Shrub climate sensitivity and the heterogeneity of the greening of the tundra

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The greening of Arctic and Subarctic regions over the last decades has been mainly associated with the rapid expansion of erect shrub species. Observed throughout the circumpolar region, shrub expansion deeply alters the structure and function of tundra ecosystems by reducing overall albedo in terrestrial ecosystems, resulting in a positive feedback to climate change. Shrubs also affect patterns of snow deposition, accumulation and snowmelt and contribute to deepening the active layer and accelerating permafrost degradation. Shrub expansion is however highly heterogeneous throughout the Arctic and the Subarctic regions and the drivers of such heterogeneity are still poorly understood. While climatic drivers of erect shrub growth have been partially identified, recent advances in shrub dendrochronology suggest that shrub climate sensitivity is site- and species-specific, with evidence of higher sensitivity in Eurasia and in wetter regions. However, these findings are based on studies that rely on samples collected at different positions on shrub individuals (root collars or stems) and therefore, they did not consider the impacts of the multi-stem growth form of erect shrub species on their climate sensitivity. Differential climate sensitivity within a single individual might hamper analysis and interpretation of large dendrochronological datasets. ******Therefore, the long-term objectives of this research program aim to disentangle the effects of climatic, topographic and edaphic factors on the growth of shrub species and, by extension, on the heterogeneity of the greening trend observed in the tundra over the last decades. Over the next five years, my team will test the following research hypotheses: 1. The heterogeneity in the greening trend observed throughout northern North America results from the differential climate sensitivity of shrubs at various spatial scales; 2. The climate sensitivity of shrubs is closely associated with both edaphic and topographic variables; 3. The multi-stem growth form of erect shrub species influences their sensitivity to climate.******This research program linking shrub expansion, greening of the tundra and shrub climate sensitivity will generate results of significant interest not only for the scientific community but also for governmental agencies responsible for the development of the North. By determining if the greening trend heterogeneity is associated with differential shrub climate sensitivity, our results will feed the models that aim to predict vegetation changes over the next century and will contribute to the development of shrub dendrochronology by, at the very least, laying the foundations for standard sampling protocols. Through the different research projects, this program will contribute significantly to the training of Highly Qualified Personnel (2 Ph.D., 2 M.Sc. and 5 undergraduate students).