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Boreal forests store substantial amounts of carbon in vegetation and soil pools. The magnitude of these pools is related to fire regime attributes. Climate change is expected to alter boreal fire regimes, leading to changes in the amount of stored carbon. Quantifying these changes is of importance to understanding and managing global carbon budgets. We investigate how fire return interval (FRI) interacts with seasonal variation in fire intensity and severity to affect carbon stocks and fluxes in the boreal black spruce forests of Quebec, Canada. A size-class structured population model of stand dynamics was coupled with an established model of boreal carbon dynamics and linked to a simplified representation of fire regime that simulates the occurrence of fires and their direct effects on canopy tree mortality, surface fuels combustion and regeneration. We simulated carbon stocks and fluxes under seven levels of FRI and two fire seasons: spring and summer. We tested for an effect of these fire regime parameters on equilibrium mean C stocks. All dead organic matter and biomass carbon stocks were sensitive to FRI between 60 and 300years. These C stocks were lower for summer fires that occurred under shorter FRIs. Net primary production was highest at FRIs between 150 and 300years. Total C stocks were highest for FRIs from 150 to 700years, varying little over this range. There was a small but significant difference of C pool sizes between stands with even and uneven tree-diameter distributions. This difference was greatest for FRIs of 150years or less. Reductions in equilibrium C storage are forecasted for nearly 27% of the study region under expected end-of-century climates.