The impact of forest fire on permafrost slopes Klondike area, Yukon Territory

Abstract

Numerous forest fires occurred during the summer of 2004 in the Klondike Goldfields region of the Yukon Territory, an area of extensive discontinuous permafrost. More than 35 shallow detachment failure landslides developed in subsequent weeks in Steele Creek, a small drainage basin located about 60 km south of Dawson City. Preliminary observations of the failures and near-surface thermal regime were made through freeze-up of 2004 and continued in the summers of 2005 and 2006. Detachment failures were mapped and individual sites were surveyed. Air and ground temperatures were measured in burned and unburned areas. In addition, two-dimensional DC resistivity transects were used to examine subsurface conditions in the area. Forest fire contributed to detachment failure activity on permafrost slopes by destroying the surface organic mat, causing burned surface temperatures to rise, thawing active layers by up to 20 cm (+31%) deeper than unburned slopes and weakening the surface root structures. Deeper thaw melted transient layer ground ice, raising soil porewater pressures. The thermal differences between burned and unburned sites were greater at the north-facing than south-facing sites, and active layer freezing and thawing processes varied according to both aspect and burned status. More southerly-facing and/or burned sites generally thawed earlier, refroze later and had warmer temperatures than more northerly and/or unburned sites. Thaw of burned areas with high ground surface temperatures can be expected to continue, depending on climatic conditions, until sufficient revegetation occurs to shade the surface and rebuild the insulating organic mat. The detachment failures occurred from a few weeks to two years after forest fire, and only on slopes where permafrost was extensive. They were not similar to others in the literature in that almost all occurred in coarse-gained soils and had failure planes elevated above the permafrost table. These landslides were flow-type failures that rafted portions of the organic mat on top of deforming, non-cohesive sediment. They occurred in areas of deeper thaw but their distribution and the resistivity data suggest that they were associated with supra-permafrost taliks which concentrated groundwater flow. In an unglaciated area like the Klondike region this landsliding process has likely occurred thousands of times during the Pleistocene and may be responsible for elements of the form of the region's slopes. Predicted increases in the frequency and magnitude of forest fire in the boreal forest due to warming climates may increase incidence of these types of failures.