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Boreal forest ecosystems play a significant role in the global C cycle. Over the current century, these forests are expected to undergo one of the largest increases in temperatures. Climate change is expected to affect boreal C storage through changes in fire regimes, tree growth and decomposition rates. We developed a diameter-size structured model, coupled with a boreal soil C dynamics model and a fire effects model to investigate the short-term responses of boreal carbon storage to climate change. We applied this model to 1.0 ha patches of mono specific black spruce stands under four climatic periods: 1980-2010, 2010-2040, 2040-2070 and 2070-2100. Forest inventory and historical fire intensity records representative of our study area were used to calibrate the model. Historical and future fire return interval (FRI) maps and projected weather data estimated by CanESM2 RCP8.5 climate scenario were used to drive historical and future disturbance frequency, forest growth, decomposition rates, and C dynamics. Black spruce trees showed amplified decreases in growth and temperature-sensitive decomposition rates of soil C pools were enhanced as projected air temperature increased. This study indicates that the short-term effects of climate change on black spruce forests productivity and C storage are substantially negative. In our simulation experiments, ecosystem C storage was reduced by 10% by the end of 2100. We predicted a marked and widespread change from C sink to C source, which has already started and will persist at least until 2100. The results of this study indicate that black spruce forest of northern Quebec could be losing their capacity to sequester and store organic C over the next coming decades due to climate change.