Great Basin Naturalist


Numerous basins of the intermountain area often have extensive playa surfaces that are nearly devoid of vegetation. Margins of these playas support sparse communities dominated by chenopod shrubs Allenrolfea occidentalis (iodine bush) and Sacrobatus vermiculatus (black greasewood). These plants establish and persist in an environment where halomorphic soils induce extreme osmotic stress and atmospheric precipitation is very low and erratic and occurs largely during the winter when temperatures are too low for growth. We measured net CO2 assimilation rates, leaf conductances, transpiration rates, water-use efficiencies, and stem xylem potentials for these two C3 species. Data were collected in above-average (1991) and below-average (1992) precipitation years. Net CO2 assimilation rates for Allenrolfea were statistically similar in 1991 and 1992 but in general declined for Sarcobatus in 1992. For both species, leaf conductances and leaf transpiration rates declined significantly from 1991 to 1992, with the decline greater for Sarcobatus. Water-use efficiencies doubled from 1991 to 1992 for both plant species. Predawn xylem water potentials were −2.2 and −3.3 MPa for Allenrolfea and −1.8 and −2.6 MPa for Sarcobatus by September 1991 and 1992, respectively. Afternoon xlem water potentials were −3.1 and −2.0 MPa for Allrolfea and −2.6 and −2.2 for Sarcobatus beginning in May 1991 and 1992, respectively. Xylem water potentials dropped to −5.0 MPa for Allenrolfea and −3.4 MPa for Sarcobatus by September for both 1991 and 1992. For Allenrolfea, in general, the total soil water potential within the zone of maximum root activity is more negative than the plant's predawn xylem potential, which suggests that the plant is partially phreatophytic and/or has a large capacitance due to its extensive woody root system.