Western Mojave Desert Research Articles

Swainson’s Hawk Nesting Population in the Antelope Valley of the Western Mojave Desert, California

The Swainson’s Hawk (Buteo swainsoni) has a long history of breeding in California, but a severe decline in the statewide breeding population was identified in 1979, when in all of southern California only two pairs were found, one in the Antelope Valley of the western Mojave Desert. That area was little studied until we began banding Swainson’s Hawks there in 1997. Over 20 breeding seasons between 1979 and 2022, we documented in the Antelope Valley 124 attempts to nest, in which the mean clutch and brood sizes were 2.49 and 2.37, respectively. From 2004 through 2006, we observed two to four breeding pairs annually; from 2009 through 2022, three to 14 breeding pairs. The rate of success of the 91 nests revisited to determine if any young fledged was 64%. Nest trees consisted of 81.5% non-native species, 13.7% native species, including Joshua trees (Yucca brevifolia), and 4.8% unidentified deciduous trees. Between 1997 and 2022, in 50 nests, we recorded 170 vertebrate prey items, of which 90 were gophers (Thomomys bottae). Though the Antelope Valley population has grown since 1980, its nesting and foraging habitat now face multiple threats. To conserve occupied nesting territories, we recommend creation of nesting and foraging habitat reserves that include both native desert and cultivated alfalfa close to existing conserved land.

Distribution, morphology, and karyotype of San Joaquin pocket mice from the western Mojave Desert

The San Joaquin pocket mouse (Perognathus inornatus) is endemic to California and currently includes three subspecies (P. i. inornatus, neglectus, and psammophilus). P. inornatus from the western Mojave Desert have been assigned to the subspecies neglectus based on similar morphology. During the course of live-trapping projects between 1990 and 2017, we captured P. inornatus at several locations in the Mojave Desert and gathered information on habitat associations, relative abundance, seasonal activity, and field identification. P. inornatus was found at elevations ranging from 668–1,109 m above mean sea level in creosote bush scrub, allscale scrub, Joshua tree woodland, rubber rabbitbrush scrub, spinescale scrub, and California juniper woodland. The capture locations and our review of museum specimens indicate that, at a minimum, the species’ range in the Mojave Desert encompasses approximately 7,000 km2 primarily in the Antelope Valley, extending at least as far north as the Rand Mountains, east to the vicinity of Hinkley, south to Palmdale, and west to near Gorman. P. inornatus capture rates were typically low relative to other nocturnal rodents, and it was not found consistently at localities at which it had been captured previously. Adults were sexually dimorphic, with males significantly larger than females. P. inornatus was significantly larger than the sympatric little pocket mouse (P. longimembris) for length of head-body, total length and body mass, and possessed a relatively shorter tail. We collected 15 specimens of P. inornatus from nine localities in the western Mojave Desert for cytogenetic analysis and each possessed the same karyotype, characterized by a diploid complement of 52 chromosomes, which has not been previously described for the genus. We argue that the 52-chromosome form should be considered distinct for conservation purposes, because it occupies a relatively small geographic area that is being subjected to increasing habitat loss and fragmentation due to residential, commercial, and renewable energy development.

Addressing the raven food subsidy challenge by engaging restaurants to close their dumpsters.

Ravens have benefitted from resource subsidies provided by humans so much that their population has increased by over 800% in the western Mojave desert over the last 50 years. Our food waste is an especially large subsidy. Raven predation is one of the greatest threats to desert tortoise survival in the California desert. We sought to create and evaluate a behavioral change program among restaurants in Yucca Valley and Twentynine Palms, California in the western Mojave with elevated raven populations. Half of the 60 restaurants in these two communities received an intervention encouraging them to close their dumpsters while the other half served as controls with no intervention. Treatment restaurants received two in-person visits to discuss the manifold importance of dumpster closure and laminated information signs to display and extend communication to staff. We surveyed all dumpsters for open/closed status five times before the intervention and five times again 3 months after the intervention. We found a significant increase in closure rates among treatment restaurants due to the interventions, with an average of 9.5% increase toward maximum possible lid closure. Restaurants achieving 80%+ closure received "Gold Star Awards" to further stimulate community-wide behavior change by publicly recognizing and encouraging desired behaviors. We summarize dumpster closure rates from similar interventions in nearby Joshua Tree and across the Coachella Valley to illustrate how local social norms may influence behavior.

The triple argon isotope composition of groundwater on ten-thousand-year timescales

Understanding the age and movement of groundwater is important for predicting the vulnerability of wells to contamination, constraining flow models that inform sustainable groundwater management, and interpreting geochemical signals that reflect past climate. Due to both the ubiquity of groundwater with order ten-thousand-year residence times and its importance for climate reconstruction of the last glacial period, there is a strong need for improving geochemical dating tools on this timescale. Whereas 14C of dissolved inorganic carbon and dissolved 4He are common age tracers for Late Pleistocene groundwater, each is limited by systematic uncertainties related to aquifer composition and lithology, and the extent of water-rock interaction. In principle, radiogenic 40Ar in groundwater acquired from decay of 40K in aquifer minerals should be insensitive to some processes that impact 14C and 4He and thus represent a useful, complementary age tracer. In practice, however, detection of significant radiogenic 40Ar signals in groundwater has been limited to a small number of studies of extremely old groundwater (>100 ka). Here we present the first high-precision (<1‰) measurements of triple Ar isotopes (40Ar, 38Ar, 36Ar) in groundwater. We introduce a model that distinguishes radiogenic 40Ar from atmospheric 40Ar by using the non-radiogenic Ar isotopes (36Ar, 38Ar) to correct for mass-dependent fractionation. Using this model, we investigate variability in radiogenic 40Ar excess (Δ40Ar) across 58 groundwater samples collected from 36 wells throughout California (USA). We find that Δ40Ar ranges from ~0‰ (the expected minimum value) to +4.2‰ across three study areas near Fresno, San Diego, and the western Mojave Desert. Based on measurements from a network of 23 scientific monitoring wells in San Diego, we find evidence for a strong dependence of Δ40Ar on aquifer lithology. We suggest that Δ40Ar is fundamentally controlled by the weathering of old K-bearing minerals and thus reflects both the degree of groundwater-rock interaction, which is related to groundwater age, and the integrated flow through different geological formations. Future studies of Late Pleistocene groundwater may benefit from high-precision triple Ar isotope measurements as a new tool to better interpret 14C- and 4He-based constraints on groundwater age and flow.

Current status of the Mohave ground squirrel: an update covering the period 2013–2020

The Mohave ground squirrel (Xerospermophilus mohavensis) is endemic to the western Mojave Desert of California. It is restricted to a small geographic area and is listed as Threatened under the California Endangered Species Act. Human development has resulted in loss and degradation of its desert habitat and climate change is believed to pose an additional threat. To determine its current conservation status and geographic distribution, I have assembled all available data from field studies during the years 2013-2020. These data confirm that the species is still present in 4 core areas and that it continues to be widespread in the northern and central portions of its historical range. However, the recent data also confirm earlier conclusions that the Mohave ground squirrel is now extirpated from the southernmost portion of its range. Recent surveys raise concerns about its status in other areas as well. A large-scale trail camera survey on the South Range unit of China Lake Naval Air Weapons Station in 2019 failed to detect the Mohave ground squirrel over much of this large installation. Recent trail camera surveys on Fort Irwin strongly suggest that the closely-related round-tailed ground squirrel (Xerospermophilus tereticaudus) has now replaced the Mohave ground squirrel over most of the base. There is additional evidence that the round-tailed ground squirrel is present in disturbed habitats to the west of Barstow and that hybridization with the Mohave ground squirrel is occurring there. It will be important to protect and conserve currently occupied Mohave ground squirrel habitat in view of this new information.

The Catastrophic Decline of Tortoises at a Fenced Natural Area

ABSTRACTAgassiz’s desert tortoise (Gopherus agassizii), a threatened species of the southwestern United States, has severely declined to the point where 76% of populations in critical habitat (Tortoise Conservation Areas) are below viability. The potential for rapid recovery of wild populations is low because females require 12–20 years to reach reproductive maturity and produce few eggs annually. We report on a 34‐year mark‐recapture study of tortoises initiated in 1979 at the Desert Tortoise Research Natural Area in the western Mojave Desert, California, USA, and provide substantive data on challenges faced by the species. In 1980, the United States Congress designated the Research Natural Area and protected the land from recreational vehicles, livestock grazing, and mining with a wildlife‐permeable fence. The 7.77‐km2study area, centered on interpretive facilities, included land both within the Natural Area and outside the fence. We expected greater benefits to accrue to the tortoises and habitat inside compared to outside. Our objectives were to conduct a demographic study, analyze and model changes in the tortoise population and habitat, and compare the effectiveness of fencing to protect populations and habitat inside the fence versus outside, where populations and habitat were unprotected. We conducted surveys in spring in each of 7 survey years from 1979, when the fence was under construction, through 2012. We compared populations inside to those outside the fence by survey year for changes in distribution, structure by size and relative age, sex ratios, death rates of adults, and causes of death for all sizes of tortoises. We used a Bayesian implementation of a Jolly Seber model for mark‐recapture data. We modeled detection, density, growth and transition of tortoises to larger size‐age classes, movements from inside the protective fence to outside and vice versa, and survival. After the second and subsequent survey years, we added surveys to monitor vegetation and habitat changes, conduct health assessments, and collect data on counts of predators and predator sign. At the beginning of the study, counts and densities for all sizes of tortoises were high, but densities were approximately 24% higher inside the fence than outside. By 2002, the low point in densities, densities had declined 90% inside the fence and 95% outside. Between 2002 and 2012, the population inside the fence showed signs of improving with a 54% increase in density. Outside the fence, densities remained low. At the end of the study, when we considered the initial differences in location, densities inside the fence were roughly 2.5 times higher than outside. The pattern of densities was similar for male and female adults. When evaluating survival by blocks of years, survivorship was higher in 1979–1989 than in 1989–2002 (the low point) and highest from 2002 to 2012. Recruitment and survival of adult females into the population was important for growing the population, but survival of all sizes, including juveniles, was also critical.Major events and activities driving the decline in populations both inside and outside the fence included illegal collecting, upper respiratory tract disease, and hyperpredation by the common raven (Corvus corax) on juvenile tortoises. Other sources of death were gunshots, vehicles, and predation by mammals. Outside the fence, fragmentation and deterioration of habitat was a critical driver. Between the first and last surveys, 2 different ecosystem processes were underway: recovery of vegetation and soils from grazing and vehicles inside the fence and continued deterioration outside the fence. Habitat outside the fence became increasingly denuded of shrubs and fragmented by roads and trails, and habitat fragments increased 50‐fold. Outside the fence, biomass of non‐native annual plants was higher and the cover of shrubs was lower, a reflection of ongoing deterioration. These changes and losses of habitat resulted in loss of shrub cover and sites for burrows, reduction in preferred food plants, and greater exposure to predators and extremes in temperature. Overall, the tortoise population and habitat inside the fence appeared to benefit from protection and showed signs of recovery at the end of the study. Fencing, control of vehicular access, and removal of livestock grazing were among several recommended management actions for critical habitat in the first recovery plan in 1994. At the end of the study, the Natural Area remained as 1 of 2 fenced, official protected areas for the species in the geographic range. We attribute fencing to continuing higher densities of adults inside the fence compared with outside the fence and promising signs of recovery. Densities of adults at the Natural Area also were 2.3 to 5.5 times higher than in 16 of the 17 Tortoise Conservation Areas (critical habitat units) within the geographic range. © 2020 The Authors.Wildlife Monographspublished by Wiley Periodicals, LLC on behalf of The Wildlife Society.

Ecology of the Western Queen Butterfly Danaus gilippus thersippus (Lepidoptera: Nymphalidae) in the Mojave and Sonoran Deserts.

The purpose of this study was to assess the ecological knowledge surrounding the western queen butterfly, Danaus gilippus thersippus (H. Bates). Specifically, our objectives were to synthesize existing data and knowledge on the ecology of the queen and use results of this assessment to inform the direction of future research on this understudied species. We identified six core areas for assessment: distribution, the biodiversity of plant resources, western queen and their host plant phenology, chemical ecology, and four key life history traits. We mapped the distribution of D. g. thersippus from museum specimen records, citizen science (e.g., iNaturalist) and image sharing app-based observations, along with other observational data enumerating all current known plant resources and long-range movements. We assembled 14 larval food plants, six pyrrolizidine alkaloids plants and six nectar plants distributed in the western Mojave and Sonoran Desert regions of the United States and Baja California. We report on its phenology and its long-range movement. Butterfly species have declined across the western US, and western monarch populations have declined by 97%. Danaus g. thersippus has received little research attention compared with its famous congener D. plexippus L. Danaus g. thersippus’ desert distribution may be at its temperature limits for the species distribution and for its rare host plant Asclepias nyctaginifolia.

Selected trace-elements in alluvium and rocks, western Mojave Desert, southern California

Concentrations of twenty-seven elements, including naturally-occurring water-quality contaminants arsenic, chromium, and uranium, were measured in 217 samples of alluvium and rock from the western Mojave Desert, southern California, using portable (pXRF) and laboratory (LXRF) X-ray fluorescence. Comparison of measurements with NIST-traceable standards was good, although pXRF overestimated iron compared to LXRF. Results suggest pXRF survey data are sufficiently accurate to assess regional geochemical differences in geologic-source terrains. Principal component analysis showed rubidium and potassium were associated with alluvium eroded from felsic terrain, while iron, copper, chromium, and to a lesser extent titanium, manganese, and nickel were associated with alluvium eroded from mafic terrain. Zinc, vanadium, and arsenic were associated with alluvium eroded from hydrothermal terrain. Elemental assemblages associated with different source terrains were traced spatially to identify the source and composition of alluvium composing aquifers pumped for water supply. Changes in geologic source terrain to the Mojave River, associated with movement along the San Andreas Fault over the past one to five million years, reduced the mafic fraction and increased the felsic fraction of alluvium deposited to the regionally important floodplain aquifer along the Mojave River—lowering chromium concentrations in alluvium through geologic time. Comparison of pXRF and sequential extraction data from 40 samples showed arsenic and uranium were more abundant on the surfaces of mineral grains, while chromium and vanadium remained mostly within unweathered mineral grains—suggesting arsenic and uranium may be more readily mobilized into groundwater with changes in pH, redox, or ionic strength than chromium or vanadium.

Genetic Structure Across a Contact Zone betweenXerospermophilusGround Squirrels in Southern California

The Mohave ground squirrel (Xerospermophilus mohavensis) is endemic to the western Mojave Desert of California and is state-listed as Threatened. This species is of conservation concern because of the potential for large-scale renewable energy development within its range. Recent evidence suggests that this threatened species may at least occasionally hybridize with the closely related round-tailed ground squirrel (Xerospermophilus tereticaudus) where the species come into contact. As a result, there is great interest in the distribution of the Mohave ground squirrel, especially where it may come in contact with the round-tailed ground squirrel. One of the least understood portions of the Mohave ground squirrel distribution is around the Hinkley Valley, west of the city of Barstow, where its eastern range limits come into proximity with the round-tailed ground squirrel. To document what is known of the fine-scale distribution of these congeners in this region, we assemble and review all available trapping survey data from this area and combine the field survey data with genetic analyses of recently collected samples. Our evaluation of trapping records confirms that there are no reliable recent reports of Mohave ground squirrels in the Hinkley Valley itself, which has been developed for agriculture. The species appears to be largely restricted to the relatively undisturbed desert habitat west of Hinkley. In contrast, a number of round-tailed ground squirrel records exist for the Hinkley Valley, some dating back to 1977. Using 13 nuclear microsatellite loci from 55 ground squirrel samples collected across this region, we confirm the presence of genetically pure Mohave and round-tailed ground squirrels. However, 3 individuals collected in the Hinkley Valley that were identified as round-tailed ground squirrels on the basis of external characteristics show evidence of Mohave ground squirrel ancestry. In similar fashion, 3 animals from the desert region west of Hinkley were found to have some round-tailed ground squirrel genetic ancestry in spite of external features typical of Mohave ground squirrels. While round-tailed ground squirrels seem to be the dominant species in Hinkley Valley and Mohave ground squirrels seem dominant in the desert environment to the west, hybridization does appear to occur occasionally. Further, when these species hybridize, at least some hybrids are fertile, and backcrossing appears to occur in both parental directions. It is important to investigate this phenomenon more closely to evaluate possible threats to the genetic integrity of the Mohave ground squirrel.

Subsurface fault geometries in Southern California illuminated through Full-3D Seismic Waveform Tomography (F3DT)

More precise spatial descriptions of fault systems play an essential role in tectonic interpretations, deformation modeling, and seismic hazard assessments. The recent developed full-3D waveform tomography techniques provide high-resolution images and are able to image the material property differences across faults to assist the understanding of fault systems. In the updated seismic velocity model for Southern California, CVM-S4.26, many velocity gradients show consistency with surface geology and major faults defined in the Community Fault Model (CFM) (Plesch et al. 2007), which was constructed by using various geological and geophysical observations. In addition to faults in CFM, CVM-S4.26 reveals a velocity reversal mainly beneath the San Gabriel Mountain and Western Mojave Desert regions, which is correlated with the detachment structure that has also been found in other independent studies. The high-resolution tomographic images of CVM-S4.26 could assist the understanding of fault systems in Southern California and therefore benefit the development of fault models as well as other applications, such as seismic hazard analysis, tectonic reconstructions, and crustal deformation modeling.

Pieces of the puzzle: Lack of significant C4 in the late Miocene of southern California

The stable carbon isotope ratios of tooth enamel from 89 herbivores from the Dove Spring Formation (DSF) of the El Paso Basin, western Mojave Desert, California, were analyzed to determine if C4 plants may have been present in the late Miocene of southern California. Taxa analyzed include antilocaprids, castorids, camelids, equids, gomphotheriids, merycoidodontids, and rhinocerotids. The atmospheric CO2-δ13C value has varied significantly in the geologic past, leading to changes in the δ13C values of plants and tooth enamel. The estimated δ13C value of atmospheric CO2 in the middle to late Miocene is −5.8±0.2‰, making the pure C3 diet cutoff value higher than the modern by ~2‰. Given this, much of the evidence of an early spread of C4, which inappropriately uses the modern C3 diet cutoff value, may be a false signal from water-stressed C3 plants and the amount of C4 biomass in the geologic past may have been over-estimated. The enamel-δ13C values of all taxa sampled are −10.1±1.1‰ (n=89). Accounting for the changes in the δ13C values of atmospheric CO2, enamel-δ13C values from the DSF indicate that herbivores from that area had C3 diets and lived in an environment dominated by C3 plants. Serial enamel isotope data from the DSF revealed negligible intra-tooth variations in the δ13C values, confirming that these herbivores consumed C3 plants year-round. If the C4 photosynthetic pathway did spread to southern California prior to the rapid, global rise in C4 in the late Miocene, the taxa sampled did not incorporate C4 biomass into their diets. The results of the stable carbon isotope analysis, along with the evidence from the rock record and the life habits of the fossil taxa present, indicate that the El Paso Basin consisted of diverse habitats including C3 grasslands, wooded C3 grasslands, and riparian forests.

Diet of the Mohave Ground Squirrel (Xerospermophilus mohavensis) in Relation to Season and Rainfall

The Mohave ground squirrel (Xerospermophilus mohavensis) is endemic to the western Mojave Desert of California. It is listed as threatened under the California Endangered Species Act, yet there is little published information on its habitat requirements. We studied the diet of Mohave ground squirrels at 4 sites in desert scrub habitat in Inyo County, California, primarily by microhistological analysis of 754 samples of fecal pellets collected from live-trapped animals. Over all sites and seasons, shrub foliage was the largest component of the diet (39.8% relative density) and mainly derived from several taxa of Chenopodiaceae: winterfat (Krascheninnikovia lanata), spiny hopsage (Grayia spinosa), and saltbushes (Atriplex spp.). Forb leaves were next in importance (34.1% relative density), especially from Fabaceae (Astragalus and Lupinus), Polemoniaceae (Gilia and Linanthus), and Asteraceae. Flowers, pollen, and seeds were also major components (20.3% relative density). Leaves composed nearly all of the diet in spring, whereas pollen, flowers, and seeds made up about a third of the diet in summer. Following dry winters when annual forbs were limited, Mohave ground squirrels depended primarily on foliage from perennial shrubs and forbs. Following wet winters when spring annuals were abundant and most plant species flowered and set seed prolifically, squirrels consumed a high proportion of leaves plus flowers, pollen, and seeds of annual forbs. Mohave ground squirrels reproduced only after winter rainfall >80 mm that resulted in a standing crop of herbaceous annuals ≥100 kg · ha-1. Mohave ground squirrels consumed very little of the nonnative annual plant biomass present on our study sites (Erodium, Salsola, Bromus, and Schismus contributed <3% overall to the diet). Conservation implications include the following: (1) priority should be given to protecting habitats supporting preferred perennial forage plants, including winterfat and spiny hopsage; (2) habitats with an understory dominated by native annual forbs have higher value than those dominated by nonnative plants, especially annual grasses; and (3) if climate change results in lower and less regular winter precipitation, suitable habitat for Mohave ground squirrels may be reduced and fragmented in the drier portions of the geographic range.

Large-Scale Land Development, Fugitive Dust, and Increased Coccidioidomycosis Incidence in the Antelope Valley of California, 1999-2014.

Ongoing large-scale land development for renewable energy projects in the Antelope Valley, located in the Western Mojave Desert, has been blamed for increased fugitive dust emissions and coccidioidomycosis incidence among the general public in recent years. Soil samples were collected at six sites that were destined for solar farm construction and were analyzed for the presence of the soil-borne fungal pathogen Coccidioides immitis which is endemic to many areas of central and southern California. We used a modified culture-independent nested PCR approach to identify the pathogen in all soil samples and also compared the sampling sites in regard to soil physical and chemical parameters, degree of disturbance, and vegetation. Our results indicated the presence of C. immitis at four of the six sites, predominantly in non-disturbed soils of the Pond-Oban complex, which are characterized by an elevated pH and saltbush communities, but also in grassland characterized by different soil parameters and covered with native and non-native annuals. Overall, we were able to detect the pathogen in 40% of the soil samples (n=42). Incidence of coccidioidomycosis in the Antelope Valley was positively correlated with land use and particulate matter in the air (PM10) (Pearson correlation coefficient>0.5). With the predicted population growth and ongoing large-scale disturbance of soil in the Antelope Valley in coming years, incidence of coccidioidomycosis will likely further increase if policy makers and land developers continue to ignore the risk of grading land without implementing long-term dust mitigation plans in Environmental Impact Reports.

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.

Community Structure and Demography in a Saline Pan-Dune Mosaic in the Western Mojave Desert

A unique saline ecological system formed by an extensive mosaic of small clay pans and low stable dunes exists within Edwards Air Force Base in the western Mojave Desert of California. This ecosystem lies between the large Rosamond and Rogers dry lakes on the old Pleistocene lakebed of Lake Thompson. Plant communities on the low and relatively stable dunes were broadly classed as saltbush scrub, with a total canopy cover of 30–36%. Atriplex confertifolia was the typical dominant, with Ericameria nauseosa as an important associate. Taller dunes of younger age and less saline soils had similar plant cover, but a distinct plant community with Atriplex canescens and Krascheninnikovia lanata as dominants and Yucca brevifolia as a common associate. Flat areas adjacent to the dunes were dominated by a virtual monoculture of Atriplex confertifolia. Aboveground shrub and bunchgrass biomass including dead material varied from 503 to 1204 kg ha−1, low in comparison to similar plant communities in the Great Basin. The absence of small saplings and seedlings of many shrub species suggests successful establishment is highly episodic. Seedlings were abundant only in Isocoma acradenia.

Plant Phenology and Water Relations in a Saline Pan-Dune Mosaic in the Western Mojave Desert

An extensive mosaic of small clay pans and low stable dunes exists within Edwards Air Force Base in the western Mojave Desert of California. This pan-dune ecosystem, positioned between the large Rosamond and Rogers dry lakes on the old Pleistocene lakebed of Lake Thompson, provides an opportunity to study the seasonal patterns in growth and flowering phenology and water relations in a saltbush scrub community dominated by a mix of C3 and C4 species. C3 shrubs initiated vegetative growth in February, with current shoot and leaf production largely ceasing in most species by mid-April. The deeply rooted Tetradymia stenolepis and Ericameria nauseosa continued vegetative growth through May. C4 species including Atriplex and the grass Distichilis spicata typically began vegetative growth in mid- to late March, 2–6 weeks later than the first group and continued this growth in most cases through June and July. Reproductive growth commonly did not begin until June and continued into July and/or August. Shrub species showed a general pattern of slowly declining water potentials (increased stress) through the late spring and summer months, reflecting more limited soil water availability. Smaller changes in predawn water potential over this period were seen in the deeply rooted species. Soil moisture availability in the widespread Atriplex confertifolia showed a pattern of variation between pan, dune and open flat microhabtats.

Impact of Ground Water Depletion on the Mesquite Community at Edwards Air Force Base, Western Mojave Desert, California

Impact of Ground Water Depletion on the Mesquite Community at Edwards Air Force Base, Western Mojave Desert, California

Assessing site occupancy of Mohave ground squirrels: Implications for conservation

ABSTRACTTo better understand the distribution and habitat relationships of the Mohave ground squirrel (Xerospermophilus mohavensis), a California state threatened species, I investigated site occupancy of squirrels using remote‐sensing wildlife cameras (i.e., camera traps). In 2012, I conducted camera trap surveys at 96 randomly selected sites within and adjacent to the Desert Tortoise Research Natural Area, in the western Mojave Desert, California. In 1,105 camera trap days, there were 91 detections of squirrels at 30 out of 96 sites. I used single‐season, single‐species occupancy models to explore several survey and site variables hypothesized to influence detection probability (p) and site occupancy (ψ) of the species. Detection probability of squirrels varied with survey date and between 2 study areas, peaking in early April (p = 0.23, SE = 0.05 and p = 0.13, SE = 0.05, respectively) but decreasing approximately 4‐fold by the end of the study season in mid‐May. Site occupancy was positively related to the length of ephemeral stream channel(s) within a site, increasing from 0.24 (SE = 0.07) to 0.89 (SE = 0.10) with the range of stream channel lengths observed. Vegetation transects conducted at the study sites in 2014 indicated higher percent canopy cover, species richness, and diversity of perennial shrubs along ephemeral stream channels than in the surrounding upland habitat. This study provides the first estimates of site occupancy and detection probability of Mohave ground squirrels using the non‐invasive method of camera trapping. The findings have implications for design and timing of future studies, and highlight the importance of protecting ephemeral stream channels for Mohave ground squirrel conservation. © 2015 The Wildlife Society.

Sioen buys Le Comptoir Zouloo

Cryptobiotic soil crusts form the dominant cover of a unique system of dunes and intermittently inundated pans and playas situated on the Pleistocene Lake Thompson bed, in the western Mojave Desert, CA. Recent work has reported on basic ecological parameters including distribution, abundance, and species composition of these algal (cyanobacterial)-dominated crusts as well as on their photosynthetic rates in laboratory studies. As a step toward building an ecological model for a natural resource management plan, we determined the photosynthetic rate of the crusts under field environmental conditions. We examined the relationship between the environmental conditions of crust moisture content (MC), temperature, and ambient CO2 concentration on photosynthetic rates during the mid-spring and the late autumn.Maximal photosynthetic rates (Pmax), which did not vary greatly with season, were 3.57 and 3.99 μmol CO2 m−2 s−1 for crusts on dunes and in pans, respectively. Mean photosynthetic rates for three types of crust on dunes, smooth and flat, pedicillated, and unconsolidated, were 1.31, 1.47, and 1.29 μmol CO2 m−2 s−1, respectively. Pmax of crusts on dunes and pans occurred at soil MCs of 110% and 20%, respectively, both lower and higher MCs resulted in decreased rates of photosynthesis. The relation between atmospheric CO2 concentration and net photosynthesis was linear (r2=0.88 and 0.97 for crusts on dunes and pans, respectively) through at least 1000 ppm CO2. We measured positive net photosynthesis at temperatures from 0 to 46 °C and found no clear optimum within this range when crusts were provided with optimal moisture and light conditions. Photosynthetic rates in this field study were somewhat lower than those measured in the laboratory, but both optimum MC and the relation between ambient CO2 and net photosynthetic rate were similar. Respiration rates were generally less than 10% those of photosynthetic rates. Based on data herein and climatic information, we estimated the potential net primary productivity of crusts on dunes to be 11.7 g C m−2 yr−1. The photosynthetic rate of the unconsolidated crust type, for which no data have been published, was similar to the other crust types. This finding, together with previous information that the chlorophyll values of this and other crust forms were similar, suggests that ecological studies, as well as conservation and restoration efforts, give these the same attention as the other crust types.

Huber acquires Safire flame retardants technology from Floridienne/Catena

Cryptobiotic soil crusts form the dominant cover of a unique system of dunes and intermittently inundated pans and playas situated on the Pleistocene Lake Thompson bed, in the western Mojave Desert, CA. Recent work has reported on basic ecological parameters including distribution, abundance, and species composition of these algal (cyanobacterial)-dominated crusts as well as on their photosynthetic rates in laboratory studies. As a step toward building an ecological model for a natural resource management plan, we determined the photosynthetic rate of the crusts under field environmental conditions. We examined the relationship between the environmental conditions of crust moisture content (MC), temperature, and ambient CO2 concentration on photosynthetic rates during the mid-spring and the late autumn.Maximal photosynthetic rates (Pmax), which did not vary greatly with season, were 3.57 and 3.99 μmol CO2 m−2 s−1 for crusts on dunes and in pans, respectively. Mean photosynthetic rates for three types of crust on dunes, smooth and flat, pedicillated, and unconsolidated, were 1.31, 1.47, and 1.29 μmol CO2 m−2 s−1, respectively. Pmax of crusts on dunes and pans occurred at soil MCs of 110% and 20%, respectively, both lower and higher MCs resulted in decreased rates of photosynthesis. The relation between atmospheric CO2 concentration and net photosynthesis was linear (r2=0.88 and 0.97 for crusts on dunes and pans, respectively) through at least 1000 ppm CO2. We measured positive net photosynthesis at temperatures from 0 to 46 °C and found no clear optimum within this range when crusts were provided with optimal moisture and light conditions. Photosynthetic rates in this field study were somewhat lower than those measured in the laboratory, but both optimum MC and the relation between ambient CO2 and net photosynthetic rate were similar. Respiration rates were generally less than 10% those of photosynthetic rates. Based on data herein and climatic information, we estimated the potential net primary productivity of crusts on dunes to be 11.7 g C m−2 yr−1. The photosynthetic rate of the unconsolidated crust type, for which no data have been published, was similar to the other crust types. This finding, together with previous information that the chlorophyll values of this and other crust forms were similar, suggests that ecological studies, as well as conservation and restoration efforts, give these the same attention as the other crust types.