Widespread Ecological Impacts of Shrub Expansion in Alpine and Arctic Tundra in the Yukon

The Arctic is experiencing a change in vegetation boundaries, particularly the spread of tall shrubs into tundra ecosystems. Growing evidence from repeat aerial photography and satellite imagery shows an expansion of tall, canopy forming woody shrubs up mountain slopes and northward into Arctic tundra. The correlation between warming and greening has been used to link climate change with shrub expansion; however, the exact mechanisms driving shrub increase are probably a more complex interaction between nutrients, snow, soil temperatures and disturbance.

Woody shrubs will likely change the structure of the tundra and by altering albedo, nutrient turnover times, carbon cycling and biodiversity. Enhanced nutrient cycling associated with warmer winter soil conditions may provide a positive feedback mechanism that could promote further expansion of shrubs in the Arctic.

We have surveyed shrub abundance in the mountains of the Kluane Region and on the Arctic Coast of the Yukon, and found evidence of shrub expansion including younger age distributions of shrubs at higher elevations and expansion of individual willow patches identified in historic photographs. For willows across the Yukon, we have found positive correlations between annual growth and summer temperatures, leading us to attribute the observed shrub expansion to warming over the last century. Here, we present experimental quantifications of the ecosystem level impacts of greater shrub canopy cover including changes to snow depths, soil temperatures, carbon cycling, and surface reflectance.

Understanding both the rate of change in canopy forming woody shrubs and the impacts of this change on ecosystem function will improve circumpolar estimates of future carbon storage, wildlife habitat and permafrost integrity in tundra ecosystems.