Detecting Forest-Wetland Changes in Northwestern Canada's Discontinuous Permafrost Region Using Landsat Trend Analysis

Abstract

Rapid circumpolar warming has led to accelerated permafrost thaw that has induced extensive landscape change in northwestern Canada. In northwestern Canada's discontinuous permafrost zone, forest-dominated peatland landscapes, which were largely underlain by permafrost, are transitioning to treeless, wetland-dominated landscapes that are permafrost-free. Previous studies that focused on permafrost thaw-induced landscape transitions in boreal peatlands have used aerial photographs and/or high-resolution satellite images acquired only at end nodes or decadal intervals across each study's time frame. This study uses a dense near-annual resolution of mosaicked Landsat imagery to determine if changes can be detected to peatland landscapes over a 37-year period for 10 study sites across a latitudinal transect through the southern Northwest Territories and northern British Columbia, Canada. Tasselled cap (TC) transformations (brightness, greenness and wetness) are applied to the Landsat imagery and compared with forest cover changes (1970–2010) derived from high resolution areal imagery to visually assess the performance of this method in revealing forest-wetland changes driven by permafrost thaw. While methods based on Landsat TC trends have been successfully applied in northwestern Canada with forest fires and mass wasting, this study explores pixel trends as well as time series for more gradual thaw-induced changes across peat plateaus, collapse scar wetlands and channel fens. This study demonstrates that TC transformations can be used to determine differing brightness, greenness and wetness trajectories for individual landcovers within permafrost peatlands (i.e., forest, collapse scar wetland and fen). However, the subtle and non-monotonic landcover and hydrologic changes resulting from decades of permafrost thaw can be more challenging to detect. At the regional scale, widespread spectral greening was observed that was not solely indicative of forest cover or density increases, whereby TC trends were not consistent with expected permafrost thaw-induced landscape changes. This work suggests that TC transformations may be limited in use for assessing broad-scale effects of gradual permafrost thaw processes that occur at high temporal and spatial frequencies.