Wind-Snow Interactions and Treeline Advance in the Medicine Bow Mountains, Wyoming: A Coupled Examination Using Dendroecology and Remote Sensing

Research suggests that broad-scale increases in temperature facilitated an abrupt initiation of upper treeline advance beginning in the 1950s at climatic treelines throughout a large portion of the southern and central Rocky Mountains. Despite this regional trend, patterns of finer scale variability often imply the likely influence of both wind-snow interactions and temperature on driving regeneration dynamics in these climatically-sensitive ecotones. This is particularly true for mountain ranges subject to consistently strong winds, such as the Medicine Bow Mountains of southeast Wyoming. A rich history of treeline work exists for this area, yet questions remain regarding how influential wind and snowpack variability are in governing climate-vegetation interactions within upper treeline ecotones and whether this varies according to the level of wind exposure. Here we present a coupled examination using dendroecology and remote sensing to test the hypothesis that sufficient snow cover is required in order for the ecological manifestation of increasing temperatures to appear at upper treeline; namely treeline advance. We used dendroecological methods to reconstruct the history of colonization on the two highest peaks in the range (Medicine Bow Peak Massif and Kannaday Peak). We sampled a total of six sites by placing nested-belt transects on two south-facing and one north-facing site for each peak. To gauge the influence of wind-snow interactions at each site, we analyzed remotely-sensed images. We selected three sets of LANDSAT images for each mountain peak based on years with maximum, minimum, and mean snowfall conditions to capture the entire range of variability. Results demonstrate that snow cover can be a critical modifier of treeline advance, especially on wind-exposed slopes and on mountain peaks with a relatively dry hydroclimatology, where a protective snow layer is only evident during high snow years. Overall, this research suggests that the role of wind-snow interactions in mediating upper treeline advance is contingent on local hydroclimatology and that future changes in these environments will most likely be driven by changes in both temperature and snowpack conditions.