Influence of Fire on Permafrost in Lowland Forests of the Tanana Flats, Interior Alaska

Abstract:

The degradation of ice-rich permafrost in lowland ecosystems may have particularly strong ecological impacts due to the potential for thaw settlement and subsequent water impoundment. We examined the effects of fire disturbance on permafrost across a chronosequence of fire scars (1930-2010) in the forested areas of collapse-scar bog complexes in the Tanana Flats of Interior Alaska, and utilized a thermal permafrost model (GIPL) to assess the roles of soil physical properties and historic climate. Field-based calculations of potential thaw settlement following the loss of ice-rich permafrost ranged from 0.4 m to 0.9 m. This subsidence would cause the surface elevations of current day forests to drop, on average, to 0.1 m below the surface water level of adjacent collapse-scar bogs, likely resulting in water impoundment. However, the vulnerability of permafrost to deep thawing and talik formation was variable among fire scars due to heterogeneity in organic layer thickness, soil texture, moisture, and associated thermal properties. Simulated reductions in organic layer thickness predicted talik formation in peat and silt loam-dominated soils, but not in sandy loams. The vulnerability of permafrost to talik formation increased under the climatic conditions since 1970, which were characterized by higher air temperatures. Pronounced permafrost thawing occurred during periods of high snow accumulation, whereas periods of low snow accumulation appeared to facilitate permafrost recovery. Simulations of the complete removal of the organic layer (high severity fire) in silt loam-dominated sites suggested the long-term loss of permafrost under the climate of the last century. Overall, the influence of fire on permafrost in these lowland ecosystems appears to be dependent on soil physical properties, fire severity, and climatic conditions.