High-latitude warming, fire, CO₂, and precipitation will increase over the 21^st century. Under these conditions, CO₂ exchanges with the atmosphere will be strongly dependent on plant and microbial nutrient constraints. Here we investigate the role of plant nutrient acquisition, use, and retention traits on 21^st century Alaskan boreal forest composition following fire using the mechanistic ecosystem model, ecosys. The model has been rigorously and successfully tested against observations from eddy covariance flux towers in many high-latitude sites across multiple years, and against large-scale remote sensing vegetation observations. We additionally tested ecosys against observations of Alaska forest composition, biomass (R²=0.59), upscaled GPP (R²=0.67), and eddy-covariance measurements at three fire chronosequence stands (R²= 0.52 - 0.79). We conducted seven sensitivity simulations to test the effects on boreal forest vegetation dynamics of area burned, fire severity, post-fire seedling regeneration, and changes in precipitation. Consistent with changes over the Holocene, 21^st century climate and fire will alter forest composition. Competition for nutrients after fire in early succession and for light later in succession explain modeled forest compositional changes. Plant traits for carbon and nutrient acquisition and retention, i.e., slower uptake and greater retention of nutrients in evergreen conifer trees compared to deciduous broadleaf trees, explain these patterns. Over the 21^st century, rapid nitrogen mineralization from climate- and fire-induced soil warming enabled post-fire deciduous broadleaf trees to sustain continued rapid nitrogen uptake and CO₂ fixation. Our study suggests the relative dominance of deciduous broadleaf trees nearly doubles by 2100, strongly affecting the carbon cycle, surface energy fluxes, and ecosystem function, and thereby feedbacks with climate.