Shrub expansion in Arctic Alaska: 50 years of change documented using aerial photography

Abstract

Evidence from arctic Alaska suggests that the terrestrial landscape is changing in response to warming. Between 1946 and 1951, several thousand low-altitude panchromatic oblique aerial photographs were taken as part of geologic reconnaissance and exploration of Alaska's Arctic Slope and Brooks Range. For this study, 202 of the landscapes in the old photographs were re-photographed. Comparison of the old and new photographs revealed an increase in shrub cover in the last half-century. The changes were observed over a 220,000 km2 tract of arctic tundra, and it is likely that they are more widespread. A quantitative method for comparing the photographs yielded an increase in alder shrub cover from 14 to 20%, with similar increases observed for willow and birch shrubs. This shrub expansion was observed in many landscape positions, including hill slopes, river terraces, and also river floodplains, where the increase in shrub vegetation has resulted in the narrowing and stabilization of floodplains. The regional expansion of shrubs documented in the photographs can only be explained by a perturbation operating on a similarly large scale. In the absence of large-scale disturbances like fire, the increase in shrubs documented here is most likely to be a product of elevated temperatures and other changes in climate favorable to shrub growth.