Ninth International Conference on Permafrost ... - IARC Research

Estimation of the Extent of Near-Surface Permafrost in the Mackenzie Delta,Northwest Territories, Using Remote SensingT.-N. Nguyen, C.R. Burn, D.J. KingDepartment of Geography and Environmental Studies, Carleton University, Ottawa, ON, CanadaS.L. SmithGeological Survey of Canada, Natural Resources Canada, Ottawa, ON, CanadaIntroductionBased on air temperature, permafrost should be continuousin the Mackenzie Delta (MD), northwest Canada (Henry &Smith 2001). However, the most recent Permafrost map ofCanada, using sparse ground temperature data, classifiesthe MD as having discontinuous permafrost (Heginbottomet al. 1995). There has been little fieldwork investigatingpermafrost extent within the delta, yet, the determination ofnear-surface permafrost (NSP) extent, defined in this studyas permafrost within 3 m from ground surface, is importantfor land-use planning, as terrain behaviour varies betweenfrozen and unfrozen ground (Smith et al. 2001).The objectives of this research were (1) to assess if thespatial distribution of near-shore vegetation associations canbe used to predict NSP presence in the MD and, if so, (2)to apply remote sensing techniques to map these vegetationcommunities and estimate the proportion of ground underlainby NSP.Vegetation and Permafrost in Mackenzie DeltaThe MD stretches 200 km north–south and 60 km east–west (Mackay 1963). It is an alluvial landscape intersectedby a network of channels and thousands of lakes, and issubjected to annual flooding. Spatial variation in channelshifting, flooding, and sedimentation is expressed by patternsof vegetation (Gill 1973).Horsetail (Equisetum spp.) communities are the typicalemergent plant associations, while Willow-horsetail (Salix-Equisetum) communities can be found with increasingdistance from stream channels. Alders (Alnus spp.) arelocated at slightly higher elevations than the Willowhorsetailcommunities, since alder species are less tolerantof sedimentation. Spruce (Picea glauca) forests representthe climax community south of tree line. North of tree line,sedges (Carex spp.) and horsetails are common adjacent tochannels, and Willow-horsetail and Salix richardsonii arewidespread on more elevated sites (Pearce 1998).In the MD, the thermal effect of water bodies and theshifting nature of channels affect the age, distribution,temperature, and thickness of permafrost (Smith 1975). Onpoint bars, taliks have been recognized near channels byvarious authors (Gill 1973, Smith 1975). The presence ofa talik, where winter freeze-back does not reach the top ofpermafrost, is due to the slow establishment of permafrostbeneath newly exposed ground, and to deep snowdriftsblown off the channels and trapped in near-shore willowstands (Dyke 2000).Vegetation and Permafrost SamplingField sites were selected so that several representativevegetation classes were present at each site. A line transectmethodology was adopted for vegetation sampling andpermafrost probing. A total of 52 transects were laid insummer 2006. The majority of transects were between 100m and 200 m long depending on the width of the vegetationzones found at each site. Transects were establishedperpendicular to the shoreline and crossing the successionalsequence of vegetation (Fig. 1). Vegetation was sampledusing point and line-intercept methods and was identified atthe genus level for shrubs and trees, and at the functionaltypelevel for herbs. Plant presence and frequency werecalculated to provide an indication of how dominant a planttype is in a community. The ground was probed using waterjetdrilling to detect NSP in each vegetation association.Since the active layer thickness rarely exceeds 1.5 m in theMD, NSP was considered absent if unfrozen ground wasrecorded to 3 m depth (Kokelj & Burn 2005).Relation Between Vegetation and PermafrostThere was a clear association between presence ofpermafrost in the upper 3 m and different vegetationcommunities. On point bars and alluvial islands, HorsetailFigure 1. In each 20 m x 20 m plot, the water-jet drilled hole in thecentre was complemented with two others. Each plot was assigned asingle NSP presence or absence label based on the rule of majority.Spruce forests were not the focus of this research since the presenceof permafrost is ubiquitous there with thin active layer thicknesses(Kokelj & Burn 2005).221