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 .
The gas_well system permanently installed in the soil was adopted for studying the dynamic relationship between CO 2 profiles and seedling root growth of Pinus sylvestris var. sylvestriformis (Takenouchi) Cheng et C. D. Chu. The study was conducted in the Open Research Station of Changbai Mountain Forest Ecological System, The Chinese Academy of Sciences from 1999 to 2001. Four treatments were arranged in the rectangular open_top chambers (OTCs): ambient CO 2+no_seedling, 700 μmol/mol CO 2+no_seedling, ambient CO 2 +seedlings, 700 μmol/mol CO 2+seedlings. By collecting and analyzing soil gas synchronously, it was found that the dynamics of CO 2 profiles were related to the biological activity of seedlings. There were more roots distributed in the top soil and the boundary layer across soil and sand, which made more contributions to the CO 2 profiles due to respiration root. Compared with the ambient CO 2, elevated CO 2 led to the peak of CO 2 concentration distribution shifted from soil surface layer to the boundary layer as seasonally growing of seedling roots. It is suggested the gas_well system is an inexpensive, non_destructive and relatively sensitive method for study of soil CO 2 concentration profiles.
