Fire-Associated Change In Surface and Frozen Ground Conditions In Nunatsiavut, Labrador

• Main Author: Wang, Yifeng
• Other Authors: Lewkowicz, Antoni G.
• Format: Thesis
• Language: English
• Published: Université d'Ottawa / University of Ottawa 2020
• Subjects: Permafrost; Fire; Boreal; Nunatsiavut; Labrador; Canada; Nain; Postville; permafrost; Subarctic; Tundra
• Online Access: http://hdl.handle.net/10393/40791; https://doi.org/10.20381/ruor-25017

Forest fires are a significant natural surface disturbance to permafrost landscapes. However, post-fire permafrost literature is mainly derived from research in the western North American boreal forest. This thesis research, conducted in collaboration with a multi-disciplinary, multi-institutional team of researchers, is the first investigation of post-fire frozen ground conditions in the coastal boreal forests of the eastern Canadian subarctic, where precipitation amounts are greater and fire intervals are much longer than in the West. Ground thermal conditions and related environmental variables were evaluated from summer and winter measurements at three historic forest fire sites in Nunatsiavut, coastal Labrador. Electrical resistivity tomography was used to assess the presence and distribution of frozen ground. Abiotic and biotic variables, thought to be associated with post-fire permafrost persistence, were subject to a series of exploratory data analyses, including a Principal Components Analysis (PCA). Based on the literature, it was hypothesized that ground temperatures would be lower in the undisturbed, closed-canopy forest than in the adjacent burned areas, due to greater interception of snow, a thick and undisturbed organic mat, and greater shading of the ground. Burned areas were expected to have lesser organic mat thicknesses and greater shrub cover. It was expected that permafrost, if present, would be located beneath forest cover, while degrading in the burned areas. Numerical modelling was also performed for two of the sites to investigate future change in post-fire ground thermal conditions as the climate warms. Frozen ground conditions varied at the three sites. Near Nain, some patches of perennially frozen ground were identified in the undisturbed sections along the transects, with their presence and persistence relating to the intact canopy cover and the fine-grained sediments. Thermal modelling simulations to the end of this century demonstrated the resilience of this thin permafrost to at least 2060, even when subjected to fire disturbance and continued climate warming. No permafrost was identified along the transects at the southernmost site near Postville, reflecting the warmer climate at this site. Post-fire vegetation cover at all three sites was dominated by shrubs, but their snow-trapping effect, which is commonly observed in tundra environments, was limited by the deep (>1.3 m) snow cover throughout the sites and the presence of other erect vegetation, including regenerating trees or standing dead. The PCA did not reveal consistent associations between any of the biotic or abiotic variables, reflecting the complexity of the post-fire ecosystem. However, the PCA did show that burned sections differ in terms of canopy cover and snow depth, underlining the critical role of an intact forest canopy in the ecological protection of sensitive, discontinuous permafrost. Despite the absence of clear links between the variables, this research provides important first insights into fire-associated ecosystem change and its impact on permafrost in eastern Canada.