ABSTRACT: Rate of permafrost degradation in peatlands.

CGRG Bibliography of Canadian Geomorphology
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Abstract

Due to the insulating properties of dry peat, permafrost at its southern limit occurs mainly in peatlands, where it is found in raised mounds or plateaus because of the expansion of freezing water. The dry peat on the surfaces of plateaus protects the permafrost from summer heat. However, because peat forms in wet conditions, peat plateaus are usually surrounded by wetlands, and often by standing water. The standing water acts as a heat conductor, causing the permafrost core to melt from the sides. Peat plateaus have been found to undergo cycles of aggradation and collapse. Fire is a major trigger for peat plateau collapse because it can consume the insulating peat. The peat plateau surface provides a suitable habitat for tree growth. Trees on the subsiding plateau edges die when their roots drown in the wetlands. The dead trees provide a record of the location of the plateau edge over time because their dates of death, determined by crossdating, indicate when they were deposited into the water. Study sites were at Hotchkiss (57 10 N 118 11 W) and Lutose (59 26 N 117 16 W) in northern Alberta and Sandy Lake (60 32 N 114 30 W) in the Northwest Territories. At each site, 16 transects were established across retreating plateau edges. Live and dead trees along the transects were sampled for crossdating and examination of compression wood. Dead trees were crossdated using frost rings as markers. The tree ring record extended back to about 1950. Since that time, plateau edges retreated at 0.1 to 0.2 m per year on average. The highest retreat rates occurred after 1975, probably due to changes in climatic factors such as temperature and precipitation, which affect the soil thermal regime. Year-to-year variability in retreat rates was high, reflecting variability in climate and in tree resistance to drowning. The collapse of peat plateaus brings about major changes in vegetation, decomposition, and carbon storage and release. Peatlands are a major carbon sink, drawing more carbon from the atmosphere by photosynthesis than they return to it by decomposition. Changes in fire frequency and severity triggered by climate change could reverse this imbalance. Fire itself releases large amounts of carbon dioxide, and the collapse of peat plateaus into wetlands increases anaerobic decomposition and methane release.

Bibliography of Canadian Geomorphology