The effect of Pinus sylvestriformis seedlings density on net photosynthetic rate was studied under elevated CO 2. Atmospheric CO 2 concentration was controlled in OTC (Open Top Chamber). The results showed that elevated CO 2 not only made net photosynthetic rates (NPRs) of two Pinus sylvestriformis seedlings densities increased,but also mitigated their intra-specific competition. Meanwhile,the difference of seedling NPRs between 100 and 400 plant·m -2 under 500 μmol·mol -1 air CO 2 concentration was less than that under 350 μmol·mol -1 with the same PAR levels. When air CO 2 concentration reached 700 μmol·mol -1 , the NPRs of seedlings under both planting densities were close to each other with the same PAR levels. The intra-specific competition was minimized under air CO 2 concentration of 700 μmol·mol -1 .
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