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TitreInvestigations of effects of forest fires on the ground thermal regime and permafrost, northwestern Canada
AuteurSmith, S L; Bonnaventure, P
Source11th International Conference on Permafrost, book of abstracts; par Günther, F (éd.); Morgenstern, A (éd.); 2016 p. 489-490
Séries alt.Secteur des sciences de la Terre, Contribution externe 20150405
ÉditeurBibliothek Wissenschaftspark Albert Einstein |a Potsdam, Germany (Potsdam, Germany)
RéunionXI International conference on permafrost; Potsdam,; DE; juin 20-24, 2016
Medianumérique; en ligne
ProvinceAlberta; Territoires du Nord-Ouest
SNRC84M/11; 84M/12; 96C/07; 96C/08
Lat/Long OENS-120.0000 -119.0000 60.0000 59.5000
Lat/Long OENS-125.0000 -124.0000 64.5000 64.2500
Sujetspergélisol; trous de mine; regimes thermiques; températures au sol; géologie des dépôts meubles/géomorphologie; Quaternaire
ProgrammeInfrastructures terrestres, Géosciences de changements climatiques
LiensOnline - En ligne (PDF, 345 MB)
Résumé(disponible en anglais seulement)
Wildfires are a common occurrence in the boreal forest of northwestern Canada and can have important impacts on the permafrost environments through their influence on the ground thermal regime. Increases in ground surface temperature due to microclimatic changes associated with organic layer removal or the loss of the coniferous tree cover, can result in increased thaw penetration, permafrost degradation and ground instability where soils are ice-rich. Understanding the long-term effects of environmental disturbance such as fire on permafrost environments is important for assessments of ecosystem change and for planning infrastructure in the context of a changing climate. The Geological Survey of Canada has maintained a number of field sites in the Mackenzie transportation/transmission corridor of the Northwest Territories and northern Alberta that have facilitated the characterization of thermal effects of fires on permafrost terrain up to two decades following initial fire events for sites underlain by warm permafrost in both mineral soils and peatlands.
Study Sites and Methods
The study sites are adjacent to the Norman Wells to Zama pipeline right-of-way which extends from the central Mackenzie Valley NWT to northern Alberta. This paper focusses on two sites. The first study site is a north facing slope located at kilometre post (KP) 182 (64.3°N, 124.5° W). The site is underlain by fine-grained soils (silt and clay). Permafrost is generally warm in this area with mean annual ground temperatures above -1°C and underlain by ice-rich soils. The site was burned in 1994 but prior to the event the site was covered by a white spruce and white birch forest. Burning was more severe at the top of the slope with complete loss of the forest canopy and removal of the organic layer. Five instrumented boreholes, up to 1.9 m deep, were established at the site in 1995 to measure ground temperatures. Four of these were on the burnt area of the slope at the slope bottom (KP182B), mid-slope (KP182M), slope crest (KP182C) and top of the slope (KP182T). One borehole was also established in an unburnt area (KP182UB). Instrumentation was also installed to measure air temperature and near surface temperature (3-5 cm depth). A more detailed description of the site and instrumentation is provided in Smith et al. (2015). The second study site is located on a peat plateau at KP783 (59.7°N 119.5°W) in northern Alberta. Two boreholes (20 m deep) were instrumented in 1984 to measure ground temperatures and instrumentation to measure air and ground surface temperature was installed in 1999. The forest cover at the site originally consisted of stunted black spruce and the site was underlain by thick (7 m) ice-rich frozen peat. Permafrost was 10 to 14 m thick and was at temperatures near 0°C (-0.1 to -0.2°C). The site was severely burned in summer 2004 with burning of vegetation and the peat surface. The instrumentation at the site allowed a comparison of thermal conditions before and after the fire. Earlier analysis of ground temperature data and additional information about the site can be found in Smith et al. (2008).
Results and Discussion
The near surface ground temperature data collected about one year following the 1994 fire at KP182 indicates that before the fire, surface temperatures were below 0°C at the site. Following the fire, surface temperatures rose above 0°C (Figure 1) at the 4 burned sites with the highest temperatures found at the more severely burned sites in the upper part of the slope (KP182T, KP182C). The annual mean surface temperatures at burned sites were generally 1-2°C warmer than at the unburnt site (KP182UB). At KP783 annual mean surface temperatures following the 2004 fire were generally higher than those recorded prior to the fire (Figure 1). Decreases in surface temperature about 5 years after the fire at both KP182 and KP 783 were also observed which are assumed to be associated with regrowth of vegetation. At KP182, shallow ground temperatures increased at all burned sites while the change in ground temperature at the unburnt site was relatively small. The ground warming at burned sites was accompanied by an increase in active layer thickness (determined from shallow temperature measurements) and in the upper portion of the slope where burning was more severe, thaw progressed to depths below 1.5 m shortly after the fire. At the top of the slope, permafrost is likely not sustainable as the ratio between frozen and unfrozen thermal conductivity in the upper portion of the soil is not sufficient to provide the required thermal offset (Smith et al. 2015). Ground temperature data from other recovering burn sites in mineral soils in the central Mackenzie Valley also indicate that permafrost is degrading. Although burning was severe at the KP783 site, warm permafrost still persists a decade after the fire with some increase in thaw depth following the fire which has been accompanied by surface settlement. The insulating effect of the peat likely reduces the impact of surface warming on the underlying permafrost. The large seasonal difference in thermal conductivity of the peat is able to maintain the thermal offset required to sustain permafrost at the site. There is also a high latent heat requirement for thawing the ice-rich peat.
Summary and Conclusion
The magnitude of the impact of forest fires on warm permafrost is associated with the severity of the fire and also the properties of the subsurface materials. Where severe burning occurred in areas underlain by mineral soils, degradation of permafrost is likely occurring. In contrast, at a severely burned peatland site, warm permafrost persists a decade after the fire with the thick peat reducing the impact of the higher surface temperatures. The loss of the surface buffer layer due to burning will also make the ground thermal regime additionally sensitive to increases in air temperature associated with a warming climate. Warming and thawing of ice-rich permafrost following fires can result in surface settlement and ground instability and these impacts need to be considered in planning and maintenance of infrastructure in a changing climate.
Field data collection has been supported by Natural Resources Canada and several other sources over the years including the Program for Energy Research and Development and the Northern Oil and Gas Science Research Initiative. The important contribution of our late colleague Dan Riseborough to the data analysis is also gratefully acknowledged.
Smith, S.L., Burgess, M.M. and Riseborough, D.W., 2008. Ground temperature and thaw settlement in frozen peatlands along the Norman Wells pipeline corridor, NWT Canada: 22 years of monitoring. In: D.L. Kane and K.M. Hinkel (Editors), Ninth International Conference on Permafrost. Institute of Northern Engineering, University of Alaska Fairbanks, Fairbanks Alaska, pp. 1665-1670.
Smith, S.L., Riseborough, D.W. and Bonnaventure, P.P., 2015. Eighteen year record of forest fire effects on ground thermal regimes and permafrost in the central Mackenzie Valley, NWT, Canada. Permafrost and Periglacial Processes. DOI: 10.1002/ppp.1849
Résumé(Résumé en langage clair et simple, non publié)
Cette étude se penche sur l'impact des feux de forêt sur les milieux pergélisolés. Des résultats portant sur deux sites adjacents à une emprise de pipeline, soit une pente de pergélisol tempéré située dans la partie centrale de la vallée du Mackenzie (incendiée en 1994) et une tourbière située dans le nord de l'Alberta (incendiée en 2004), sont présentés. La perte de végétation et les perturbations de la surface découlant des feux ont entraîné un réchauffement du sol et un dégel estival à plus grande profondeur. L'ampleur de l'impact des feux de forêt sur le pergélisol tempéré est liée à l'intensité du feu et aux propriétés des matériaux souterrains. Aux endroits où le feu était plus intense sur la pente, il se produit une dégradation du pergélisol. Bien que le feu ait été intense dans la tourbière, le pergélisol y est encore tempéré, probablement en raison de l'isolation fournie par la tourbe épaisse. Ces changements dans l'état du pergélisol résultant de perturbations naturelles peuvent avoir des répercussions sur la stabilité du paysage et l'intégrité des infrastructures dans les corridors de transport adjacents.