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The effect of rising atmospheric concentrations of carbon dioxide (CO2) on tree size-density relations has not been thoroughly investigated despite such relations forming the basis of contemporary forest stand stocking metrics. As an initial exploration of the effects of varying CO2 concentrations on size-density relations, we treated model stands of trembling aspen with increased concentration of CO2 (560 ppm) for 11 years in northern Wisconsin and found compelling differences in stand development compared with those in ambient atmosphere. Pure and mixed trembling aspen forests grew faster in elevated atmospheric CO2 and with developmental trajectories of self-thinning communities carrying more stand basal area at equivalent trees per hectare. Comparison of our experimental results with historical stand density data suggests forest developmental characteristics may have changed in response to historical increases in atmospheric CO2; thus, current stocking guides could underestimate maximum stand carrying capacity. Although our results may be constrained to our study species in a controlled experiment, they still suggest that comprehensive reassessments of stand growth dynamics and size-density relations may be needed across suites of species and spatial extents because of global atmospheric CO2 increases that have already occurred.