Phreatophytes under stress: Review of studies on transpiration and stomatal conductance of Tamarix across a Western US floodplain

Transpiration (E_L) and stomatal conductance (G_S) were measured with stem heat-balance and Granier sap flux sensors on the dominant phreatophyte, saltcedar (Tamarix spp.), growing at six sites on a floodplain on the Lower Colorado River, US. Plant-specific leaf area index (LAPS) of shrubs was measured by leaf harvesting and Licor 2000 Plant Canopy Analyzer, and fractional cover (f_c) was estimated by aerial imagery. Ground-area transpiration (E_G) was calculated as E_L × LAPS × f_c. The sites presented environmental gradients with respect to distance from the river (0.2-1.5 km), depth to groundwater (2.4-3.5 m), groundwater salinity [1.9-24.0 g L^-1 Total Dissolved Solids (TDS)] and soil texture (ranging from sand to clayey silts). E_L aried from 1-3 mm m^-1 leaf d^-1 across sites, while LAPS ranged more narrowly, from 2-4, and f_c varied from 0.5-0.95. Due to differences in E_L, LAPS and f_c, E_G ranged from 1.2-9.5 mm d^-1, nearly a 10-fold range. Only one site had E_G characteristic of unstressed condition. We concluded that saltcedar is capable of high E_G rates, but is also a stress adapted species, with E _G highly variable over nonflooding riparian zones typical of regulated rivers. Salinity of the aquifer and vadose zone were identified as key constraints on saltcedar E_G. Mean E_G over the floodplain was only 40% of potential ET, contrary to earlier assumptions that saltcedar is invariably a high-water use plant.