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Water use by the introduced shrub Tamarix ramosissima and three co-occurring, native phreatophytes was measured in the lower Virgin River floodplain (southern Nevada) using the stem-heat-balance method. During the 1993 growing season, measurements were conducted on Tamarix in a closed, monospecific stand and in a mixed community with the native species Pluchea sericea, Prosopis pubescens, and Salix exigua. Our objectives were (1) to determine whether leaf-area-based water use of Tamarix is higher than that of co-occurring native riparian taxa, (2) to assess the role of Tamarix stand leaf area index (LAI, leaf area per unit ground area) on Tamarix water loss, and (3) to verify whether Tamarix is capable of using large amounts of water under the extreme evaporative demands that characterize arid environments. Leaf-area-based sap flow rates were comparable in the four species despite large differences in individual leaf area and total water loss. Daily water use of Tamarix (in grams per day) weighted by the daily potential evapotranspiration (PET) increased linearly with the total leaf area per plant, suggesting that water uptake was sufficient to compensate for water loss at the leaf level, even during times of extreme atmospheric water demand. Under high PET, maximum sap flow rates of Tamarix on a leaf-area basis were significantly higher at locations where Tamarix LAI was lower, indicating that highly transpiring Tamarix stands may reduce leaf-level evaporative demand. However, daily estimates of transpiration of dense Tamarix stands (LAI up to 3.5 m(2)/m(2)) in our study site exceeded PET by a factor ranging from 1.6 to 2.0, which confirms that mature Tamarix stands can lose very high quantities of water due to the maintenance of high leaf area. Results from this and other studies demonstrate that, at least under moderate to high water tables, key variables controlling water use by riparian stands include structural characteristics such as LAI and density. Management practices aimed at conserving water should be geared towards avoiding the development of dense Tamarix thickets along arid water courses, particularly if it is confirmed that these tend to attain higher LAI than native riparian communities.