Winter Methane Fluxes Over Boreal and Arctic Environments

Abstract Unprecedented warming of Arctic–boreal regions (ABR) has poorly understood consequences on carbon cycle processes. Limited winter data add uncertainty to annual methane (CH 4 ) budgets. In this study, winter CH 4 flux measurements were conducted using the snowpack diffusion gradient method over five ABR ecosystem types in Canada and Finland: closed–crown and open–crown coniferous boreal forest, boreal wetland, erect–shrub and prostrate–shrub tundra. Boreal forest uplands acted as net CH 4 sinks with soil liquid water content identified as an important environmental control, while the boreal wetland acted as net CH 4 source during winter. We identified several wetland tundra CH 4 emission hotspots and large spatial variability in boreal wetland CH 4 emissions. Snow cover conditions and near‐surface soil temperature did not impact significantly CH 4 exchanges. Our results indicate non–negligible winter CH 4 flux, which must be accounted for in annual carbon balance and terrestrial biosphere models over ABR.