1. The form and function of terrestrial plants is largely governed by the availability of water, with plants in water limited environments expressing traits minimizing water loss and tissue damage during drought. Areas with high salinity are analogous to those with low water availability, even where water is abundant. We evaluated variation in salinity tolerance and water use strategies in an introduced halophytic shrub, Tamarix spp by measuring gas exchange rates, biomass accumulation, and turgor loss points. We hypothesized that salinity tolerance is not uniformly expressed, with high salinity ecotypes expressing more conservative water use traits that maximize hydraulic safety over high conductance rates.

2. We studied two populations of Tamarix spp. along the lower Colorado River where groundwater salinity differed by nearly six-fold. Cuttings collected from the sites were grown in a greenhouse with five salinity levels from 0 to 32 parts per thousand (ppt), imposing increasing water limitation.

3. In situ measurements of leaf water potentials () and branch xylem anatomy were taken on ecotypes sourced from high- and low-salinity sites. In the greenhouse, biomass and gas exchange rates were evaluated over the 0-32 ppt salinity gradient, and a separate dry down experiment was performed to determine turgor loss points and stomatal responses to drying soils.

4. In the greenhouse, the low salinity population accumulated 72% more biomass when grown at 4 ppt compared to 16 ppt, while the high salinity population produced 50% more biomass when grown at 16 ppt. Net carbon assimilation was greater at lower salinities in the low salinity population but independent of salinity in the high salinity population. The high salinity population had a lower turgor loss point and exhibited greater stomatal control relative to the low salinity population.

5. Synthesis. Results provide evidence for divergence of traits related to plant water use across salinity gradients in this recently introduced halophyte. Local adaptation to increased salinity has implications in aridland riparian ecosystems where water management or drought may lead to altered soil salinities. The interaction of trait variation within Tamarix spp. and increasing salinity is likely to favor its continued dominance.