FIELD TRIP Stanley D. Smith David J. M. Bradney

ABSTRACT At this site, we also examined well-developed desert pave­ ment that had desert varnish surfaces estimated to be The all-day Mojave Desert field trip described in this at least 8,000 years old. The mosaic structure of Mojave summary includes four main stops. These cover an eleva­ Desert vegetation (-clump, fertile islands surrounded tiona[ range from 715 to 1,510 m and species compositions by bare areas of desert pavement) and caliche formation varying from a simple creosotebush () and were also discussed at this site. bursage (Ambrosia dumosa) community to dramatically The third stop was at 1,087 m and was in a blackbrush more complex zonal desert-mountain communities. The ( ramosissima)-dominated community. Black­ field trip also featured postburn community composi­ brush forms a distinct elevational belt between creosote­ tions and fossil packrat middens for the interpretation of bush desert along the lower bajadas and pinyon-juniper past plant distribution. woodland in the mountains over much of the Mojave Desert region. Subordinate in the blackbrush community INTRODUCTION were Joshua tree ( brevifolia), winterfat (Eurotia lanata), Mohave yucca, creosotebush, turpentine bush An ali-day field trip was taken along an elevational gra­ ( montana), desert almond (Prunus fascicu­ dient from Mojave Desert shrub associations in the Las latus ), and purple sage (Salvia dorrii). A large wash near Vegas Valley to pinyon-juniper woodland in the Spring the site had a distinct community dominated by rabbit­ Range west of Las Vegas. The field trip traveled from brush (Chrysothamnus nauseosus), catclaw acacia, cheese­ Las Vegas along the Blue Diamond-Pahrump Highway. bush (Hymenoclea salsola), and desert willow (Chilopsis Various desert communities were examined, including linearis). several recent burn sites along the gradient. Additionally, Just past the third stop, we took a detour by hiking up a side trip was made to a 15,000-year-old packrat midden a steep south-facing slope to a series of caves in a dolomite that shows evidence of pinyon-juniper woodland occurring outcrop (1,160 m elevation). In these caves are located a where desert shrub associations occur today. Figure 1 series of woodrat (genus Neotoma) middens dating from the depicts the gradient in vegetation and shows the four pri­ present to 15,000 years old. We examined a present-day mary community types that were examined and are dis­ woodrat midden and the process of packrat behavior lead­ cussed below. ing to midden formation. The active midden was found to contain plant materials similar to those observed within a 30-m radius of the caves: yucca leaf parts, twigs from vari­ THE FIELD TRIP ous shrubs, and cactus spines. We then examined several The first stop on the field trip was at a simple two­ intermediate-age fossil midden materials, which also con­ species Mojave Desert community at the bottom of the tained desert-type plant material. Last, we examined the Blue Diamond gradient. The site was at 715 m elevation 15,000-year-old midden deposit, and observed primarily and included creosotebush (Larrea tridentata) and white pinyon and juniper needles in the deposits, indicating that bursage (Ambrosia dumosa) as the only woody on this cave site was once pinyon-juniper woodland. These the site. The site was extremely open and thus typical of observations led to a discussion of Southwestern biogeogra­ the Lower Sonoran Zone of the Mojave Desert. phy and the migration of plant communities as a function The second stop, at 805 m elevation, was a mixed Mojave of climatic change. Desert community that was again dominated by creosote­ About 2 miles above the packrat midden area, we stopped bush and white burs~e. However, in this community we again within the blackbrush community to examine a also found a significant number of subordinate perennials, 10-year-old burn. The burn site was completely devoid of includiQg ephedra (), Rose­ blackbrush, but was instead dominated by small subshrubs mary eriogonum (Eriogonum fasciculatum), Mohave yucca such as brittlebush (Encelia virginensis) and desert mallow (Yucca schidigera), catclaw acacia (Acacia greggii), beaver­ (Sphaeralcea ambiqua). The site also had a denser cover of tail cactus (Opuntia basilaris ), pencil cholla ( Opuntia ra­ brome grasses (both Bromus tectorum and B. rubens) than mosissima), and indigo bush ( fremontii). did the adjacent, nonburned blackbrush community. The group took some time to discuss fire ecology in the lowland Mojave Desert and the role of introduced brome grasses in the fire cycle. Field trip held in conjunction with the Symposium on Cheatgrass Inva­ The fourth stop was a canyon site at 1,510 m elevation sion, Shrub Die-Off, and Other Aspects of Shrub Biology and Management, near the base of Mount Potosi in the Spring Mountains. Las Vegas, NV, April 5-7, 1989. Stanley D. Smith is Associate Professor and David J. M. Bradney is a The zonal vegetation on the slopes of the mountains was Graduate Student in the Department of Biological Sciences, University of an association of singleleaf pinyon (Pinus monophylla) Nevada, Las Vegas, NV 89154. and juniper (Juniperus osteosperma), both of which

350 This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. 6,000 Singleleaf Pinyon Spring Mountain Pass Utah Jumper 5,502 ft (1 ,677m) Big Sagebrush Apache Plume Rabbitbrush Green Mormon Tea Singleleaf Ash Pinyon-Juniper 5,000 Woodland Joshua Tree 'i Stop#4 . -> Winterfat I ..9! Turpentine Bush 4,960 ft (1,512 m~ ca Desert Trumpet Q) 0 Blackbrush Q) Creosotebush > Ground Thorn .c0 Desert Almond ca Purple Sage 4,000 JoshuaT Tree­ Desert Willow Biackbrush -I Rabbitbrush e. Desertscrub c Creosotebush 0 White Bursage i Nevada Mormon Tea Rosemary Eriogonum 1.. Q)> jjj Mohave Yucca Catclaw Acacia Beavertail Cactus 3,000 Pencil Cholla Indigo Bush

Creosotebush Stop#2 Desertscrub 2,640 ft (805 m) Stop#1 2,350 ft (716 m) 2,000

Figure 1-Field trip stops and a depiction of elevational changes in plant community composition along the Blue Diamond-Pahrump Highway.

extended into the canyon bottom. Also abundant in the 2-year-old burn that had eliminated virtually all the canyon were big sagebrush (Artemisia tridentata), Apache Joshua trees, pinyons, and junipers from the area. We plume (Fallugia paradoxa), rabbitbrush, desert almond, examined and discussed this early successional stage, green Mormon tea (Ephedra viridis ), and singleleaf ash and compared it to the older burn on the other side of (Fraxinus anomola). We stopped at Spring Mountain Pass the Spring Mountains. (1,675 m elevation) for lunch, taking the time to enjoy the midday shade of a well-developed pinyon-juniper woodland. ACKNOWLEDGMENT Mter lunch, we descended into the Pahrump Valley west of the Spring Mountains. Along the western slope of the The figure used to describe the Mojave Desert gradient mountains, in a transitional area between blackbrush and was drawn and kindly donated by Dr. Joseph McAuliffe of pinyon-juniper woodland, we stopped at a more recent, UNLV's Department of Biological Sciences.

351 iT U.S. GOVERNMENT PRINTING OFFICE: 1990-573-041/21012