Soil moisture extraction by evergreen and drought-deciduous shrubs in the Mojave Desert during wet and dry years

Annual and seasonal evapo-transpiration (ET) were compared among Mojave Desert shrubs with different leaf phenologies over a 3-year period during which annual precipitation varied from well below average to more than twice average. During the wet year, soil wetting fronts reached maximum depths of 0)75 m to '1)95 m, depending on soil texture at the study sites. The evergreen shrubs Larrea tridentata and Ephedra nevadensis, and the drought-deciduous shrub Ambrosia dumosa, were able to extract soil water in a uniform manner to depths '1 m. For stands of the deciduous shrub Lycium pallidum, a soil texture change at c. 0)75 m impeded percolation of water below that depth. There were no significant differences (p(0)05) in annual ET between the evergreen shrubs Larrea and Ephedra relative to the drought-deciduous shrubs Ambrosia and Lycium during the 3 years of the study. Early in the growing season, extraction of soil water from beneath plant canopies was slightly greater than from shrub interspaces for Ambrosia, Ephedra, and Lycium, but not for Larrea. For all species, annual soil water extraction from beneath plant canopies was not significantly different than that from shrub interspaces. The lower limit of soil water extraction (Le) for the study sites varied from 4 to 10 volumetric per cent, depending on soil texture, and did not differ significantly among species. For all species, Le was reached within 6 to 12 months following twice average precipitation during the period of November 1994 to March 1995. We conclude that ET in the Mojave Desert is dependent largely on winter precipitation and the amount of soil water available during the growing season rather than on species composition.