Impacts of Forest Fire on Permafrost in the Discontinuous Zones of Northwestern Canada

• Main Author: Holloway, Jean
• Other Authors: Lewkowicz, Antoni G.
• Format: Thesis
• Language: English
• Published: Université d'Ottawa / University of Ottawa 2020
• Subjects: Permafrost; Climate change; Forest fire; Thermokarst; Canada; Northwest Territories; Active layer thickness; Ice; permafrost; taiga; Taiga shield; Alaska
• Online Access: http://hdl.handle.net/10393/41186; https://doi.org/10.20381/ruor-25410

Climate change is causing increases in the frequency, severity, and extent of fires in the boreal forest, which in turn is expected to change historical cycles of permafrost response and recovery to disturbance. A review of recent literature (forming part of this thesis) shows that there are disparities in current knowledge of post-fire permafrost response. First, the majority of studies on permafrost-fire interactions have been conducted in Alaska, leaving regional gaps for the boreal forest across Canada. Second, there are limited direct measurements of certain variables which affect post-fire permafrost dynamics. These include snow depth, burn severity, and soil moisture, as well as ground ice content and quantified subsidence and thermokarst development. Third, the majority of post-fire permafrost studies address near-surface impacts, neglecting permafrost conditions at depth. Finally, there is a lack of long-term information and regional investigations over a broad range of environmental conditions, particularly how permafrost responds across a variety of ground ice contents. This thesis addresses these knowledge gaps through in-situ measurements and analysis of permafrost conditions following fires occurring over the last half century and in particular in 2014 and 2015, along a 650 km latitudinal transect spanning the discontinuous zones, from isolated patches (57.8°N) to extensive discontinuous permafrost (63.1°N), in northwest Canada. A variety of monitoring techniques were used to evaluate permafrost change, including ground and air temperature measurements, direct current electrical resistivity tomography (ERT) surveys, measurements of frost table depth, snow depth, organic layer thickness, burn severity, and ground subsidence. Samples of frozen and unfrozen soil were collected by coring or pit digging, and laboratory analyses conducted to establish soil characteristics. Laboratory experiments were also performed to establish a relationship between resistivity and temperature, and to generate a threshold between frozen and unfrozen soil that could assist in the interpretation of ERT surveys. A total of 68 sites along the Mackenzie Highway in northern Alberta and southern Northwest Territories (NWT) were examined to evaluate permafrost change due to climate warming and forest fire since an initial survey in 1962. The transect extends through the isolated patches and sporadic discontinuous permafrost zones, including 11 sites which burned at various times between 1971 and 2012. Overall, there has been significant permafrost degradation, especially at sites with thin organic layers and coarse-grained soils. This occurred preferentially at the southern end of the transect, where nearly 2°C of climate warming has occurred, such that even undisturbed sites experienced degradation. However, permafrost has persisted at about half of the sites where black spruce (Picea mariana) canopies with organic layers generally >40 cm thick overlie fine-grained sediments. Permafrost even persisted at the majority of burned sites, but greater frost table depths were observed at those which were burned in 2012. A second transect was established to examine permafrost change following the abnormally severe fire year of 2014 (and more limited fires in 2015) that affected sites across a wide range of conditions in the southern NWT. Eleven monitoring sites were established in the sporadic and extensive discontinuous zones between 2015 and 2016, and annual field surveys were conducted through to 2019, including the first repeat ERT surveys conducted following fire. Permafrost change occurred at all sites, including unburned ones, indicating the ongoing impacts of climate change in the region. Snow-depth days, maximum snow depth, and the nival offset were all greater at burned sites. Permafrost change was more pronounced at burned sites, with greater relative decreases in average apparent resistivity and increases in frost table depths and ground temperatures, particularly at sites with low gravimetric moisture content, coarse soil textures, and organic layers <40 cm thick. These changes are pronounced in the near surface (<5 m depth), with deeper permafrost appearing relatively unaffected within the 5-year post-fire time-frame. The field observations indicate that permafrost can still persist following fire at a significant percentage of locations in the discontinuous zone. However, slow degradation is occurring at both burned and unburned sites due to the warming climate, and particularly at dry sites with coarse-grained soils and thin organic layers. Post-fire permafrost change is evident at sites which burned in the last 10 years, but over the long-term, frozen ground appears resilient to fire, with characteristics like active layer thickness returning to pre-fire levels. Similarly, cold permafrost on the taiga shield is resilient to fire, even with thin residual organic layers. At high ice-content sites, however, where ground subsidence and thermokarst develop, water inundation and permafrost thaw can occur, particularly in areas which have been severely burned. This thesis underlines, therefore, the importance of monitoring and modelling a variety of landscape types to establish post-fire permafrost impacts and temperature trajectory, and more specifically the effects of heterogeneity of drainage conditions, substrate, and organic layer thicknesses on the fate of permafrost in the boreal forest.