ECOPHYSIOLOGY OF THE TEMPERATE DESERT HALOPHYTES - ALLENROLFEA-OCCIDENTALIS AND SARCOBATUS-VERMICULATUS

Numerous basins of the intermountain area often have extensive playa s urfaces that are nearly devoid of vegetation. Margins of these playas support sparse communities dominated by the chenopod shrubs Allenrolfe a occidentalis (iodine bush) and Sarcobatus vermiculatus (black grease wood). These plants establish and persist in an environment where halo morphic soils induce extreme osmotic stress and atmospheric precipitat ion 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 collect ed in above-average (1991) and below-average (1992) precipitation year s. Net CO2 assimilation rates for Allenrolfea were statistically simil ar in 1991 and 1992 but in general declined for Sarcobatus in 1992. Fo r both species, leaf conductances and leaf transpiration rates decline d significantly from 1991 to 1992, with the decline significantly grea ter for Sarcobatus. Water-use efficiencies doubled from 1991 to 1992 f or 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 beginning i n May 1991 and 1992, respectively, and dropped to 3.8 and -4.2 MPa for Allenrolfea and -1.8 and -2.8 MPa for Sarcobatus by September 1991 an d 1992, respectively. Afternoon xylem water potentials were 3.1 and -2 .0 MPa for Allenrolfea and -2.6 and -2.2 MPa 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 of b oth 1991 and 1992. For Allenrolfea in general, the total soil water po tential 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 ext ensive woody root system.