Erect shrub encroachment in the High Arctic: evidence and consequences

关于低北极地区灌木覆盖增加的报告越来越多，而低北极和高北极之间过渡的情况仍未得到充分研究。然而，我们最近强调了灌木在其纵向极限附近对气候变率的敏感性。在只有匍匐植被生长的地方，增加直立灌木覆盖（20至70厘米高或更高）将彻底改变北极高地。与树线相似，我把这种转变称为直立灌木线。这些灌木的孤立种群在这些纬度维持了多年，但最近的变暖可能促进了更好的生长和/或产生更多的可存活种子。如果是这种情况，直立灌木可能会入侵邻近的匍匐群落。

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.