Erect shrub encroachment in the High Arctic: evidence and consequences

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Reports of increasing shrub cover in the Low Arctic are mounting whereas the situation at the transition between Low and High Arctic, remains understudied. Yet we recently highlighted the sensitivity of shrubs to climate variability near their longitudinal limit. An increase in erect shrub cover (20 to 70cm high or more) where only prostrate vegetation grows would drastically transform the High Arctic. Similarly to the treeline, I refer to this transition as the erect shrubline. Isolated populations of these shrubs have maintained themselves for years at these latitudes but the recent warming may have promoted better growth and/or production of more viable seeds. If this is the case, erect shrubs may invade adjacent prostrate communities. My long-term research objective is to contribute to a better understanding of the dynamic of arctic vegetation, its pivotal role in northern ecosystems and the ecological processes involved in this transformation. Over the next five years, with my students, we will combine field observations, shrub ring counts (dendrochronology) and a field experiment with snow manipulation (snow fences) and nutrient addition to address the status of erect shrubs at the northern limit of their range in Canada. We will analyse functional traits (basic characteristics that can be compared among sites and also can be used to model plant growth) of dominant shrub species and determine the distribution, reproductive status and colonisation pattern of erect shrub populations in high arctic ecosystems of Baffin and Bylot Islands, Nunavut. We will first try to answer two questions about the response of shrubs to current changes : Q1. Are erect shrub populations increasing at the northern limit of their range? Q2. What are the plant functional strategies of dominant shrub species in the High Arctic? Then, we will quantify the effect of erect shrub growth on local ecosystem transformation in the High Arctic to address a third question : Q3. How is a shift in growth form impacting high arctic ecosystems? A shift from herb and prostrate shrub tundra to erect shrub tundra will require redrawing of the current limit of the High Arctic, and will critically alter the productivity of the system (more shrubs, less lichens) impacting directly foodwebs (herbivores and decomposers), ecosystem processes (nutrient cycling) and the global system (albedo, carbon flux). Taller vegetation will trap snow altering winter processes (including insulation of permafrost, albedo, shelter of small mammals and traffic routes). Whether the progression of erect shrub tundra is slow or rapid is currently unknown, and the proposed project will be the first to address this question at the landscape level in critical locations in High Arctic Canada.