Where does the water come from? A case study of two endangered plant species

Over the past decades, increased human pressures have greatly altered the hydrologic regime of the Amargosa Valley, which in turn has drastically reduced the amount of water available to plants. The main objective of this study was to use stable isotopes in order to determine the water source for both Nitrophila mohavensis and Grindelia fraxino-pratensis, both of which are federally protected plants growing in the Lower Carson Slough area of Amargosa Valley. As a result of being federally protected, we created a new technique to collect plant xylem water for isotopic analysis by condensing plant transpiration water. Throughout the 2005 growing season δ¹⁸O values of G. fraxino-pratensis ranged from -10.3‰ to -13.9‰, suggesting vertical partitioning of water sources during the year. Based on a two-compartment mixing model, G. fraxino-pratensis appears to utilize water found near the soil surface (0-20cm) during early spring, yet as the soil dried out during the summer months these plants utilized deeper water sources. In contrast, water uptake by N. mohavensis appeared largely restricted to the upper soil layer (0-20cm), with little utilization of deeper water sources. The difference in where these two plants access water may be explained by difference in soil volumetric water content between these two sites. Throughout the growing season VWC at the site where N. mohavensis occurs was significantly greater than soil VWC at the site where G. fraxino-pratensis occurs (P<0.001).