Impacts of wildfire on the permafrost in the boreal forests of Interior Alaska

Abstract

[1] The impact to the permafrost during and after wildfire was studied using 11 boreal forest fire sites including two controlled burns. Heat transfer by conduction to the permafrost was not significant during fire. Immediately following fire, ground thermal conductivity may increase 10-fold and the surface albedo can decrease by 50% depending on the extent of burning of the surficial organic soil. The thickness of the remaining organic layer strongly affects permafrost degradation and aggradation. If the organic layer thickness was not reduced during the burn, then the active layer (the layer of soil above permafrost that annually freezes and thaws) did not change after the burn in spite of the surface albedo decrease. Any significant disturbance to the surface organic layer will increase heat flow through the active layer into the permafrost. Approximately 3-5 years after severe disturbance and depending on site conditions, the active layer will increase to a thickness that does not completely refreeze the following winter. This results in formation of a talik (an unfrozen layer below the seasonally frozen soil and above the permafrost). A thawed layer (4.15 m thick) was observed at the 1983 burned site. Model studies suggest that if an organic layer of more than 7-12 cm remains following a wildfire then the thermal impact to the permafrost will be minimal in the boreal forests of Interior Alaska.

Comparison of geophysical investigations for detection of massive ground ice (pingo ice)

“…Severe fire removes most of the surface organics and surface thermal offset [Romanovsky and Osterkamp, 1995] changes, thawing permafrost and increasing the active layer moisture content. As a result, severe fire has caused 3 to 4 m of active layer thawing around the Fairbanks region [Yoshikawa et al, 2002]. A pingo may be destroyed if the depth of the massive ice is shallower than the maximum thawing depth.…”

High-latitude cooling associated with landscape changes from North American boreal forest fires

“…Thus, despite more energy absorption from low summer albedos, net radiation and heat fluxes are suppressed (Chambers et al, 2005;Liu and Randerson, 2008). Although black carbon has a substantial impact on summer surface energy budgets, its persistence is limited to the first few years as it is washed away and degraded, and grasses, herbs, and shrubs colonize and/or re-sprout (Amiro et al, 1999;Yoshikawa et al, 2003). Re-growing landscapes during the first two decades are subsequently dominated by shrubs and tree saplings that exhibit higher summer albedos than pre-fire mature ecosystems.…”

Measuring thermal conductivity in freezing and thawing soil using the soil temperature response to heating

“…The thermal conductivities of the shallow surface layers are highly dependent on the composition and state of the soil and vegetation. Changes in water and ice content produce the greatest changes (by a factor of ten or more) in thermal conductivity temporally (Yoshikawa et al, 2003) and generally correspond to drying/wetting or freezing/thawing events. Soil composition in the periglacial landscape is highly variable spatially due to the agency of cryoturbation.…”