Water sources used by riparian trees varies among stream types on the San Pedro river, Arizona. Agricultural and Forest Meteorology 105

Variation in the sources of water used by tree species has important ramifications for forest water balances. The fraction of tree transpiration water derived from the unsaturated soil zone and groundwater in a riparian forest was quantified for Populus fremontii, Salix gooddingii, and Prosopis velutina across a gradient of groundwater depth and streamflow regime on the San Pedro River in southeastern Arizona, US. The proportion of tree transpiration derived from different potential sources was determined using oxygen (18O) and hydrogen (D) stable isotope analysis in conjunction with two- and three-compartment linear mixing models. Comparisons of 18O and D of tree xylem water with that of potential water sources indicated 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 fremontii, 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 transpiration water from upper soil layers. Similarly, at the ephemeral stream site during the summer rainy period, Prosopis velutina derived a greater fraction of its transpiration water from upper soil layers, than at a perennial stream site where groundwater depth was less than 2 m. Measurements of transpiration flux combined with stable isotope data revealed that Populus fremontii transpired a greater quantity of water from upper soil layers at the ephemeral stream site than at the perennial stream site. These results imply that transpiration from groundwater and unsaturated soil layers by riparian vegetation may depend on the interaction between site conditions and species assemblage