Boreal Forest Wildfire in a Changing Climate

Abstract: The boreal region contains 40% of the earth’s carbon (C) that is stored in vegetation and soils with its forests accounting for almost 30% of the terrestrial C sink. Boreal forests are experiencing some of the most rapid rates of climatic warming and increases in fire activity, threatening to release large amounts of their dense C reserves to the atmosphere. While climate and wildfire place strong controls on ecosystem function, few observations have been made regarding their potential synergistic effects. This thesis utilized 50 separately occurring summer 2018 wildfires, each paired with an unburnt control and spread across the near-entire climatic range of boreal Fennoscandia, providing the first field campaign targeting regional-scale variation in boreal wildfire activity. It was found that climate determined the prefire quantity and structure of forest fuel, which in turn controlled both the immediate and extended fire-induced removal rates of C and nitrogen (N) to the surrounding land and atmosphere. Reorganization of ecosystem structure through wildfire burning strengthened climatic control over soil microbial community structure and nutrient cycling, with warmer regions experiencing enhanced nutrient mobilization under oligotrophic processing of remaining burnt organic soil. Greater warmth and soil fertility together stimulated growth of broadleaf overstory species under a restricted recovery capacity of previously established coniferous and ericaceous plant communities, though this transition to novel growth patterns was limited by residual, biodiversity inhibiting plant-soil feedbacks. Together these results suggest that continued climate change and wildfire can serve to remove C and nutrients from burnt boreal forests under a limited capacity for their reaccumulation. This postfire resource drain is capable of influencing longer-term patterns of ecosystem regrowth and the boreal forest contribution to the global land-atmosphere C balance that is deserving of further study.