The Vegetation and Flora of Edwards Air Force Base , Western Mojave Desert , California

Edwards Air Force Base extends over 121,000 ha in the Antelope Valley of the western Mojave Desert, with much of the area part of a closed endorheic basin that held the Pleistocene Lake Thompson. Notable topographic features are Rogers, Rosamond and Buckhorn dry lakes, while rounded domes and scattered hills are present to the north and east. Elevation gradients are limited, ranging from a low of 690 m to 1044 m near the eastern margin. Diverse communities of saltbush scrub dominate the lower plains, while creosote bush scrub and Joshua tree woodlands are present away from the old lake basin. In many ways EAFB is a biogeographic crossroads with the broader central Mojave Desert to the east, Owens Valley and Great Basin to the north, and cismontane Central Valley and foothills to the west. The flora as currently known contains 403 vascular plant taxa, with 324 (80%) of these native. The eight largest families comprise more than 68% of the flora, led by the Asteraceae with 84 taxa (72 native). Annual plants make up over two-thirds of the total flora.


INTRODUCTION
Edwards Air Force Base (EAFB) lies in the Antelope Valley, a low-lying basin at about 700 m elevation that occupies the western edge of the Mojave Desert.The Antelope Valley is roughly triangular in shape and bordered on the southwest by the San Gabriel Mountains and on the northwest by the Tehachapi Mountains (Fig. 1).This western valley is a closed endorheic basin that was the site of the large Pleistocene Lake Thompson (Orme 2008).The boundaries of EAFB include approximately 121,000 ha of the Antelope Valley, extending over portions of Kern, Los Angeles and San Bernardino counties.Geographically, EAFB is approximately 160 km northeast of Los Angeles, 140 km northwest of San Bernardino and 130 km southeast of Bakersfield.
The topography of EAFB can be characterized by broad expanses of flat to gently sloping plains interspersed with broad, rounded domes and a scattered occurrence of steeper, more rugged hills that rise sharply above the surrounding plains.The domes and hills consist mostly of outcrops of granite and quartz monzonite, with non-granitic volcanic rock forming some of the smaller features (EAFB 2008).Limestone is absent.Elevations on base range from 690 m at Rogers Dry Lake to 1044 m at Red Buttes near the eastern boundary.
The base can be divided into three distinct physiographic sections.The first physiographic area occupies the central and southwestern lowlands of the base.This region extends from the southern to the northern boundary of the base and has a low relief of approximately 120 m, with elevations ranging from 690 to 815 m (EAFB 2008).The greatest portion of this area is composed of Rogers Dry Lake (11,453 ha), Rosamond Dry Lake C 2017, The Author(s), CC-BY.This open access article is distributed under a Creative Commons Attribution License, which allows unrestricted use, distribution, and reproduction in any medium, provided that the original author(s) and source are credited.Articles can be downloaded at http://scholarship.claremont.edu/aliso/.
(5589 ha), Buckhorn Dry Lake (739 ha), and playa complexes north and northeast of Buckhorn Dry Lake (690 ha).In the late Pleistocene, Lake Thompson rose to 710 meters above sea level and covered 950 km 2 (95,000 ha) of the Antelope Valley (Orme 2008).The lake slowly desiccated following the Pleistocene and the old lakebed is today represented by Rogers, Rosamond and Buckhorn Dry Lakes.Beyond these playas the former lake basin is characterized by exposed lake beds and beach ridges as well as areas mantled by aeolian and fluvial deposits (Orme 2008).Rogers Dry Lake has a long history of aviation use ranging from the development of special aircraft for the U.S. Air Force to its service as a recovery site for early Space Shuttle landings.
The second region is an upland area in the northwest portion of the base north of Rosamond Dry Lake and west of Rogers Dry Lake.This area is characterized by low, rounded hills, including the Rosamond and Bissell Hills, with elevations ranging between 690 and 975 m.The third physiographic area is formed by the highlands to the east of Rogers Dry Lake and extending to the eastern boundary of EAFB.This upland area is similar in landform to that in the northwestern corner of the base except for two prominent relief features.Leuhman Ridge in Kern County reaches 1034 m in elevation, and Haystack Butte in San Bernardino County has an elevation of 1030 m.
The climate of EAFB is typical of the western Mojave Desert with a precipitation regime dominated by winter rainfall (Rundel and Gibson 1996).The presence of the San Gabriel and Tehachapi Mountains to the southwest and northwest of the Antelope Valley limits, but does not prevent, the passage of moist air from the Pacific Ocean.Cyclonic winter storms bring the majority of precipitation, with 80% falling from November through March.A summer weakening of the subtropical high pressure center of the eastern pacific, however, can occasionally allow low pressure systems from the Gulf of Mexico or tropical storms from the subtropical Pacific Ocean to penetrate and bring August or September rains.Mean annual precipitation at Rogers Dry Lake was 122 mm for 1933-1989(Londquist et al. 1993) ) or about 125 mm for 1944-2003(EAFB 2008)).Figure 2 VEGETATION Saltbush scrub with a dominance of single or multiple species of Atriplex is widespread in the western Mojave Desert and forms the dominant vegetation on EAFB in the areas around Rosamond, Buckhorn and Rogers dry lakes (Fig. 3A; Lichvar and Sprecher 1996;Lichvar et al. 2004).A complex interaction of topography, geology and soils creates special microsite conditions with a diverse set of identifiable associations including sand fields, various dune types, beach ridges and bare clay pans (Fig. 3B; Sharifi et al. 2017a).Dominance by Atriplex canescens (four-wing saltbush) is generally associated with deep sand along washes or on dunes.In contrast, stands dominated by Atriplex confertifolia (shadscale) occur in areas of deflated dunes with sandy hummocks and pans.Associations of Atriplex Vegetation and Flora of Edwards Air Force Base spinescens (spinescale) are limited to areas of the Pleistocene bed of Lake Thompson that have been eroded by flood events to produce shallow sands over the soil surface.Clay drainages exhibit associations of Atriplex torreyi (Nevada saltbush) with Ericameria nauseosa (rabbitbrush), Gutierrezia microcephala (matchweed) and Artemisia tridentata subsp.parishii (Parish's sagebrush) as co-dominants.Atriplex parryi (Parry saltbush) is restricted to depressions and small pans.Associations dominated by Atriplex polycarpa (allscale) are typically found in less saline transitional ecotones between playa, oligotrophic soils and upland plant communities.Allscale also occurs on rhyolitic outcrops and clay pans disturbed by trenching.
The edges of clay pans in the southwestern portion of the base where there is a thin veneer of sand that has been sorted into the pan and dune habitat are dominated by stem succulent shrubs such as Suaeda nigra (inkweed), Kochia californica (gray molly) and Arthrocnemum subterminale (Parish's glasswort).The adjacent low remnant dunes are dominated by Atriplex confertifolia and Sporobolus airoides (alkali sacaton) (Sharifi et al. 2017b).
Creosote bush dominates the moderate elevations above the saltbush communities.The lower-elevation boundary of this community roughly marks the extent of Pleistocene Lake Thompson.Larrea tridentata (creosote bush) will not grow on the saline and fine-textured soils of the old lakebed.The broad creosote bush community includes areas of both relatively low and high species diversity.The portions with low species richness are dominated by L. tridentata and/or Ambrosia dumosa (burrobush).The more species-rich communities include Ephedra nevadensis (Nevada tea), Grayia spinosa (spiny hopsage) and Krascheninnikovia lanata (winterfat) as important associated shrubs.These are the same species that make up the understory shrubs in the Joshua tree woodland.The annuals are much more diverse than in the saltbush scrub and are most commonly observed in the sandy soils on the flats near the Rosamond Hills and slopes of the west range.
Joshua tree woodland (Fig. 3C) occurs on deep, sandy soils or in shallow as well as rocky soils on steep hillsides of higher elevation areas of EAFB where surface or subsurface water is available.This distribution along lines of subsurface water distribution can be seen with Yucca brevifolia (Joshua tree) north of Leuhman Ridge which appears to follow areas with subsurface moisture.Joshua trees also occur at the lower elevations where water is trapped in sand dunes above the playa lakebed.Among these habitats, Joshua trees are densest and largest where a layer of sand 5-10 feet deep overlays the clay pan.Joshua tree woodland does not have a distinctive understory of shrubs.The most common associated species are Ambrosia dumosa, Atriplex canescens, Ephedra nevadensis, Grayia spinosa, Krascheninnikovia lanata and Larrea tridentata.Joshua tree woodland differs from the more diverse communities of creosote bush scrub largely on the basis of the presence of sufficient numbers of Y. brevifolia to create an image of a woodland.Stands of Joshua tree woodland at EAFB are among lowest-elevation stands anywhere in the range of the species.
Mesquite woodlands (Fig. 3D) on EAFB are largely concentrated in the south-central area of the base directly south of Rogers dry lake in an area with relatively shallow ground water availability (Campbell et al. 2017).In spring, much of the herbaceous understory is formed by non-native annual grasses, which have increased in abundance and cover in years of early rainfall and have greatly transformed the herbaceous community by eliminating most native species.The most common species are Bromus madritensis subsp.rubens (red brome), B. tectorum (cheatgrass), B. berteroanus (Chilean chess), Hordeum murinum subsp.leporinum (barley) and Schismus barbatus (splitgrass).
Beyond the major plant communities described above, there are several azonal communities of significance at EAFB.These are communities of limited geographic area that are controlled in distribution by local soil conditions or substrate geology rather than by larger landscape gradients.These include the plant cover on clay pans, sand fields and stable sand which support distinct communities of salt bush scrub.A rich diversity of annual species is associated with the ecotone between the clay pan and the sand dunes where soil-water content is highest as the water evaporates from the clay pans.Several species of Phacelia are only found at the base of the deflated dunes when the pans contain several inches of water.This is also the primary habitat for the endangered Calochortus striatus (alkali mariposa lily).
Another azonal community can be seen in the sinuous sandy washes that wind through the base.The soil in these washes is formed by layers of fluvial deposits associated with runoff from higher mountain areas.Nutrient content is low in these soils but regular infiltration provides for soil water resources for plants with appropriate root systems.Examples of perennial plant species that are largely restricted to such habitats are Petalonyx thurberi (sandpaper plant) and Lycium cooperi.Notable for their absence from this habitat at EAFB, however, are several species characteristic of similar sandy washes across the central and eastern Mojave Desert.These include Ericameria paniculata (black-banded rabbitbrush) and Scutellaria mexicana (Mexican bladder sage).
Alkaline meadows with high soil salinity and pH form distinct local communities at EAFB in the seepage zone to the north of Piute Ponds and adjacent to other local wetland areas such as near water outfall pipes and ditches along Main Base and in the housing areas.These alkaline soils typically support a dense coverage of Distichlis spicata (saltgrass) and taxa of the Juncus balticus-mexicanus complex (wiregrass).The native bunch grass Sporobolus airoides (alkali dropseed) may also be locally common in meadows and at springs.In wet years local areas of the clay pans on the base may be salinized and support colonization by Distichlis and Juncus as well as the rare native annual Puccinellia simplex (California alkali grass).
The shores of Pleistocene Lake Thompson are thought to have once contained springs and seeps with wetlands, freshwater meadows and willow, but these were gradually lost as the lakebed dried.The last three existing springs on EAFB were related to faults and were dry by 1950.One was destroyed quarrying for road construction at Red Hill.Evidence for another was provided by a belt of mesquite trees, and the third was on the lakebed and marked by a single surviving tree.Today, the main wetlands are at the base wastewater treatment ponds and at Piute Ponds, both of which are man-made wetlands.Piute Ponds form the largest freshwater marsh in Los Angeles County and an important stop for migratory birds on the Pacific Flyway.The wetlands are designated as an Important Bird Area by the Audubon Society and identified as a Significant Ecological Area by the County of Los Angeles (EAFB 2008) as they provide a significant habitat for waterfowl and an important stop for migratory birds.Freshwater marsh forms around the edges of the man-made ponds at Piute Ponds and the South Base retention ponds.The vegetation is dominated by Typha angustifolia (cattail), Bulboschoenus maritimus (alkali bulrush) and Schoenoplectus acutus (tule rush).Piute Ponds are a series of interconnected impoundments constructed in 1961 to evaporate effluent from the Lancaster Water Reclamation Plant (LWRP).The ponds currently receive more than 2400 million gallons of treated wastewater from the LWRP each year.At peak levels the ponds can cover as much as 160 ha of water area.In wet years there can be an internal flow of water from Amargosa Creek into Piute Ponds and occasionally Rosamond dry lakebed.

HUMAN IMPACTS
Native Americans probably had very little impact on the vegetation in the Antelope Valley and more specifically what is now EAFB.They likely harvested edible bulbs, collected the seeds of annuals, made cakes from the mesquite beans and cleared areas near existing springs to build villages.Past evidence of Native Americans has been observed in the rock flakes, beads and fire rings observed during botanical surveys of the base, but Native American populations were small (EAFB 2008).Spanish military expeditions crossed the area of EAFB in the early part of the 18th century, but no native settlements were recorded (EAFB 2008).Mineral prospecting began in the late 19th century, but major impacts were largely limited to the mountain areas.Nevertheless, exploratory pits and mines are widespread across the base.The main focus of early mining activity at EAFB was related to precious metals in the Kramer Hills to the east and later bentonite clay from the dry lakebeds for use as a sealant and lubricant in oil exploration.
Homesteads became widely established on what is now the base in the early 20th century, with a focus on raising livestock and prospecting for minerals.Productive deposits of borates brought more settlers and increased the travel across the dry lakebeds (EAFB 2008).However, land-use changes were largely limited to agriculture and grazing associated with homesteaders who built small ranches in what is now the southern margin of the base.Land was bladed for farming, rice cultivation was attempted on Rosamond Dry Lake, wells were dug for irrigation and cattle were grazed in the area.
A railroad line which crossed the base was one of the first large-scale construction projects and resulted in a long band of disturbance, introduced weeds and caused changes in sheet flow water movement.However the major military mission began with the establishment of the Muroc Bombing and Gunnery Range on the east side of Rogers Dry Lake in 1931 and expanded rapidly with the onset of World War II.Large portions of Leuhman Ridge and North Base were bladed at this time as evidenced by archived photographs in the base files.Nearly every hill on base has numerous trails to the top as these were used for training activities as observation and communication posts.Gradually, many of the WWII era facilities were removed and new facilities built as postwar projects came and went.Most significant of this development was the clearing of the area near Rosamond lakebed for buildings and the main base runways.
Nevertheless, the presence of military security at EAFB has protected much of the historic vegetative cover from changes that have impacted other parts of the Mojave Desert.Since the development of military facilities in WWII there has been only limited ORV vehicle activity and grazing (EAFB 2008).In those areas where grazing has occurred, there is a characteristic dominance today of native and non-native annual species such as Erodium cicutarium (red-stem filaree), Bromus tectorum, Amsinckia tessellata (fiddleneck) and Bromus madritensis subsp.rubens.
Historically, the open vegetation of the Mojave Desert did not allow fires to carry over significant distances.However, in recent decades the non-native Bromus madritensis subsp.rubens and B. tectorum have become increasingly abundant.In wet years these species provide enough biomass to provide a continuity of fuel between woody shrubs and can facilitate large fires that can have major impacts on shrub survival (Brooks 1999;Brooks and Matchett 2011).More recently, Brassica tournefortii (Sahara mustard) has increased alarmingly in abundance and can also provide sufficient fuel to carry a ground fire.Two other ubiquitous non-native species, Schismus barbatus and Erodium cicutarium, do not normally produce enough biomass to support a fire, but they increase their abundance in the first spring after a fire and can outcompete native annuals by germinating before the native annuals.Before 1995, wildfires were an extremely rare event but fire frequency in the Mojave Desert overall has increased over the last two decades.Only two large fires have occurred on the base, with both located in the northwestern portion of the base and appear to have a human origin.There have been natural lightning fires but these have remained small and are concentrated in the east.
Groundwater has been drawn from wells at EAFB since the first settlements.By the middle of the 20th century an expanding agricultural enterprise utilized large amounts of water.Eventually the drawdown of groundwater level produced cracking in the dry lakebeds, subsidence and environmental concerns.Over time, as military activities expanded, agricultural water use decreased but was replaced to a large extent by increased urban and industrial use.Today, groundwater drawdown remains greater than aquifer recharge (Campbell et al. 2017).
Some of the most significant environmental changes since Ernest Twisselmann carried out his systematic plant collecting in the 1950s and 1960s have been the loss of natural surface springs at Rosamond Lakebed and loss of the cottonwood grove north of Branch Park.The continued lowering of the water table has affected the mesquite and saltbush and the abundance of some of the rare perennial species.Long-term drought has affected many of the rare annuals whose populations are extremely small in average rainfall years.The health of mature Joshua trees shows evidence of decline in recent years although recruitment is still occurring.

BIOGEOGRAPHY
In many ways EAFB represents a biogeographic crossroads in its position near the western extent of the Mojave Desert.To the north there are open corridors to the southern Owens Valley with connections to the Great Basin bioregion and flora.For example, Cymopteris deserticola (desert cymopteris) represents a range extension with closely related species centered in Utah.Similarly, a number of perennial Mojave Desert species with southern origins do not occur on base because of the occasional years when temperatures drop as low as -12 • C.These episodes of low temperature also impact non-native species.For example, many of the Tamarix aphylla (athel tree) on the base are killed or die back to the ground every decade.Low winter temperatures and an almost complete absence of summer rain may well explain the absence of several typical Mojave species.Plants from the central Mojave Desert that do not occur on EAFB include Yucca schidigera and Encelia farinosa (brittlebush), and populations of Psorothamnus arborescens (Mojave indigo bush) are rare.Unreliable summer precipitation at the western margin of the Mojave Desert also means that many C 4 summer annuals are either absent or uncommon.Beyond these factors, the absence of some central Mojave taxa at EAFB may be related to the absence of niches associated with high mountain ranges and carbonate substrates in the western Mojave.The higher mountains receive more rainfall and have more microclimate niches for species.The lack of carbonates removes a whole group of species that specialize in that sterile habitat which results in less competition.
To the west, the relatively short distance from the San Joaquin Valley results in annuals such as Lasthenia californica and Layia glandulosa which are relatively common on the base but much less abundant in the central Mojave.Elevations on the base are too low to support Pinus monophylla (single-leaf pinyon) or any species of oak.Juniperus californica (California juniper), rare on the base, is only common in the western Mojave.It is replaced by Juniperus osteosperma (Utah juniper) in the eastern Mojave.

FLORA
The flora of Edwards Air Force Base as currently known includes 403 vascular plant taxa with 324 of these (80%) native.A relatively small number of the 56 families make up a disproportionate share of the flora.The eight largest families comprise more than 68% of the total flora and over two-thirds of the native taxa.These are the Asteraceae (84 taxa, 72 native), Poaceae (35 taxa, 13 native), Boraginaceae (32 taxa, all native), Polygonaceae (29 taxa, 27 native), Brassicaceae (28 taxa, 15 native), Chenopodiaceae (27 taxa, 21 native), Fabaceae (22 taxa, 19 native) and Polemoniaceae (19 taxa, all native).
Annual plant species represent the most common life form in the flora with 258 taxa when eight taxa of facultative annuals that occasionally live into a second year are included.Of this total, 205 taxa (79%) are native.The second largest group is comprised of herbaceous perennials with 86 taxa with 64 (75%) of these native.These herbaceous perennials include seven species of geophytes, a diverse group representing four different families.
Many species of herbaceous perennials and annuals are restricted to wetland habitats around Piute Ponds and there are additionally three species of aquatic plants-Stuckenia pectinata, Najas marina and Lemna minor.There are four species of perennial succulent plants present, all Cactaceae.There are 47 shrub and subshrub species, all but one of which are native.Finally, there are 12 tree species present with eight of these native and four introduced.The native trees include four species of Salix and Populus fremontii, as well as the widespread Prosopis glandulosa and a single individual of Juniperus californica.
The EAFB flora includes 80 taxa of non-native plants, comprising 20% of the total taxa present.These non-native taxa are heavily concentrated in just three families-Poaceae (22 taxa [21 species]), Brassicaceae (13 species) and Asteraceae (12 species)-which together make up almost 60% of all nonnatives.The great majority of non-natives are annuals (55 taxa) or herbaceous perennials (22 taxa).
ENDANGERED SPECIES Sixteen plant taxa on EAFB are listed or have been listed in the rare and endangered plant inventory of the California Native Plant Society (CNPS 2017).Astragalus preussii var.laxiflorus and Eriastrum rosamondense have the highest classification of 1B.1, indicating that they are seriously threatened because of a limited distribution.Calochortus striatus, Cymopterus deserticola, Eriophyllum mohavense, Eschscholzia minutiflora subsp.twisselmanii and Puccinellia simplex have a rank of 1B.2, indicating that they are moderately threatened.Loeflingia squarrosa var.artemisiarum recently recovered its 2B.2 rank, indicating that it is rare in California but more common outside of the state; Baldwin et al. (2012) treat the variety as a synonym of L. squarrosa.Canbya candida, Castilleja plagiotoma, Chorizanthe spinosa, Cleomella brevipes, Goodmania luteola, Muilla coronata, Nemacladus gracilis and Psorothamnus arborescens var.arborescens are listed as 4.2 or 4.3, indicating species of limited range but inadequately studied.

HISTORY OF PLANT COLLECTING
Plant collecting at what is now EAFB had a significant moment when Catherine Brandegee collected the rare Cymopterus deserticola on an unpaved road on the east range south of the original Kramer Junction in 1913.Sporadic collecting occurred in the area prior to the formation of EAFB during World War II.Most notable of the modern collections were those from 1964-1966 by Ernest Twisselmann, the rancher turned botanist famous for his Kern County Flora (Twisselmann 1967), who began systematic collection on the base.He was followed in 1977-1978 by Larry Heckard from the Jepson Herbarium at Berkeley and Maynard Moe from California State University Bakersfield (Moe 2016).
In January 1989, Computer Sciences Corporation hired David Charlton as part of their NEPA team for completing small environmental documents called AF Form 813's.Unfortunately 1989 and 1990 were below average rainfall years and very little was accomplished botanically.In 1991 and through several high rainfall years later, a base plant list was produced along with a vegetation map and rare plant identification guide.At this time a single-case herbarium was established.Charlton left EAFB for eight years in 1996 returning from 2004 to 2009 to develop management plans for the 10 sensitive species.During this second appointment, an electronic pictorial flora of the base and a vegetation map at the alliance level were completed, and many new and non-native species were collected and added to the EAFB Herbarium.This small herbarium, designated here as EAFB, lacks accession numbers.Thus, the EAFB flora had begun to be assembled independently before the query system of the Consortium of Southern California Herbaria made it possible to more easily determine species that had been previously collected by others.The following is a compilation of 17 seasons of observation by the on-site botanist and collections by others over the years.Botanists Dave Silverman, Denise Laberteaux and Mark Bagley have done surveys on base but were mostly restricted to mapping rare plant populations.A few species reliably identified as occurring on EAFB currently lack vouchers but are included with high confidence.
ANNOTATED CHECKLIST OF THE FLORA Family classification adheres to FNA (2016), whereas genera and species follow Baldwin et al. (2012) unless otherwise noted.Selected synonymy is provided.Frequency of taxa is denoted by the terms: common, locally common, occasional, uncommon and rare.Rarity follows the California Native Plant Society's Rare and Endangered Plant Inventory (CNPS 2017).In general, the most recent collection voucher is the only one listed.(*) denotes non-native taxa and ( †) denotes rare taxa.Charlton 1621 (EAFB).ERIOGONUM MACULATUM A. Heller.Spotted skeletonweed.Annual; uncommon, primarily on the northeastern quarter of the base, often in loose sand along roadsides.Charlton 3260b (EAFB).

Fig. 1 .
Fig. 1.Location of Edwards Air Force Base in Antelope Valley in the western Mojave Desert, California.Base boundaries are indicated by a dot-dash line.

Fig. 3 .
Fig. 3. Plant communities at Edwards Air Force Base, California.-A.Saltbush scrub.-B.Pan-dune mosaic near Buckhorn Dry Lake.-C.Open stand of Joshua tree woodland where sand has accumulated above the old lakebed.-D.Mesquite woodland.