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Climate change necessitates research into interactions between elevated carbon dioxide (CO2) concentrations and drought on plant photosynthetic physiology. This study describes the physiological properties of Platycladus orientalis (Chinese thuja) and Quercus variabilis (Chinese cork oak) saplings cultivated through orthogonal treatments of four CO2 concentrations combined with five soil volumetric water contents (SWC). It highlights the differences between the interactive effects from the treatments. Water stress had little effect on photosynthetic traits until the soil volumetric water contents exceeded 70-80 or 100%. Similar variations in carbon-13 isotope abundance (delta C-13) of water soluble compounds (delta C-13(WSC)) extracted from leaves of two species have been observed. Whether soil volumetric water contents exceeded or fell below the water threshold values (70-80% of field capacity for P. orientalis and 100% of field capacity for Q. variabilis), instantaneous water use efficiency decreased. Elevated carbon dioxide could increase iWUE and enhance drought tolerance, depending on stimulating net photosynthetic rates and declining stomatal conductance and transpiration rates. Augmenting either drought, excess water, or ambient carbon dioxide could alleviate the physiological inhibition caused by the stresses described above.