Twenty-Fourth Annual Meeting and Symposium of the Desert Tortoise Council, March 5-8, 1999
Abstracts

Exotic plants comprise a relatively small proportion of the Mojave Desert flora, but a few species dominate many plant communities and have negatively affected or threaten to affect ecosystem integrity. The most studied exotic plant species in the Mojave Desert is the riparian perennial, Tamarix spp. Riparian habitats comprise less than 3% of the entire region, and the remaining upland area is often dominated by exotic annuals. Exotic annuals can comprise 66-97% of the total annual plant biomass, and are present at approximately 50% of all wildland sites. Because annuals are currently the most common exotic plants in the Mojave Desert, we will focus on the ecology and management of exotic annuals in this paper.

Widespread and common exotics include four species that are dominant in plant communities throughout the Mojave Desert. Bromus madritensis ssp. rubens [Bromus rubens] is recognized as a potential wildland pest, but relatively little is known about its ecology. It is considered an invasive weed in its Mediterranean home range and appears to be limited in dominance primarily by rainfall and soil nitrogen in the Mojave Desert. Bromus tectorum has ecological effects that are well documented in the Great Basin Desert, but not in the Mojave Desert. Distribution of Bromus tectorum is generally confined to elevations above 5,000 feet, yet encroachment into and dominance at lower elevations has been observed throughout the Mojave Desert. Schismus spp (S. arabicus and S. barbatus) is recognized as a potential wildland pest but very little is known about its ecology in North America. It is not invasive in its home range, but it is well adapted for arid conditions and is not limited by low water and nutrient levels in the Mojave Desert. Erodium cicutarium is widespread and has been shown to outcompete native plants in the Mojave and Sonoran Deserts.

Locally common exotic annuals include seven species that are dominant in plant communities in certain regions or habitats. Brassica tournefortii became dominant in cis montane southern California during the 1980's. It is currently spreading north and east along roads into the Mojave Desert, but since 1995 it has been observed spreading away from roads into wildland areas. Hirschfeldia incana [Brassica geniculata], Descurania sophia, Sisymbrium irio, Sisymbrium altissimum, and Salsola spp., are often locally abundant along roadsides, livestock watering sites, and off-highway vehicle staging areas. Bromus trinii appears to be ecologically similar to other brome grasses, and where it is abundant, it can fuel the spread of fires and may compete with native plants. Other exotic annuals are either very limited in their distributions or are confined to urban or agricultural areas.

Exotic annuals may affect ecosystem integrity by competing with native plants, altering soil characteristics, and/or promoting wildfires. Increased levels of atmospheric CO₂ and nitrogen may promote the dominance of exotic annual species in the Mojave Desert. Management of exotic plants must begin by preventing invasions. Once they become established, the dominance of exotic annuals may be minimized with biological agents, application of herbicides, manual removal, or protection of habitat from human disturbances.

We identify several research areas important to the control and management of exotic annuals in the Mojave Desert: 1) determine the distributions of exotic species and consolidate this information on a public domain database; 2) identify environmental variables that facilitate the invasion and dominance of alien species; 3) identify physiological and ecological characteristics of exotic annual species that promote their invasiveness; 4) prioritize the most vulnerable habitats to receive immediate protection from exotic plant invasion and prioritize the exotics of greatest concern based on their invasive attributes; and 5) control exotic species and restore degraded habitats using well-designed and replicated experiments.

References

Beatley, J. C. 1966. Ecological status of introduced brome grasses (Brome spp.) in desert vegetation of southern Nevada. Ecology 47:548-554

Brooks, M. L. 1998. Ecology of a Biological Invasion: Alien Annual Plants in the Mojave Desert. Dissertation, University of California, Riverside. 186 pp.

Hunter, R. 1991. Bromus invasions on the Nevada Test Site: present status of B. rubens and B. tectorum with notes on their relationship to disturbance and altitude. Great Basin Naturalist 51: 176-182.

Kemp, P., and M. L. Brooks. 1998. Exotic Species of California Deserts. Fremontia 26:30-34.

Melgoza, G. and R. S. Nowak. 1991. Competition between cheatgrass and two native species after fire: implications from observations and measurements of root distribution. Journal of Range Management 44:27-33.