Abstract

Macmillan Pass, at 1350 m asl (above sea level), is located in the Selwyn Mountains at the Yukon–Northwest Territories border (63 °15′N, 130°02′W). This area lies within the discontinuous but widespread permafrost zone. Palsa–peat plateau complexes cover 0.7% of the 235 km2 study area and are found in bog and fen depressions at elevations from 1285–1690 m. Palsa heights range from 0.15–9.75 m and diameters from 3.25–75.0 m; peat plateaus have maximum heights of 2.5 m and maximum diameters of 225 m. Both features are vegetated by Cladina-Betula glandulosa, Cladina-Polytrichum-Cetraria, and crustose lichens-Polytrichum plant communities.Palsas and peat plateaus are windswept during winter. On surfaces which support recumbent (5–15 cm tall) plant communities there was an average of only 7.5 cm of snow during late winter 1978. Snow cover was thinner by a ratio of 1:4 compared to control areas.These permafrost features have formed since the White River volcanic ash fall of 1220 BP. On palsas and peat plateaus this ash occurs at an average depth of 21 cm and has an average thickness of 11.6 cm.Shrinkage and (or) total decay of palsas and peat plateaus has occurred during the past 34 years. In one palsa field this represents a 34% reduction of area whereas in two others, 100%. The areal extent of some peat plateaus has also been reduced.

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