Pairwise comparisons of shrub change across alpine climates show heterogeneous response to temperature in Dall's Sheep range

Abstract

Encroachment of woody vegetation into alpine and high latitude systems complicates resource use for specialist wildlife species. We converted Landsat imagery to maps of percent shrub cover in alpine areas of Dall's sheep (Ovis dalli dalli) range. We then compared percent cover to interpolated climate data to infer drivers of shrub change between the 1980s and 2010s and determine if that change is occurring at different rates in climatically distinct alpine areas. We identified areas spatially interconnected by their mean July temperature intervals and compared their rates of shrub change, finding net rates of shrub growth were higher at temperatures notably above shrub growing season minimums. Along a climatic gradient, high precipitation areas had highest net shrub change, Arctic areas followed, while alpine areas of interior Alaska and the cold Arctic showed the least amount of net shrub change at these higher temperatures. Despite the requirement of higher temperatures for shrub growth, temperature and net shrub change displayed different relationships across the range wide climatic gradient. In areas of rapid climate warming, such as the Arctic and cold Arctic, the linear correlation between shrub change and temperature was highest. In the high precipitation areas where temperatures have been largely above growing season minimums during the study period, precipitation had the strongest linear correlation with shrub change. High latitude studies on shrub change focus primarily on expansion in the Arctic, where increased greening trends are linked to higher rates of warming. We provide the broadest climatic examination of shrub change and its drivers in Alaska and suggest shrub expansion 1) occurs more broadly than just in areas of notable climate warming and 2) is dependent on different environmental factors based on regional climate. The implications for Dall's sheep are complicated and further research is necessary to understand their adaptive capacity in response to this widespread vegetative shift.