Water sources used by riparian trees varies among stream types on the San Pedro River, Arizona

Variation in the sources of water used by tree species has important ramifi cations for forest water balances. The fraction of tree transpiration water derived from the unsaturated soil zone and groundwater in a riparian fores t was quantified for Populus fremontii, Salix gooddingii, and Prosopis velu tina across a gradient of groundwater depth and streamflow regime on the Sa n Pedro River in southeastern Arizona, US. The proportion of tree transpira tion derived from different potential sources was determined using oxygen ( delta O-18) and hydrogen (deltaD) stable isotope analysis in conjunction wi th two- and three-compartment linear mixing models. Comparisons of delta O- 18 and deltaD of tree xylem water with that of potential water sources indi cated that Salix gooddingii did not take up water in the upper soil layers during the summer rainy period, but instead used only groundwater, even at an ephemeral stream site where depth to groundwater exceeded 4 m. Populus f remontii, a dominant 'phreatophyte' in these semi-arid riparian ecosystems, also used mainly groundwater, but at the ephemeral stream site during the summer rainy season this species derived between 26 and 33% of its transpir ation water from upper soil layers. Similarly, at the ephemeral stream site during the summer rainy period, Prosopis velutina derived a,greater fracti on of its transpiration water from upper soil layers, than at a perennial s tream site where groundwater depth was less than 2 m. Measurements of trans piration flux combined with stable isotope data revealed that Populus fremo ntii transpired a greater quantity of water from upper soil layers at the e phemeral stream site than at the perennial stream site. These results imply that transpiration from groundwater and unsaturated soil layers by riparia n vegetation may depend on the interaction between site conditions and spec ies assemblage. (C) 2000 Elsevier Science B.V. All rights reserved.