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First published online February 12, 2009; doi:10.3732/ajb.0800181 American Journal of Botany 96: 620-626 (2009) © 2009 Botanical Society of America, Inc.

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Increasing demands on limited water resources: Consequences for two endangered plants in Amargosa Valley, USA¹

Center for Conservation Biology, University of California-Riverside, Riverside, California 92521 USA

ABSTRACT

Recent population expansion throughout the Southwest United States has created an unprecedented demand for already limited water resources, which may have severe consequences on the persistence of some species. Two such species are the federally protected Nitrophila mohavensis (Chenopodiaceae) and Grindelia fraxino-pratensis (Asteraceae) found in Amargosa Valley, one valley east of Death Valley, California. Because both species are federally protected, no plant material could be harvested for analysis. We therefore used a chamber system to collect transpired water for isotopic analysis. After a correction for isotopic enrichment during transpiration, ¹⁸O values of plant xylem water were significantly different between N. mohavensis and G. fraxino-pratensis throughout the study. Using a multisource mixing model, we found that both N. mohavensis and G. fraxino-pratensis used soil moisture near the soil surface in early spring when surface water was present. However, during the dry summer months, G. fraxino-pratensis tracked soil moisture to deeper depths, whereas N. mohavensis continued to use soil moisture near the soil surface. These results indicate that pumping groundwater and subsequently lowering the water table may directly prevent G. fraxino-pratensis from accessing water, whereas these same conditions may indirectly affect N. mohavensis by reducing surface soil moisture and thus its ability to access water.

Key Words: Amargosa Valley (USA) • Asteraceae • Chenopodiaceae • desert plants • Grindelia fraxino-pratensis • groundwater • Nitrophila mohavensis • stable isotopes • water uptake patterns

Received for publication 28 May 2008. Accepted for publication 21 November 2008.

FOOTNOTES

¹ The authors thank E. Maher and R. Vargas for field assistance and data collection and J. I. Querejeta and T. Dawson for helpful insights with stable isotope analyses. This research was supported by a grant from the BLM and NSF biocomplexity grant EF0410408.

² Author for correspondence (e-mail: nhass001{at}ucr.edu)

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