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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. [ABSTRACT FROM AUTHOR]

Peatlands in the Western Boreal Plains act as important water sources in the landscape. Their persistence, despite potential evapotranspiration (PET) often exceeding annual precipitation, is attributed to various water storage mechanisms. One storage element that has been understudied is seasonal ground ice (SGI). This study characterized spring SGI conditions and explored its impacts on available energy, actual evapotranspiration, water table, and near surface soil moisture in a western boreal plains peatland. The majority of SGI melt took place over May 2017. Microtopography had limited impact on melt rates due to wet conditions. SGI melt released 139mm in ice water equivalent (IWE) within the top 30cm of the peat, and weak significant relationships with water table and surface moisture suggest that SGI could be important for maintaining vegetation transpiration during dry springs. Melting SGI decreased available energy causing small reductions in PET (<10mm over the melt period) and appeared to reduce actual evapotranspiration variability but not mean rates, likely due to slow melt rates. This suggests that melting SGI supplies water, allowing evapotranspiration to occur at near potential rates, but reduces the overall rate at which evapotranspiration could occur (PET). The role of SGI may help peatlands in headwater catchments act as a conveyor of water to downstream landscapes during the spring while acting as a supply of water for the peatland. Future work should investigate SGI influences on evapotranspiration under differing peatland types, wet and dry spring conditions, and if the spatial variability of SGI melt leads to spatial variability in evapotranspiration. [ABSTRACT FROM AUTHOR]