| Summary: | Dissertation (Ph.D.) University of Alaska Fairbanks, 2016 A warming climate is expected to cause widespread thawing of discontinuous permafrost, and the co-occurrence of wildfire may function to exacerbate this process. Here, I examined the vulnerability of permafrost to degradation from fire disturbance as it varies across different landscapes of the Interior Alaskan boreal forest using a combination of observational, modeling, and remote sensing approaches. Across all landscapes, the severity of burning strongly influenced both post-fire vegetation and permafrost degradation. The thickness of the remaining surface organic layer was a key control on permafrost degradation because its low thermal conductivity limits ground heat flux. Thus, variation in burn severity controlled the local distribution of near-surface permafrost. Mineral soil texture and permafrost ice content interacted with climate to influence the response of permafrost to fire. Permafrost was vulnerable to deep thawing after fire in coarse-textured or rocky soils throughout the region; low ice content likely enabled this rapid thawing. After thawing, increased drainage in coarse-textured soils caused reductions in surface soil moisture, which contributed to warmer soil temperatures. By contrast, permafrost in fine-textured soils was resilient to fire disturbance in the silty uplands of the Yukon Flats ecoregion, but was highly vulnerable to thawing in the silty lowlands of the Tanana Flats. The resilience of silty upland permafrost was attributed to higher water content of the active layer and the associated high latent heat content of the ice-rich permafrost, coupled with a relatively cold continental climate and sloping topography that removes surface water. In the Tanana Flats, permafrost in silty lowlands thawed after fire despite high water and ice content of soils. This thawing was associated with significant ground surface subsidence, which resulted in water impoundment on the flat terrain, generating a positive feedback to permafrost ...
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