JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecological Applications.Abstract. 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 evapotranspirati-on (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 leafarea 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 m2/m2) 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.

Riparian plants have been classified as "drought avoiders" due to their access to an abundant subsurface water supply. Recent water-relations research that tracks water sources of riparian plants using the stable isolopes of water suggests that many plants of the riparian zone use ground water rather than stream water, and not all riparian plants are obligate phrcatophytes (dependent on ground water as a moisture source) but may occasionally be dependent on unsaturated soil moisture sources. A more thorough understanding of riparian plant-water relations must include water-source dynamics and how lhose dynamics vary over both space and time. Many rivers in the desert Southwest have been invaded by the exotic shrub Tamarix ramosissima (saltcedar). Our studies of Tamarix invasion into habitats formerly dominated by native riparian forests of primarily Populus and Salix have shown that Tamarix .~ucccssfully invades these habitats because of its (1) greater tolerance to water stress and salinity, (2) status as a facultative, rather than obligate, phreatophyte and, therefore, its ability to recover from droughts and periods of ground-water drawdown, and (3) superior regrowth after fire. Analysis of wate~loss rates indicate that Tamarix-dominated stands can have extremely high evapetranspiration rates when water tables are high but not necessarily when water tables are lower. Tamarix has leaf-level transpiration rates that are comparable to native species, whereas sap-flow rates per unit sapwood area are higher than in natives, suggesting that Tamarix maintains higher leaf area than can natives, probably due to its greater water stress tolerance. Tarnarix desiccates and salinizes floodplains, duc to its salt exudation and high transpiration rates, and may also accelerate fire cycles, thus pre disposing these ecosystems to further loss of native taxa. Riparian species on regulated rivers can be exposed to seasonal water stress due to depression of floodplain water tables and elimination of annual floods. This can potentially result in a community shift toward more stress-tolerant taxa, such as Tamarix, due to the inability of other riparian species to germinate and establish in the desiccated floodplain environment. Management eflbrts aimed at maintaining native forests on regulated rivers and slowing the spread of Tamarix inva.~ion must include at least partial reintroduction of historical flow regimes, which favor the recruitment of native riparian species and reverse tong-term desiccation of desert floodplain environments.

Tamarix ramosissima (Tamaricaceae) is a woody phreatophyte that has invaded thousands of hectares of floodplain habitat in the southwestern U.S. In this study, we examined the response of gas exchange and stem sap flow of Tamarix and three co-occurring native phreatophytes (Pluchea sericea (Asteraceae), Prosopis pubescens (Fabaceae) and Salix exigua (Salicaceae)) to drought conditions in an early successional floodplain community in the Mojave Desert of southern Nevada. In an analysis of a size/age series of each species across the whole floodplain (both mature and successional stands), stem growth rate was lowest for Tamarix. However, along the same successional chronosequence, Tamarix came to dominate the 50+ year old stands with dense thickets of high stem density. Xylem sap flow, when expressed on a sapwood area basis, was highest in Tamarix under early drought conditions, but comparable between the four species toward the end of the summer dry season. Multivariate analysis of the gas exchange data indicated that the four species differentiated based on water use under early drought conditions and separated based on plant water potential and leaf temperature (indices of drought effects) at the end of the summer dry season. This analysis suggests that the invasive Tamarix is the most drought tolerant of the four species, whereas Salix transpires the most water per unit leaf surface area and is the least tolerant of seasonal water stress. Therefore, Salix appears to be well adapted to early successional communities. However, as floodplains in this arid region become more desiccated with age, Tamarix assumes greater dominance due to its superior drought tolerance relative to native phreatophytes and its ability to produce high density stands and high leaf area.

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