The Ecology of Gypsophile Endemism in the Eastern Mojave Desert

Abstract

The vegetation on gypsum outcrops in the eastern Mojave Desert, USA, is characterized by the presence of a suite of gypsophile endemic species. Shrub species dominant on adjacent upland alluvial soils occur at much reduced densities or are absent. Soil factors that could influence species distribution and abundance were investigated using field correlation techniques. Except for the presence of calcium sulfate in solution, gypsum soils appear to be chemically similar to alluvial soils. Little evidence for excessive salinity, toxic ion effects, or increased nutrient deficiency relative to alluvial soils was found. The gypsum soils had low bulk densities due to the presence of sponge gypsum. They usually presented indurated or heavily cryptogamic surface crusts, while the alluvial soils had high bulk densities and rock pavement surfaces. Within the gypsum environment, the factors most highly correlated with species abundances were rock cover, cryptogam cover, soil depth, and total gypsum content. Surface gravel was associated with the establishment of many alluvial soil species on gypsum. Cryptogam cover was positively correlated with abundance for some gypsovag species and for many endemic species. Abundance for some other gypsophiles was positively correlated with total gypsum content, while gypsovag abundances showed little correlation with gypsum content. It is concluded that plant densities remain low on gypsum because its unfavorable surface crust characteristics present problems at the establishment stage. Rock cover and cryptogam cover ameliorate these problems in different ways. The chief advantage on gypsum for the gypsophile species may be reduced moisture stress during the early summer drought, due either to reduced competition for water because of low densities or to intrinsic properties of the gypsum soil. Mineral nutrition factors appear to play a relatively minor role.