Abstract：The use of large-scale tree plantations has provoked increasing concern regarding the negative effects on local environments in different ecosystems.However, the physiological mechanism underlying the reduction in soil water by tree plantations in wetlands is not clear. The aims of this study were to investigate the effects of poplar (Populus deltoides)plantations on soil water content and to elucidate the underlying physiological mechanisms. To this end, we conducted a 1-year fixed-plot investigation of soil water content (SWC), plant photosynthetic rate (Pn),stomatal conductance (Gs), transpiration rate (Tr), and water-use efficiency (WUE) of individual leaves of 11- and 5-year-old poplars and of reed (Triarrherca sacchariflora, a native herbaceous plant) in the Dongting Lake wetlands, China. SWC was highest in reed, intermediate in 11-year-old poplar, and lowest in 5-year-old poplar, suggesting that poplar plantations produce a lower soil water content in wetlands. From May to July, Pn was significantly higher in reed than in the two poplar stands, but did not differ between the different-aged poplars. As a whole, Gs and Tr were higher, but WUE was lower, in the poplar stands than in reed during the growing season, indicating that Gs and Tr are the key physiological mechanisms associated with the lower soil water in poplar stands. Relationships among Pn, Gs, and Tr showed positive correlations (P\0.01) for each type of vegetation. These data suggest that poplar plantations may cause the transformation of wetlands into dry land due to a lower WUE leading to a massive water loss from soil. This, in turn, would have an influence on community composition and ecosystem function after establishment of the plantations.