Geomorphology, Vegetation Succession, Soil Characteristics and Permafrost in Retrogressive Thaw Slumps near Mayo, Yukon Territory

Three retrogressive thaw slumps of varying age have been initiated by erosion of ice-rich glaciolacustrine sediments on a bend of Stewart River, 3 km upstream from Mayo, Yukon Territory. Two of the slumps are presently active; the third stabilized before 1944. The rate of retreat of the active slump headwalls between 1949 and 1987, determined from aerial photographs and ground surveys, is up to 16 m/yr. Floors of the active thaw slumps contain well-defined vegetation successional communities that are distinct from the local, mature boreal forest. Although a few clumps of mature forest vegetation survive the fall into the slump, a birch/white spruce sere, similar to the original forest, is re-established after a period of 35-50 years. Changes in soil calcium carbonate and soil structure profiles on disturbed surfaces of varying age demonstrate the initiation of pedogenesis in the floor of the stabilized slump, but assays of pH, orgainic carbon and total nitrogen indicate that after about 40 years the new soils remain immature. Comparison of ground temperatures in the stabilized thaw slump and at undisturbed sites in the area indicates that the ground thermal regime may return to local conditions a century or more after disturbance.Key words: permafrost, terrain disturbance, retrogressive thaw slump, vegetation succession, Yukon Territory ; Mots clés: pergélisol, perturbation du terrain, décrochement de fonte régressif, succession végétale, territoire du Yukon