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Populus fremontii (Fremont cottonwood) was once a dominant species in desert riparian forests but has been increasingly replaced by the exotic invasive Tamarix ramosissima (saltcedar). Interspecific competition, reduced flooding frequency, and increased salinity have been implicated in the widespread decline of P. fremontii. To elucidate some of the multiple and interacting mechanisms of this decline, we examined ecological processes in a control stand of P. fremontii along the Colorado River in Utah, USA, as well as a disturbed stand characterized by high groundwater salinity and invasion of T. ramosissima. Sap flux data showed that P. fremontii at the saline site experienced large reductions in afternoon canopy stomatal conductance relative to the control. Thus, average daily stand transpiration was 4.8 +/- 0.1 mm/d at the saline site in comparison to 9.3 +/- 0.2 mm/d at the control site over a two-month period. Light-saturated photosynthesis and apparent quantum yield were also reduced in saline P. fremontii. Stable isotope analysis indicated that trees at the saline site utilized evaporatively enriched groundwater that was likely derived from a nearby pond of irrigation runoff; this was also the probable source of high salinity. Interspecific competition for water at the saline site is unlikely, as T. ramosissima is still a minor species that is present only in the understory. However, reduced tissue N content in P. fremontii at the saline site suggested that physiological stress during salinity and halophyte invasion may be exacerbated by altered N relations.