In the Arctic, a widespread transition from tundra to shrub-dominated vegetation appears to be underway. Winter processes could be critical to this tundra-to-shrub transition, because larger and denser shrubs trap more snow, which leads to warmer soil temperatures during winter and could permit greater decomposition and mobilization of nitrogen from soil organic matter. The goals of this research project include determining 1) whether snow trapped by shrubs in winter promotes biologically significant mobilization of nitrogen from soil, 2) whether shrubs take up nitrogen mobilized during winter and increase their growth, thus promoting shrub expansion, 3) whether shrub expansion promotes carbon loss from soil, as has been seen previously in fertilized tundra plots that have become dominated by shrubs, and 4) understanding how shrubs recover from being laid down by winter snow, and how snow affects their growth and mortality. These goals will be addressed by field experiments manipulating snow accumulation and shrub architecture, and by measuring plant and soil carbon and nitrogen where shrubs have naturally expanded over the last 50 years. Expansion of shrubs may be the most widespread land surface change in the Arctic as climate warms. Our results will help reveal the mechanisms underlying shrub expansion and their consequences for ecosystem cycling of carbon and nitrogen. Arctic and boreal regions have large stores of soil carbon that could greatly increase atmospheric CO2 concentration if released, promoting further climate warming.
