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

      Melham, Mark; Valentine, Dave; Panda, Santosh; Brinkman, Todd (2019-12)
      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.
    • Plant succession in the Arctic Brooks Range: floristic patterns from alpine to foothills, along a glacial chronosequence and elevation gradient

      Kasanke, Shawnee A.; Walker, Donald A.; Chapin, F. Stuart III; Mann, Daniel H. (2019-08)
      In the wake of rapid glacial retreat, alpine habitats in the Arctic are expanding as freshly exposed surfaces become vegetated. Many glaciers in alpine cirques have nearly disappeared, and little is known about the rate of colonization or pioneer communities that develop following deglaciation. Newly developed habitats may provide refugia for sensitive Arctic flora and fauna, especially in light of polar warming. To assess this process, vegetation communities developing on two recently deglaciated moraines in the Central Brooks Range were surveyed and compared with communities along a transect spanning both a glacial chronosequence (40-125,000 years since deglaciation) and an elevation gradient (1700-500 m) into the Arctic foothills. Results show that primary succession begins almost immediately following deglaciation. Within forty years fine-grained and rock substrates hosted small communities of 8-13 vascular and nonvascular plant species. Many pioneer taxa, especially lichens, persist into later stages of succession. Overall succession is directional and slow, increasing in species richness for about 10,000 years, after which richness decreases and communities stabilize. This is the first vegetation study on primary succession in the high Central Brooks Range, providing a missing link to a vegetation transect along the Arctic Bioclimatic gradient.