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In riparian zones along the Tarim River in northeastern China, the co-dominance by Populus euphratica and Tamarix ramosissima at the early succession stage shifts to P. euphratica dominance in the late stages. However, little is known about how this shift is mediated by the highly variable water conditions in riparian zones. Here we conducted a mesocosm experiment in which we measured the physiological and morphological traits of these two co-occuring species grown in mixtures under simulated favorable groundwater condition and no groundwater availability. Results indicated that T. ramosissima, in comparison to P. euphratica, had much lower WUE, less proportion of root biomass under favorable groundwater condition. Under no groundwater condition, T. ramosissima also showed higher maximal quantum yield of PSII which allowed it to accumulate higher aboveground and total biomass. Therefore, regardless of groundwater conditions, T. ramosissima exhibited superior competitive advantages against P. euphratica under direct competition condition, which demonstrates that the dominance shift was not resulted from the direct competition at seedling stage. Our findings further imply that a strategy of "sit and wait" in P. euphratica might favor its growth and survival when suffered flooding disturbances, thus allowing P. euphratica not being excluded through competition at early successional stage.