Stability of boreal forest stands during recent climate change: evidence from Landsat satellite imagery

Aim To detect possible expansion of boreal forest stands in response to recent warming. Previous modelling studies have concluded that major shifts in vegetation patterns, including changes in boreal forest extent, could arise during the next two centuries under global warming scenarios. However, ®eld investigations of tree stands at ecotones have so far revealed little indication of stand response to warming during the last 100 years. This study uses a c. 25-year record of Landsat satellite observations to quantify changes in forest stand extent in two areas of northern Canada. Location Two regions of northern Canada, near Richmond Gulf, Quebec, and Great Slave Lake, north-west Territories. Methods Normalized-difference vegetation index (NDVI) plots across forest-tundra boundaries were obtained from radiometrically corrected Landsat imagery acquired during the 1970s and 1990s. These curves were evaluated to look for changes over the c. 25-year period related to forest stand expansion. Results Although forest-tundra boundaries could be clearly mapped from the satellite data, no obvious change in forest boundaries was apparent during the duration of the image time series, constraining recent geographical expansion rates to <200±300 m per century. Also, no evidence for local expansion of forest stands (e.g. within sheltered valleys) was found. Main conclusions The results are consistent with ®eld observations, and suggest that, at the moment, boreal forest extents remain basically stable. This may re ̄ect inherent lags between forest response and climate change, or competitive pressures between tree stands and surrounding tundra and herbaceous vegetation.