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This study uses a process-based biogeochemistry model to simulate the carbon budget in North American boreal forests during 1986-2020 with satellite-derived burn severity data. We find that fires remove ecosystem carbon through combustion emissions of 2.4 Pg C and reduce net ecosystem production (NEP) from 32.6 Tg C/year to 0.8 Tg C/year, resulting in 3.5 Pg C loss to the atmosphere during the period, changing the boreal forests from a carbon sink to a source. When canopy’s cooling effects are considered, model estimates lower soil temperatures in summer, inducing lower net primary production due to inhibited nitrogen uptake and mineralization and lower heterotrophic respiration, reducing NEP from 1.6 Tg C/year to 0.8 Tg C/year. This reduction accounts for 50% of the simulated post-fire NEP without considering canopy influences. Our study highlights the importance of fire and its induced canopy change in boreal forest soil thermal dynamics and carbon balance.
