Down-regulation of photosynthesis in elevated CO₂ (eCO₂), could be attributed to depletions in nitrogen (N) availability after long term exposure to eCO₂ (progressive nitrogen limitation, PNL) or N dilution due to excessive carbon accumulation of nonstructural carbohydrates. To investigate this, we examined N availability, photosynthetic characteristics, and leaf N allocation in Pinus densiflora, Fraxinus rhynchophylla, and Sorbus alnifolia, grown under three different CO₂ concentrations, ambient CO₂ (aCO₂), aCO₂ × 1.4 ppm (eCO₂1.4), and aCO₂ × 1.8 ppm (eCO₂1.8), for 9 years. N availability increased under eCO₂1.8, and its allocation to chlorophyll (NF_chl) and photosynthetic N use efficiency also increased under eCO₂. The maximum carboxylation rate, leaf N per mass (N_mass), and N allocation to Rubisco (NF_Rub), however, were all lower under eCO₂1.8. There were interactions in NF_chl between canopy × species × CO₂ concentrations. The greatest changes in N allocation under eCO₂ were in the lower canopy of S. alnifolia, the most shade-tolerant species and this species have relatively high flexibility in N operations compared to shade-intolerant species. Based on the reduction in N_mass that was diluted by increased nonstructural carbohydrates and increased N availability, down-regulation of photosynthesis was found to be caused by the dilution and change in N allocations, rather than PNL.