Controls on near-surface temperatures across tree line in the Mackenzie Delta area, Northwest Territories, 2004-2009

M.J. Palmer, S.V. Kokelj, C.R. Burn

In the proceedings of: GEO2010 Calgary: 63rd Canadian Geotechnical Conference & 6th Canadian Permafrost Conference

Session: T2-E

ABSTRACT: Air and near-surface ground temperatures were recorded between 2004 and 2009, and properties of the snow cover were measured annually in late winter at eight sites along a 130-km transect across the forest-tundra transition in the uplands east of the Mackenzie Delta, NWT, to investigate the relations between these variables along the ecological gradient. Late winter snow depths decreased along the tree-line transect in association with changes in vegetation cover between sites. A gradient in near-surface ground temperature was observed along the transect, as annual near-surface ground temperatures decreased northward. The ground temperature gradient was steepest (0.1 to 0.3oC/km) between 20 and 30 km from Inuvik. The steep gradient in near-surface ground temperature was associated with large differences in late-winter snow depth between the sites. 1 INTRODUCTION The position of tree line has a central role in influencing the distribution of permafrost in Canada. The transition between forest and tundra is a significant ecological boundary characterized by variation in the nature of vegetation and snow cover, which strongly influences soil microclimate and permafrost temperatures (Rouse, 1984; Sturm et al., 2001; Kokelj et al., 2007). The consequences of climatic warming in northern Canada (Morrison et al., 2000) may include the northward migration of tree line and an increase in shrubiness of the arctic tundra (Landhäusser and Wein, 1993; Myneni et al., 1997; Sturm et al., 2001). Understanding specific relations between air and ground temperatures across the forest-tundra transition is critical to improve our understanding of how permafrost conditions vary across this gradient and which factors may contribute to future change. The Mackenzie Delta area is the location of the northern most extent of tree line in Canada. In the uplands east of Mackenzie Delta, the tree-line transition is characterized by a mix of boreal and tundra vegetation often dominated by large shrubs, and occurring as a series of ecological stages, heavily influenced by fire history (Ritchie, 1984; Timoney et al., 1992; Landhäusser and Wein, 1993; Payette et al., 2001; Lantz et al., 2010). Ground temperature conditions have been previously described in several environments of the Mackenzie Delta region (Mackay and MacKay, 1974; Smith, 1975; Burn and Kokelj, 2009; Kanigan et al., 2009;). A large difference in ground temperature has been identified between the boreal forest of Inuvik and tundra communities north of tree line, but detailed information on the transition is sparse in both the Delta and adjacent uplands (Burn and Kokelj, 2009). While relations between vegetation, snow cover, and ground temperature have been studied extensively at the local scale (Mackay and MacKay, 1974; Smith, 1975; Sturm et al., 2001), the relative influence of factors which control ground Figure 1 Map of the study area and research sites. temperatures across tree line have not been investigated in detail. This paper investigates the influence of air temperature, vegetation, and snow conditions on near-148

RÉSUMÉ: s on near-surface temperatures across tree line in the Mackenzie Delta

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