All wilderness segments within the boundaries of Joshua Tree National Park were surveyed for frequency and cover of microbiotic soil crusts. Areas within both the Mojave Desert and Colorado Desert were examined. A total of eight vegetation community types were sampled. In order of greatest sample representation, these community types were: Sonoran Creosote Bush Scrub, Mojave Creosote Bush Scrub, Blackbrush Scrub, Mojave Pinyon-Juniper Woodland, Mojave Mixed Woody Scrub, Mojave Mixed Steppe, Stabilized Sand Dunes, Sonoran Mixed Woody and Succulent Scrub. A total of 75 research sites were established in the summer of 2006. At all sites, 100 1.0 m2 frequency plots and 250 cover points were scored for shrub vegetation (by species), perennial grass and forbs, cacti, annuals (cover only), rock, and several crust categories (algal, lichen, moss, mixed crust).
Crust cover was generally low in most areas of the park in comparison to crust cover in other arid lands of the western United States. Cover for total microbiotic crust was on average only 13.1%, with most of that cover attributable to algal crust (11.4%). The remaining cover was due to lichens. Only one site of the 75 had any detectable moss cover. The vegetation zones with highest crust cover were Mojave Creosote Bush Scrub (18.7%), Sonoran Creosote Bush Scrub (14.4%), and Mojave Mixed Woody Scrub (9.9%). All other vegetation zones had less than 6% average crust cover. There was a weak correlation between the soil stability index and crust cover (R2=0.41).
