Hotter, Richer, Shrubbier: Do Climate Change Impacts on Nitrogen Availability Explain Shrub Expansion in a Warming Arctic?

Abstract

Rapid climate change is transforming Arctic ecosystems, including circumpolar increases in the abundance, range, and competitive dominance of tundra shrub species. Changes in nitrogen cycling caused by warming temperatures and earlier snowmelt are thought to contribute to this shrubification. However, empirical evidence quantifying how summer warming and earlier snowmelt impact the realized levels of nitrogen available to tundra plants remains limited. To fill this knowledge gap, this thesis investigates how summer warming, earlier snowmelt, and plant competition modulate nitrogen supply and shrub growth. We conducted experimental manipulations of summer warming, earlier snowmelt, and nutrient addition at two sites within and beyond the existing range of key expanding shrub species in Northern Alaska. We measured plant-available nutrients and the growth response of three focal shrub species: birch (Betula nana), willow (Salix sp.), and alder (Alnus viridis). Our results revealed that within the existing range of shrub species, warming and snow removal shifted the dominant form of inorganic nitrogen from nitrate to ammonium without changing total nitrogen supply. These changes in nitrogen availability did not translate into increased shrub growth for any species. When combined with nutrient addition treatments that elevated plant available nitrogen 50-fold, warming further increased shrub growth for birch and willow (but not alder). Beyond the current shrub range, nutrient availability showed high spatial variability and did not consistently respond to climate manipulations. Competition experiments showed that birch stem growth was significantly enhanced only when nutrient addition was coupled with competition removal. Collectively, these findings suggest that climate-driven increases in nitrogen availability are insufficient to drive Arctic shrub expansion, given the key importance of the magnitude of nutrient changes and interactive effects with other climate and biotic factors.