Abstract:Branch hydraulic traits under 720μmolmol-1CO2 and drought stress were measured by improved flushing method after 5 years Pinus tabulaeformis saplings have been exposed to 720μmolmol-1CO2 for 13 months. The results show that hydraulic conductivity (Kh), specific conductivity (Ks) and huber value (Hv) in their branch under 720μmolmol-1CO2 and 380μmolmol-1CO2 concentrations reduce along drought stress, while leaf specific conductivity (Lsc) under 720μmolmol-1 CO2 declines non-linearly along drought stress. In the same drought stress, Kh, Ks, Lsc and Hv under 720μmolmol-1CO2 are respectively larger than that under the lower CO2 concentration and their differences between the two treatments are significant. According to the water potential, the water condition of the trees was divided into four grades. During the normal water(-0.45--0.65MPa), light drought condition(-1.15--0.75MPa) and middle drought condition(-1.95--1.35MPa), the Kh and Ks increase which compared with the plants in the control chamber, which show that the interaction of doubled CO2 concentration and drought stress improve the hydraulic conductivity ability else the water transport efficiency. While in the heavy drought condition(＜-2.80MPa), Kh increase but Ks decrease compared with the control, which show that the interaction improve the water transport security whereas reduce water transport efficiency. It suggests that the present study focusing on the variation of the hydraulic traits and the key effect of water transport security under elevated CO2 concentration and drought stress would benefit to afforestation in different water condition soil as increasing CO2 concentration.