Growing season CO2 exchange and evapotranspiration dynamics among thawing and intact permafrost landforms in the Western Hudson Bay lowlands

Warming conditions across Canada's subarctic and arctic regions are causing permafrost landforms to thaw, resulting in rapid land cover change, including conversion of peat plateaus to wetland and thermokarst. These changes have important implications for northern ecosystems, including shifting controls on carbon uptake and release functions, as well as altering evapotranspiration (ET) rates, which form feedbacks with climatic change. Four landforms (peat plateau, sedge lawn, channel fen, and a thermokarst shoreline collapse scar) in the Hudson Bay Lowlands, northern Manitoba, were instrumented for weekly chamber measurements of carbon dioxide (CO2) and water vapor flux over a summer season (May to September 2014). Relative to other landforms, thermokarst CO2 exchange was characterized by high respiration rates early in the season, which decreased and were offset later in the season by CO2 uptake driven by sedge productivity. For all landforms, ET peaked post-snowmelt during rapid active layer thaw, and decreased throughout the growing season, controlled primarily by atmospheric vapor deficits. This work shows distinct differences in CO2 exchange and ET between intact and thawing permafrost features. While representative of small-scale processes in a single study region over one growing season, the results presented in this study have important implications for our understanding of ecohydrological and biogeochemical functioning of subarctic landscapes under future climates.