Abstract: The photosynthetic characteristics of three-year-old seedlings of coniferous Pinus koraiensis and Pinus sylvestriformis and broadleaf Fraxinus mandshurica in open-top chambers in Changbai Mountain were studied.Seedlings were planted at four different conditions respectively:elevated CO₂ concentration(700 and 500 μmol·mol^-1)and ambient CO₂ concentration (control chamber and in open field).After treating of two growing seasons in the first two years,seedlings were continuously grown under different CO₂ concentration for another eight weeks.The results indicated that the photosynthetic capacity of three species was enhanced under high CO₂ concentration in the first two growing seasons.The maximum net photosynthetic rates and light response parameters of different tree species were different at the same CO₂ concentration.No photosynthetic acclimation was observed in the third growing season except Pinus sylvestriformis seedlings grown under 500 μmol·mol^-1 CO₂.The increase or decrease of net photosynthetic rates and light response parameters were related to the time of CO₂ treatment.Elevated CO₂ concentration could alter the ability of utilizing strong light and low light of seedlings.