Quantified positive radiative forcing at a greening Canadian boreal-Arctic transition over the last four decades

Climate warming in northern and Arctic regions drives vegetation growth and shifts species distribution. In northern Quebec's Boreal-Arctic transition (forest-tundra ecotone), this is seen in the replacement of lichen by shrubs, primarily dwarf birch. These changes impact surface albedo, contributing to climate forcings with broad consequences. This study measures vegetation changes in Tasiapik valley near Umiujaq, Quebec, using a combination of (1) hyperspectral data (347–2400 nm) collected from 62 vegetation assemblages, including lichen, dwarf birch, willow, and spruce, to calculate broadband albedo, and (2) remote sensing data from Landsat satellites over 1984–2023. By combining these data, the proportion of vegetation type for each pixel was determined at the beginning and end of the 40-year period. The areal coverage of six main vegetation types was quantified over the 9.25 km² valley. The most significant change was lichen replacement by dwarf birch with lichen understory, leading to an albedo reduction from 0.233 to 0.168 and a summer shortwave forcing of 11.17 W m⁻². At the valley scale, the spatially-averaged summer forcing was 2.16 W m⁻² when considering all observed vegetation changes. These values, lower than those in previous Norwegian studies, highlight the spatial variability of shortwave forcing due to lichen replacement. We observed that the vegetation change producing the greatest positive radiative forcing also caused the strongest greening. This suggests that Landsat-based greening may be used as a proxy for surface albedo change on an Arctic scale. This unique combination of ground and satellite data allows quantification of a direct, first-order effect of Arctic shrubification.