lichen Polygons and Soil Development along a Colonization Gradient on Eolian Sands

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Abstract
The postfire recovery sequence during the last 150 to 200 yr, lichen-ground colonization pattern, and soil development in a parabolic-dune system along the east coast of Hudson Bay, northern Quebec, [Canada], were documented using fifteen contiguous vegetation belts. Differential plant colonization takes place during succession because of a sustained exposure gradient associated with site position in the dune environment. Along the colonization gradient, seven developmental stages were identified, showing distinct vegetation and soil types and particular lichen-ground polygonal patterns. Soil development proceeded successively from bare sands, regosol, and shallow eluviated dystric brunisol. Bare sands are first colonized by Polytrichum piliferum and subsequently by Stereocaulon paschale and Rhacomitrium canescens. At stage 2, Rhacomitrium forms an extensive cover, while Stereocaulon takes over at stage 3 with a significant increase of invading species. The formation of thin dessication cracks and inception of lichen polygons, resulting in the development of confined sand outcrops, are closely associated with the dominance of Cladina mitis at stage 4. The progressive dominance of Cladina alpestris and podzolic soil development toward a late liche-spruce woodland (stages 5 to 7) are characterized by a sharp decline in species diversity and the appearance of mostly ephemeral lichen polygons. It is concluded that the two lichen-polygon types are associated with vegetation and soil development during plant stabilization of eolian sands: the C. mitis type and the C. alpestris type, respectively, formed during transient successional stages and late lichen-woodland stages.