Aim To detect possible expansion of boreal forest stands in response to rec
ent warming. Previous modelling studies have concluded that major shifts in
vegetation patterns, including changes in boreal forest extent, could aris
e during the next two centuries under global warming scenarios. However, fi
eld investigations of tree stands at ecotones have so far revealed little i
ndication of stand response to warming during the last 100 years. This stud
y uses a c. 25-year record of Landsat satellite observations to quantify ch
anges in forest stand extent in two areas of northern Canada.
Location Two regions of northern Canada, near Richmond Gulf, Quebec, and Gr
eat Slave Lake, north-west Territories.
Methods Normalized-difference vegetation index (NDVI) plots across forest-t
undra boundaries were obtained from radiometrically corrected Landsat image
ry 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 exp
ansion rates to <200-300 m per century. Also, no evidence for local expansi
on of forest stands (e.g. within sheltered valleys) was found.
Main conclusions The results are consistent with field observations, and su
ggest that, at the moment, boreal forest extents remain basically stable. T
his may reflect inherent lags between forest response and climate change, o
r competitive pressures between tree stands and surrounding tundra and herb
aceous vegetation.