The Transition from Mojave Desert to Great Basin Desert on the Nevada Test Site D. J. Hansen W. K. Ostler D.B. Hall Abstract—Plant species and associations on the Nevada Test Site (Ostler and others 1999). Approximately 1,500 ecological land- are located along elevation and precipitation gradients. Associa- form units (ELUs) were delineated using aerial photography tions in the Great Basin Desert had the highest species diversity. and satellite imagery to distinguish ecological mapping Rodent sign and productivity of annual plants are least in the lower types. The boundaries of each ELU were field verified. and higher elevations and most abundant in the mid-elevations. Landforms were selected because they are highly correlated Microbiotic crusts are most abundant in fine-textured soils and of with soil types in Nevada (Peterson 1981) and have been low abundance in soils with active erosional processes. Texture of used historically to help classify habitat types in the Mojave surface soils (0 to 5 centimeters [0 to 2 inches]) differs little among Desert (Berry 1979). Vegetation and other site parameters all associations except for a few that are correlated with playas and were sampled within representative areas of each ELU. steep mountain slopes. However, differences among associations Data were analyzed using cluster analyses and descriptive are observed for deeper substrates comprised of limestone, basalt, statistics to help classify vegetation into 10 alliances and and tuff parent materials. 20 associations. Results ________________________ The Nevada Test Site (NTS) is located about 105 km (65 miles) northwest of Las Vegas in southern Nevada (see fig. 1 Results of the vegetation classification on the NTS are of companion paper, Ostler and others, 1999). The site was shown in table 1. About 23 percent and 37 percent of the created by a series of land withdrawals in the early 1950s for ELUs sampled on the NTS were located in the Mojave Desert nuclear weapons testing. It comprises a total land area of and Great Basin Desert, respectively (table 2). The remain- 350,000 ha (1,350 square miles). Despite nearly 1,000 ing 36 percent of the ELUs were located in a Transition Zone atmospheric and below-ground nuclear tests, the area is between these two deserts. About 4 percent of the 1,508 relatively undisturbed and offers an excellent location for ELUs sampled were classified as “miscellaneous” because biological studies. The area has had limited or no livestock they were unique vegetation types, burned, scraped, or grazing since the 1950s and is designated as a National disturbed by nuclear testing. In the Mojave Desert the Environmental Research Park. Larrea tridentata/Ambrosia dumosa Shrubland was the The NTS consists of three large valleys, Yucca, French- most numerous association representing about 19 percent of man, and Jackass Flats. It has two high mesas, Rainier and the ELUs on the NTS (18 percent of the total area). No other Pahute. It has a rough elevational gradient from south to association in the Mojave Desert represented more than north with the lowest point at 829 m (2,688 ft) in Jackass 4 percent of the total ELUs. In the Great Basin Desert the Flats and the highest point at 2,340 m (7,679 ft) on Rainier Artemisia tridentata-Chrysothamnus viscidiflorus Mesa. The site straddles the Mojave and Great Basin Deserts Shrubland was the most numerous association representing and provides an excellent site to observe the transition about 11 percent of the ELUs on the NTS (7.5 percent of the between these two deserts. Numerous detailed studies have total area). No other association in the Great Basin Desert been conducted on NTS biota focusing primarily on invento- represented more than 7 percent of the total ELUs. In the ries and evaluating the effects of nuclear testing. Approxi- Transition Zone between these deserts, the Coleogyne mately 730 plant species occur on the NTS, of which several ramosissima-Ephedra nevadensis Shrubland was the most are sensitive or protected. numerous association representing about 22 percent of the ELUs on the NTS (21.6 percent of the total area). No other association in the Transition Zone represented more than Methods _______________________ 6 percent of the total ELUs. Shrublands of the NTS were classified using methods de- scribed in a companion paper presented in these proceedings Distribution of Plant Alliances on the Nevada Test Site Figure 1 shows the distribution of plant alliances on the In: McArthur, E. Durant; Ostler, W. Kent; Wambolt, Carl L., comps. 1999. NTS. Also shown are the generalized boundaries for the Proceedings: shrubland ecotones; 1998 August 12–14; Ephraim, UT. Proc. Mojave Desert, Great Basin Desert, and the Transition Zone RMRS-P-11. Ogden, UT: U.S. Department of Agriculture, Forest Service, between these deserts. Plant species that dominate associa- Rocky Mountain Research Station. D. J. Hansen, W. K. Ostler, and D. B. Hall are Biologists with Bechtel tions within the Transition Zone have been historically Nevada, P.O. Box 98521, M/S NLV-081, Las Vegas, NV 89193-8521. 148 USDA Forest Service Proceedings RMRS-P-11. 1999 Table 1—Classification of vegetation on the Nevada Test Site. deciduous (e.g., blackbrush) or have essentially leafless, photosynthetic stems (e.g., Ephedra spp.), while the most Mojave Desert abundant dominant shrubs from associations in the Mojave Lycium spp. Shrubland Alliance Desert and Great Basin Desert are evergreen in habit (e.g., Lycium shockleyi-Lycium pallidum Shrubland Larrea tridentata/Ambrosia dumosa Shrubland Alliance creosote bush, big sagebrush, singleleaf pinyon, and Utah Larrea tridentata /Ambrosia dumosa -Shrubland juniper). The evolutionary adaptation of leaf reduction or Atriplex confertifolia - Ambrosia dumosa Shrubland Alliance abscission during drought and stress-induced dormancy Atriplex confertifolia - Ambrosia dumosa Shrubland may help maintain the abundance of blackbrush and Mor- mon tea (Ephedra nevadensis) in these ecotones. While the Transition Zone Hymenoclea -Lycium Shrubland Alliance abundance of species other than blackbrush are relatively Lycium andersonii - Hymenoclea salsola Shrubland low in the Coleogyne ramosissima - Ephedra nevadensis Hymenoclea salsola - Ephedra nevadensis Shrubland Shrubland, they are frequently present in small numbers Ephedra nevadensis Shrubland Alliance being found in small patches where animals have disturbed Menodora spinescens - Ephedra nevadensis Shrubland the soil horizons or fire has reduced competition with Krascheninnikovia lanata - Ephedra nevadensis Shrubland blackbrush. Eriogonum fasciculatum - Ephedra nevadensis Shrubland Blackbrush occurs at intermediate elevations. At these Ephedra nevadensis - Grayia spinosa Shrubland elevations lightning strikes, associated with storms blowing Coleogyne ramosissima Shrubland Alliance from the south, are common during the summer and occa- Coleogyne ramosissima - Ephedra nevadensis Shrubland sionally fuel loading reaches levels high enough to support Great Basin Desert wildfires. Once burned these communities reestablish very Atriplex spp. Shrubland Alliance slowly; this phenomenon is also reported by Brown (1982). Atriplex confertifolia - Kochia americana Shrubland Atriplex canescens - Krascheninnikovia lanata Shrubland Chrysothamnus-Ericameria Shrubland Alliance Species Diversity Chrysothamnus viscidiflorus - Ephedra nevadensis Shrubland Ericameria nauseosa Shrubland Alliance Species diversity (richness or the number of species) of Ericameria nauseosa - Ephedra nevadensis Shrubland perennial trees and shrubs was greatest in the Great Basin Artemisia spp. Shrubland Alliance Desert associations (mean of 56 species) compared to asso- Ephedra viridis - Artemisia tridentata Shrubland ciations in the Transition Zone (mean of 49 species) and the Artemisia tridentata - Chrysothamnus viscidiflorus Shrubland Mojave Desert (mean of 36 species). Similar species diver- Artemisia nova - Chrysothamnus viscidiflorus Shrubland sity patterns were also observed for all combined perennial Artemisia nova - Artemisia tridentata Shrubland Pinus monophylla/Artemisia spp. Woodland Alliance species on the NTS (table 2) (e.g., Great Basin Desert: 21.7 Pinus monophylla/Artemisia nova Woodland species per ELU, Transition Zone: 17.4 species per ELU, and Pinus monophylla/Artemisia tridentata Woodland Mojave Desert: 12.7 species per ELU). Elevation and Precipitation associated with either desert, and in some cases, listed as a Plant associations within the Mojave Desert and Great minor species in both deserts. Basin Desert were ordered according to increasing elevation and precipitation (fig. 5). Mean annual precipitation was Associations that were considered typical or characteris- 2 tic of the Mojave Desert were those that contained a presence determined to be positively correlated (r = 0.85) with eleva- of Shockley’s desertthorn (Lycium shockleyi), rabbit thorn tion on the NTS, based on correlation modeling and actual (Lycium pallidum), creosote bush (Larrea tridentata), or weather recording data (French 1986; other detailed meteo- white bursage (Ambrosia dumosa), but lacked species char- rological data are presented by Fransioli and Ambos 1997). acteristic of the Great Basin Desert (table 3). The distribu- The importance of elevation, slope, and substrate in account- tion of creosote bush on the NTS (fig. 2) approximates the ing for statistical variance in shrub species cover was also described for the Nellis Air Force Range consisting of boundaries for the