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Alaska Section, American Water Resources Association Richard Kemnitz, Alaska Section AWRA Northern-Region Director I would like to invite you to the September 2000 Brown-Bag presentation by Larry Hinzman, WERC, University of Alaska Fairbanks. Please note this meeting is the second Wednesday in September. We look forward to seeing you, please feel free to bring a colleague to the meeting. September 13, 2000 "Wildfire in the Subarctic Boreal ForestsHydrologic Larry Hinzman Abstract Forest fires project an immediate and long lasting impact to boreal forest ecosystems through effects on the surface water and energy balances and underlying permafrost. The immediate impacts of moderate fires include killing of the vegetation and increasing soil moisture and temperature, but the organic surface soil remains relatively intact, providing the necessary insulation to prevent the underlying permafrost from rapidly degrading. In cases of more severe fires, the surface organic layer is entirely combusted, removing the insulative layer and exposing the mineral soil beneath. Any disturbance to the surface layer will increase heat flow through the active layer (the layer of soil above the permafrost that thaws and re-freezes seasonally) into the permafrost. Preserving the surface organic layer is critical to maintaining the thermal integrity of the permafrost.The presence or absence of permafrost is the dominant physical factor controlling local ecological and hydrological conditions. In the boreal forest of North America, permafrost is not continuous under all ground surfaces as it is in arctic regions, but rather is distributed discontinuously in valley bottoms and on north-facing slopes. The thermal condition of permafrost near Fairbanks is quite unstable, as its temperature is often -1C or warmer. The active layer responds to burns of moderate severity by rapidly increasing in thickness. After approximately three to five years (depending upon site conditions) the active layer will increase to a thickness that does not completely refreeze the following winter forming a talik (a layer of thawed soil between the permafrost and the seasonally frozen soil). This represents a “turning point” in the soil moisture regime. At this time, soils can continually drain throughout the winter and the active layer soils become progressively drier. Deciduous trees and shrubs typically become established before conifers, either through clonal regeneration or seed dispersal. As time goes on and recovery continues, the surface develops adequate insulation reducing heat flow to the subsurface and the talik may again become frozen or under the influence of a warming climate, the permafrost may not recover yielding a substantially changed ecosystem.Our field observations and modeling studies have shown that in cases of severe burns where the organic layer is more completely combusted (i.e. a thinner surface organic mat remains) the active layer will thaw to a greater depth every summer as compared to unburned or moderately surfaces. The active layer will continue to grow deeper in the years following a disturbance, again eventually forming a talik. Field observations and modeling studies indicate the taliks formed in areas of severe burns will develop at a faster rate and under current climatic conditions, the permafrost may not recover but will continue to degrade. In such situations, a rapid and permanent landscape and ecosystem change will occur. Under these conditions, the forest type may change from black spruce to white spruce or more drastic changes may occur as the coniferous forest may yield to the deciduous forest. |