Birch (Betula albosinensis Burk.) seedlings were grown under two CO2 concentrations, 350 μmol mol-1 (ambient CO2) and 700μmol mol-1 (elevated CO2), and in two planting densities, 32 plants/m2 (low density, LD) and 72 plants/m2 (high density, HD). The objectives were to characterize the responses of leaf photosynthesis to long-term elevated CO2 in birch seedlings in different planting densities, and to assess whether elevated CO2 regulates the photosynthetic capacity of leaves, in terms of nitrogen concentration (N), the activity of ribulose bisphosphate carboxygenase (Rubisco), the photosynthetic efficiency of Rubisco and the concentration of nonstructural carbohydrates (TNC). In both planting densities, the leaves of birch seedlings grown under elevated CO2 but measured at 350 μmol mol-1 CO2 had a lower leaf N concentration, net CO2 assimilation rate (A), transpiration (E) and stomatal conductance (gs), and a higher water use efficiency (WUE) than those of birch seedlings both grown and measured under ambient CO2 concentration. On the other hand, the values of A, E, gs and WUE were significantly affected by planting density under ambient CO2, whereas these parameters were not affected by planting density under elevated CO2. In contrast, the levels of sucrose, soluble sugars, starch and TNC in the leaves were not significantly affected by planting density under ambient CO2, whereas these parameters were significantly affected by planting density under elevated CO2. Our results demonstrated that there are different acclimations of leaf photosynthesis in birch seedlings, as affected by elevated CO2 and planting density, and they highlighted the importance of the CO2 level and planting density for the physiological ecology of woody plants.