The evidence for shrub expansion in Northern Alaska and the Pan‐Arctic

TLDR

Climate warming is expected to increase shrub abundance and extent in Arctic tundra, with evidence of expansion already observed in Canada, Scandinavia, and parts of Russia. The study used 202 pairs of old and new oblique aerial photographs across a 620 km × 350 km area, complemented by plot and NDVI remote‑sensing analyses, to detect shrub expansion. Repeat photography and remote‑sensing analyses show that alder, willow, dwarf birch, and smaller shrubs have increased over the past 50 years across northern Alaska, and when combined with similar evidence elsewhere, indicate a pan‑Arctic vegetation transition that could reshape ecosystem structure and climate, biota, and human systems.

Abstract

Abstract One expected response to climate warming in the Arctic is an increase in the abundance and extent of shrubs in tundra areas. Repeat photography shows that there has been an increase in shrub cover over the past 50 years in northern Alaska. Using 202 pairs of old and new oblique aerial photographs, we have found that across this region spanning 620 km east to west and 350 km north to south, alder, willow, and dwarf birch have been increasing, with the change most easily detected on hill slopes and valley bottoms. Plot and remote sensing studies from the same region using the normalized difference vegetation index are consistent with the photographic results and indicate that the smaller shrubs between valleys are also increasing. In Canada, Scandinavia, and parts of Russia, there is both plot and remote sensing evidence for shrub expansion. Combined with the Alaskan results, the evidence suggests that a pan‐Arctic vegetation transition is underway. If continued, this transition will alter the fundamental architecture and function of this ecosystem with important ramifications for the climate, the biota, and humans.