Mojave Desert Of Southern California Research Articles

Co- and Early Postseismic Deformation Due to the 2019 Ridgecrest Earthquake Sequence Constrained by Sentinel-1 and COSMO-SkyMed SAR Data

Abstract The 2019 Ridgecrest earthquake sequence ruptured a series of conjugate faults in the broad eastern California shear zone, north of the Mojave Desert in southern California. The average spacing between Global Navigation Satellite System (GNSS) stations around the earthquakes is 20–30 km, insufficient to constrain the rupture details of the earthquakes. Here, we use Sentinel-1 and COSMO-SkyMed (CSK) Synthetic Aperture Radar data to derive the high-resolution coseismic and early postseismic surface deformation related to the Ridgecrest earthquake sequence. Line of sight (LoS) Interferometric Synthetic Aperture Radar displacements derived from both Sentinel-1 and CSK data are in good agreement with GNSS measurements. The maximum coseismic displacement occurs near the Mw 7.1 epicenter, with an estimated fault offset of ∼4.5 m on a northwest-striking rupture. Pixel tracking analysis of CSK data also reveals a sharp surface offset of ∼1 m on a second northwest-striking fault strand on which the Mw 6.4 foreshock likely nucleated, which is located ∼2–3 km east of the major rupture. The lack of clear surface displacement across this fault segment during the Mw 6.4 event suggests this fault might have ruptured twice, with more pronounced and shallow slip during the Mw 7.1 mainshock. Both Sentinel-1 and CSK data reveal clear postseismic deformation following the 2019 Ridgecrest earthquake sequence. Cumulative postseismic deformation near the Mw 7.1 epicenter ∼2 months after the mainshock reaches ∼5 cm along the satellites’ LoSs. The observed postseismic deformation near the fault is indicative of both afterslip and poroelastic rebound. We provide data derived in this study in various data formats, which will be useful for the broad community studying this earthquake sequence.

Hybrid Stripmap–ScanSAR Interferometry: Extension to the X-Band COSMO-SkyMed Data

This letter aims to investigate the applicability of the hybrid stripmap–ScanSAR differential synthetic aperture radar interferometry to data collected by the X-band RADAR sensors of the Italian COSMO-SkyMed (CSK) constellation. The presented analysis relies on the experiments carried out on two sets of stripmap and ScanSAR CSK images collected over the Chuquicamata copper mine in Chile and the Mojave Desert in Southern California, respectively. As a result of our investigation, a spurious phase modulation has been recognized in the generated hybrid stripmap–ScanSAR interferograms along the azimuth direction. This effect is due to the specific staggered topology of the used active phased array antennas that introduces half-wavelength residuals during the electronic antenna steering in the elevation plane. A straightforward solution to compensate for such spurious phase modulation effects has been provided, and the achieved results demonstrate its effectiveness. The outcomes of our analysis may be relevant in the perspective of the future CSK Second Generation Program.

Directly dated MIS 3 lake-level record from Lake Manix, Mojave Desert, California, USA

An outcrop-based lake-level curve, constrained by ~70 calibrated 14C ages on Anodonta shells, indicates at least 8 highstands between 45 and 25calka BP within 10m of the 543-m upper threshold of Lake Manix in the Mojave Desert of southern California. Correlations of Manix highstands with ice, marine, and speleothem records suggest that at least the youngest three highstands coincide with Dansgaard–Oeschger (D–O) stadials and Heinrich events 3 and 4. The lake-level record is consistent with results from speleothem studies in the Southwest that indicate cool wet conditions during D–O stadials. Notably, highstands between 43 and 25ka apparently occurred at times of generally low levels of pluvial lakes farther north as interpreted from core-based proxies. Mojave lakes may have been supported by tropical moisture sources during oxygen-isotope stage 3, perhaps controlled by southerly deflection of Pacific storm tracks due to weakening of the sea-surface temperature gradient in response to North Atlantic climate perturbations.

Severe Ulceronecrotic Dermatitis Associated with Mite Infestation in the Critically Endangered Amargosa Vole (Microtus californicus scirpensis)

The entire range of the critically endangered Amargosa vole (Microtus californicus scirpensis) consists of less than 20 km(2) of riparian habitat in the Amargosa River drainage of the Mojave Desert in southern California. In September 2010, deformities on ears and chiggers on the ears and genitalia were detected, with some individuals so severely affected that they were missing ear pinnae altogether. Follow-up trapping was performed to document the presence of mites and mite-associated disease, and molecular characterization was performed on the mites. Of 151 Amargosa voles sampled from February to April of 2011, 60 (39.7%) voles had hard orange mites adhered to some part of their bodies, on ears of 46 (76.7%), on genitalia of 11 (18.3%), and near mammary tissue of 13 (21.7%) voles. Gross lesions were not detected on genitalia, but 47% of all individuals examined showed pinnal lesions and deformities, which included alopecia, swelling, marginal necrosis, and ulceration, as well as scarring, scabbing, and loss of pinna mass covering 25-100% of the pinnae. Biopsies revealed parakeratotic hyperkeratosis and acanthosis with diffuse neutrophilic exocytosis and dense necrotic granulocytes in the epidermis and superficial dermis associated with focal erosion and ulceration. In the underlying dermis, there were dense pleocellular inflammatory cell infiltrates composed primarily of necrotic granulocytes and multifocal hemorrhage. In some samples, mite mouthparts could be seen penetrating the superficial epidermis associated with focal necrosis, and mite fragments were found on the surface epidermis and within hair follicles. Microscopic examination of the mites documented that they were a larval trombiculid in the genus Neotrombicula with anatomical features that most closely resemble Neotrombicula microti, based on scutal shape, setation, and texture. PCR of 2 mite pools (each consisting of 3 mites from an individual animal) amplified 331 bp amplicons, which had 92-97% homology with the 18S rRNA gene of Leptotrombidium deliense, although coverage of Trombiculidae in GenBank is sparse. The severity and prevalence of lesions due to this chigger were atypical and distinct. Severe clinical trombiculiasis in this endangered species could negatively impact individual health and fitness.

Optimal Pump and Recharge Management Model for Nitrate Removal in the Warren Groundwater Basin, California

The town of Yucca Valley located in the southwest part of the Mojave Desert in southern California relies on groundwater pumping from the Warren groundwater basin as its sole source of water supply. This significant dependency has resulted in a large imbalance between groundwater pumpage and natural recharge, causing groundwater levels in the basin to decline more than 90 m from the late 1940s to 1994. Consequently, an artificial recharge program proposed by the Hi-Desert Water District, which provides water service to the town of Yucca Valley, was implemented for the purpose of recovering the groundwater levels; however, the rise in groundwater levels has caused nitrate ( NO3 ) concentration to increase simultaneously. The purpose of this study is to develop an optimal pump and recharge strategy for a planned conjunctive-use project to remove the high-nitrate concentration while maintaining groundwater levels at desired elevations at specified locations as well as meeting water demand. An optimization/ma...

Comparison of PI‐SWERL with dust emission measurements from a straight‐line field wind tunnel

The Portable In situ Wind ERosion Lab (PI‐SWERL) was developed to measure dust emissions from soil surfaces. This small, portable unit can test the emissivity of soils in areas that are difficult to access with a field wind tunnel, and can complete a larger number of tests in less time. The PI‐SWERL consists of a cylindrical enclosure containing an annular flat blade that rotates at different speeds, which generates shear stress upon the surface. The shear stress generated by PI‐SWERL results in the entrainment of particles including dust. PI‐SWERL was developed to provide an index of dust emission potential comparable to the field wind tunnel. The PI‐SWERL dust emission results were compared against those obtained from a ∼12 m long, 1 m wide, 0.75 m high straight line suction‐type portable field wind tunnel by conducting collocated tests at 32 distinct field settings and soil conditions in the Mojave Desert of southern California. Clay‐ to sand‐rich soils that displayed a range of crusting, gravel cover, and disturbance were tested. The correspondence between dust emissions (mg m−2 s−1) for the two instruments is nearly 1:1 on most surfaces. Deviation between the two instruments was noted for densely packed gravel surfaces. For rough surfaces a correction can be applied to the PI‐SWERL that results in comparable dust emission data to the wind tunnel. PI‐SWERL can be used to complement research efforts in aeolian geomorphology aimed to quantify spatial and temporal patterns of dust emissions as well as air quality research related to dust emissions.

The spatial and temporal variability of aerosol optical depths in the Mojave Desert of southern California

The Mojave Desert of southern California is under constant pressure from anthropogenic influences on atmospheric pollutants and land management. Previously, the spatial, temporal, and source characteristics of aerosols over the Mojave Desert have been examined using in situ observations; however, in situ observations lack spatial coverage, and the only long-term measurements of aerosol optical depth (AOD) needed for aerosol impact studies on the local climate and biota is the AERONET site at Rogers Dry Lake. In this study, we provide the first moderately high spatial resolution (17.6 × 17.6 km 2) study of AOD over the Mojave Desert of southern California using satellite data from the Multi-angle Imaging SpectroRadiometer (MISR) for the period of March 2000 to October 2005. We have demonstrated the seasonality of AOD over the entire Mojave Desert, and the spatial and temporal variability of AOD from the average AOD at national parks, military installations, and dry lakes and playas. Statistically significant differences from the Mojave Desert mean AOD can be attributed to proximity to urban sources (e.g., Rogers Dry Lake is near the Los Angeles metropolitan area) and local sources (e.g., mineral extraction at Bristol Dry Lake). The western Mojave, generally around Rogers Dry Lake and Harper Dry Lake exhibited the most sustained pattern of seasonally high aerosol optical depths throughout the year. As a result, the AERONET site at Rogers Dry Lake should not be used as a site representative of the aerosol conditions over the entire Mojave Desert of southern California.

Kin recognition and the paradoxical patterns of aggression between colonies of a Mojave desert Pheidole ant

Populations of the desert seed-harvesting ant Phei- dole xerophylla are often characterized by high nest density leading to competitive interactions between foragers from different nests. We investigated the inter-nest aggression, spatial distribution and genetic structure of a P. xerophylla population of the Mojave Desert in Southern California. Inter-nest aggression was quantifi ed by standardized staged encounters in a neutral arena. Genetic relatedness within nests and relatedness between nests were calculated using al- lelic frequencies at four microsatellite-DNA loci. We found a bimodal distribution of inter-colony aggression levels with a fi rst mode at low aggression levels and another mode at much higher aggression levels. Inter-colony aggression levels were largely non-transitive. No effect of geographical distance on inter-nest aggression levels was detected. De- spite high amounts of variation in inter-colony relatedness (-0.24 to 0.37) this variable did not correlate with the level of aggression between nests. Intra-nest relatedness ranged from 0.40 to 0.75 and close inspection of worker genotypes within colonies revealed a high proportion of polygynous colonies or a mixture of polygyny and polyandry. Aggression levels among nests was found to decrease with increasing intra-nest relatedness. These results do not support the idea that ag- gression is modulated by a nestmate recognition mechanism based on overall genetic similarity. Instead, the absence of transitivity found in inter-colony aggression and bimodal distribution of aggression levels are compatible with a com- mon label acceptance model of nestmate recognition and suggest that label diversity may be encoded by a limited number of loci.

A Procedure to Extrapolate Vegetation Cover Estimates Over Large Arid and Semi‐arid Regions Using Multiple Spatial Resolution Imagery

Typically, land degradation on arid and semi‐arid military training and testing lands in the desert Southwestern United States is associated with a decrease in total vegetative cover. Long‐term field surveys have been established by the military to measure and monitor total vegetative cover over time. However, field methods are not cost effective due to the large area of training lands, nearly 3 million acres in the California deserts alone, that must be sampled. Measurements recorded at individual transects must still be spatially extrapolated to produce a complete census of the installation. Remote sensing has been used to spatially extrapolate such field measurements over larger areas, but in the past, imagery has lacked sufficient spatial resolution to accurately estimate total vegetative cover. In this study, nested, high resolution imagery was collected at different spatial resolutions for study sites at the Marine Corps Air Ground Combat Center in the south‐central Mojave Desert of southern California. Vegetation cover estimates derived from these multiple resolution images were examined as a possible surrogate for sampling total vegetative cover in the field. Results indicate that a high correlation exits between field measurements of cover and estimates of cover derived from imagery. However, similar to field measurements, if the intent is to estimate total vegetative cover across large geographic areas, high resolution imagery is also costly in terms of collection, processing, and interpretation because a single image generally covers a small geographic area. Therefore, a protocol to scale detailed observations of total cover from high resolution imagery to lower resolution imagery that covers a larger geographic area was developed and tested. The results of this study indicate that a relatively high correlation exists between vegetation cover estimates derived from high resolution imagery and image brightness derived from a nested, lower resolution image. This protocol now allows military land managers to sample site‐specific areas with high resolution imagery and extrapolate absolute estimates of vegetation cover across training and testing lands with more cost effective, lower resolution imagery that provides complete coverage of an installation.

SYSTEMATICS OF SALVIA PACHYPHYLLA (LAMIACEAE)

Most populations of Salvia pachyphylla occur along mountain ranges adjacent to the Mojave Desert of southern California, southwestern Nevada, and northern Baja California, Mexico. A smaller disjunct group occurs in eastern Arizona near the southern edge of Navajo and Hopi reservation lands near Winslow, AZ. This study was undertaken to determine whether there are morphologically and genetically distinct geographical groups within S. pachyphylla and whether these groups form a cohesive unit easily separated from S. dorrii. Specimens of S. pachyphylla and broadly sympatric taxa in S. dorrii were examined in a morphometric analysis of twelve characters. A preliminary molecular analysis using the nuclear ribosomal DNA internal transcribed spacers (ITS-1 and ITS-2) and the embedded 5.8 S subunit was also performed on the same taxa. Morphometric analysis supports the continued recognition of S. pachyphylla and S. dorrii as distinct species and the recognition of three subspecies within S. pachyphylla, requiring two new subspecies, eremopictus and meridionalis, described here. The molecular data support the recognition of the S. dorrii species complex as a whole, but do not support the separation of S. dorrii and S. pachyphylla as distinct species, although the Mexican populations of S. pachyphylla appear genetically distinct.

Community Structure Comparison Using FAME Analysis of Desert Varnish and Soil, Mojave Desert, California

While a number of studies have shown that a close association exists between microorganisms and varnished rocks, there is little hard evidence to support the existence of either specific desert varnish communities, or any role these microbes might play in the genesis of the varnish layers. To this end, we analyzed fatty acid methyl esters (FAMEs) of samples collected from the Mojave desert of southern California to compare the microbial community structure of desert varnish with the adjacent desert soil. These analyses indicated prokaryotic and fungal communities in both desert varnish and soil samples. FAMEs specific to gram-positive bacteria were found more often, and in greater abundance in varnish samples than in adjacent soils. This may represent a higher preservation potential of gram-positive bacteria fatty acids in varnish, a source area of varnish microorganisms dominated by gram-positive bacteria, or a varnish community dominated by gram-positive microorganisms. Heterogeneity in fatty acids was documented between varnished rocks and soils from different localities, as well as between samples collected from the same locality. This heterogeneity suggests that there are significant differences in the community structure of the microbial fauna found in varnish samples compared to the adjacent soil, and that desert varnish in the Mojave desert is not characterized by a unique and ubiquitous microbial community. These results suggest that the varnish is not a homogeneous and unique environment for biota, and provide no support for the hypothesis that the varnish layers are biogenic in origin.

Seismic refraction evidence for steep faults cutting highly attenuated continental basement in the central Transverse ranges, California

A 2-D upper crustal structural profile with seven steeply dipping faults was constructed from the Los Angeles Region Seismic Experiment phase II (LARSE II) seismic refraction data recorded over the north central Transverse ranges and adjacent Mojave desert in southern California. The profile extends to a depth of approximately 4 km. The faults were identified from distinctive features in the seismic first arrival data, while the velocity structure was determined from travel times and refined by forward modelling. The resulting seismic velocity structure is correlative to the geological structure along the profile and is used to help constrain a geological cross-section. Six of the seven faults detected from the seismic data correspond directly to geologically mapped faults in the central Transverse ranges and adjacent Mojave desert. From south to north, they are the Pelona fault, the San Francisquito fault, the Clearwater fault, the San Andreas fault and two faults that cut the Portal ridge. The seventh fault is buried beneath Quaternary deposits of the western Mojave desert and appears to correlate with an additional fault that cuts Portal ridge out of the section line. The southernmost fault (the Pelona fault) separates the younger Vasquez formation in the south from the Pelona schist in the north and is determined to be an intermediate (40°–60°) south-dipping normal fault. This coupled with field observations indicate that the Pelona fault was instrumental in the late Oligocene—early Miocene formation of the Soledad basin, and the structural ascent and exhumation of the Pelona schist along the proto-Sierra Pelona intrabasin ridge. Analogous Neogene extensional deformation is found to have affected the western Mojave desert region, where faults 6 and 7 appear to have had north side down normal displacement and to have been instrumental in forming the western Antelope basin. A 6.0 km s−1 basal layer to our shallow seismic structure is correlated with the Pelona schist south of the San Andreas fault and with the tectonically related Rand schist beneath the western Mojave desert. Granitic and gneissic crystalline rocks, which form regional upper plate complexes along the Vincent thrust above the Pelona schist and along the Rand thrust above the Rand schist, yield consistently low seismic velocities as compared with the expected velocities for the constituent rock types. These anomalous velocities are reconciled by the degraded structural and textural state of the upper plate crystalline rocks that resulted from extreme shearing, brittle fracturing and related retrogressive hydration reactions starting during or shortly after the latest Cretaceous—early Palaeocene underthrusting of the schists. Extension, translation along the San Andreas transform system and compressional faulting with the uplift of the Transverse ranges further accentuated the basement deformations. The integrated result is that the survival of the Precambrian through Mesozoic granitic crust of the region became a rootless and highly attenuated upper crustal layer.

Redescription of Strophopoda aprica Van Duzee and the Description of Two New Genera and Five New Species from the Southwestern United States and Northern Mexico (Heteroptera: Miridae)

The obscure species Strophopoda aprica Van Duzee, 1921 is redescribed. Calidroides, n.gen., is erected to accommodate the type species C. schaffneri, n.sp., from Arizona, Texas, and Mexico and C. negro, n.sp., from southern Mexico. Chlamyopsallus, n.gen., is described to accommodate the type species, C. lycii, n.sp., from the western portion of the Mojave Desert of southern California. Two new species of Pseudopsallus Van Duzee are described, P. tiquiliae, n.sp., from southern California and P. greggii, n.sp., from New Mexico and southwestern Texas. Systematic relationships in the Phylini and Pseudopsallus are discussed and digital habitus illustrations, illustrations of male genitalia, scanning micrographs of diagnostic features, and new distributional records are provided.

WATER AVAILABILITY AFFECTS CLUTCH SIZE IN A DESERT SPARROW

Birds that breed in hot, arid environments experience unique physiological demands resulting from efforts to maintain positive water balance when ambient temperatures are high, but humidity and water availability are low. We provided water to Black-throated Sparrows (Amphispiza bilineata) on breeding territories in the Mojave Desert of southern California to examine the effects of water availability on reproduction in desert birds, independent of the increase in food availability commonly associated with increased precipitation in arid ecosystems. In both years of the study (2000 and 2001), water-supplemented (treatment) pairs laid significantly larger clutches than did control pairs. In 2000, the drier of the two years, the mean clutch size of treatment pairs was almost 0.5 egg larger, or about 16% larger, than control pairs. Overall, the mean egg volume per nest in treatment and control pairs was not significantly different, and the mean proportion of eggs hatched was not significantly different at α = 0.05, but was nearly so. Daily nest survival probabilities were not greater in treatment nests than in control nests in 2000 or in 2001. Treatment nests did not produce a greater mean number of young surviving to day 8 than did control nests, although the trend is of greater means in treatment nests. The results of our study indicate that for species like the Black-throated Sparrow that are abundant in deserts, are successful in breeding there, and can survive without drinking water, water availability in arid environments can nevertheless impose limitations on reproduction.

Calibrated, late Quaternary age indices using clast rubification and soil development on alluvial surfaces in Pilot Knob Valley, Mojave Desert, southeastern California

The orange coating (varnish) that forms on the undersides (ventral sides) of clasts in desert pavements constitutes a potential relative-age indicator. Using Munsell color notation, we semiquantified the color of the orange, ventral varnish on the undersides of clasts from 15 different alluvial fan and terrace surfaces of various ages ranging from less than 500 to about 25,000 yr. All of the surfaces studied are located along the central portion of the left-lateral Garlock fault, in the Mojave Desert of southern California. The amount of left-lateral offset may be used to determine the relative ages of the surfaces. The previously published slip rate of the fault may also be used to estimate the absolute age of each surface. The color of the ventral varnish is strongly correlated with surface age and appears to be a more reliable age-indicator than the percentage coverage of dorsal varnish. Soil development indices also were not as strongly correlated with age, as were the colors of the ventral varnish. In particular, rubification appears to be more useful than soils for distinguishing relative ages among Holocene surfaces. Humidity sensors indicated that the undersides of clasts condensed moisture nightly for a period of several days to over a week after each rain. These frequent wet-dry cycles may be responsible for the rapid development of clast rubification on Holocene surfaces.

Aeolian Destabilization Along the Mojave River, Mojave Desert, California: Linkages Among Fluvial, Groundwater, and Aeolian Systems

A notable aspect of ecosystem deterioration in arid regions is sand movement subsequent to the loss of plant cover. The Lower Mojave Valley in the Mojave Desert of southern California is undergoing rapid environmental change owing to groundwater drawdown, causing the death of phreatophytic vegetation and the reactivation of aeolian sand. Historical documentation shows that in the early twentieth century, sand derived from the open river channel accumulated to form large stabilized nebkhas, anchored by mesquite (Prosopis glandulosa), a plant that requires the water table to lie 9 m or less below the surface. The natural balance between winds, sand supply, and vegetation changed in the later twentieth century, as increasing demand for groundwater caused the water table to drop from a near-surface elevation to as much as 30 m below the surface. The mesquite that anchored the dunes declined in vigor or died. Riparian vegetation, including cottonwood and willow, was scoured from the channel during flood events, and did not recolonize as the low water table prevented the establishment of seedlings. Extensive new areas of the channel were laid open to wind erosion. Sand derived from the channel and cannibalized from the degrading nebkhas began to migrate downwind. Long sand stringers extended from west to east across the dunefield. In the 1990s, barchans developed atop the sand stringers, growing rapidly from incipient low-amplitude forms to well-developed barchans 60 m in width with sharply defined slipfaces 6 m in height. The dunefield has changed from a predominant mode of deposition to one of erosion and transportation, in which the water table and vegetation no long exert an influence. Dune encroachment from migrating sand streaks and dunes buried equipment, rendered buildings and homes unusable, blocked the entrances to property and destroyed pasture areas. Changes to the aeolian environment of the Lower Mojave Valley reflect complex interactions between the wind regime, river channel morphology and sediment, surface and subsurface hydrology, geology, vegetation, and human influence. This study demonstrates the sensitivity of aeolian systems to land-use changes, particularly in environments where wind is not a limiting factor, and illustrates the rapidity with which such changes can occur.

The 1999 Mw 7.1 Hector Mine, California, Earthquake: A Test of the Stress Shadow Hypothesis?

We test the stress shadow hypothesis for large earthquake interactions by examining the relationship between two large earthquakes that occurred in the Mojave Desert of southern California, the 1992 M w 7.3 Landers and 1999 M w 7.1 Hector Mine earthquakes. We want to determine if the 1999 Hector Mine earthquake occurred at a location where the Coulomb stress was increased (earthquake advance, stress trigger) or decreased (earthquake delay, stress shadow) by the previous large earthquake. Using four models of the Landers rupture and a range of possible hypocentral planes for the Hector Mine earthquake, we discover that most scenarios yield a Landers-induced relaxation (stress shadow) on the Hector Mine hypocentral plane. Although this result would seem to weigh against the stress shadow hypothesis, the results become considerably more uncertain when the effects of a nearby Landers aftershock, the 1992 M L 5.4 Pisgah earthquake, are taken into account. We calculate the combined static Coulomb stress changes due to the Landers and Pisgah earthquakes to range from –0.3 to +0.3 MPa (–3 to +3 bars) at the possible Hector Mine hypocenters, depending on choice of rupture model and hypocenter. These varied results imply that the Hector Mine earthquake does not provide a good test of the stress shadow hypothesis for large earthquake interactions. We use a simple approach, that of static dislocations in an elastic half-space, yet we still obtain a wide range of both negative and positive Coulomb stress changes. Our findings serve as a caution that more complex models purporting to explain the triggering or shadowing relationship between the 1992 Landers and 1999 Hector Mine earthquakes need to also consider the parametric and geometric uncertainties raised here.

MORPHOLOGICAL AND MOLECULAR CHARACTERIZATION OF FOUR NOSTOC SPECIES ISOLATED FROM DESERT SOILS

Kubecková, K.1, Johansen, J. R.1, Li X.2 & Vincent, J.1 1Department of Biology, John Carroll University, University Heights, OH 44118 USA; 2Botany Department, Miami University, Oxford OH 45056 USAFour strains of Nostoc were isolated and characterized with regards to morphology and life cycle. Three strains were isolated from microbiotic soil crusts in the Mojave Desert of Southern California, and one strain was isolated from the lichen Collema tenax from crusts in Canyonlands National Park. All four strains were found to be morphologically distinct, with differences in: macrocolony appearance, microcolony shape and size, density of trichomes within the microcolony, and hormogonia production. The most commonly reported species from desert soils is Nostoc commune Vaucher. However, our strains all differ from that taxon in the size and shape of akinetes and heterocysts. Collema tenax is reported to have Nostoc commune as its phycobiont, but our observations on the phycobiont indicate that it is very distinct from that species and other Nostoc we have observed in desert soils. We subsequently sequenced a partial sequence of the 16SrRNA gene and the associated internal transcribed spacer (ITS) between the 16SrRNA and 23SrRNA genes for all four strains. Evidence of three multiple rRNA operons each was recovered for two strains, based upon configuration and sequence differences in the ITS regions. ITS with the following configurations were observed: no tRNA genes, tRNAIle and tRNAAla genes, and the beginning portion of tRNAIle immediately followed by the end portion of tRNAAla. Although our strains appear distinct from the descriptions of species in Geitler (1932) and Starmach (1966), further molecular characterization of species in this genus is probably warranted before they are assigned to existing or new species.

REGRESSION-TREE MODELING OF DESERT TORTOISE HABITAT IN THE CENTRAL MOJAVE DESERT

This paper describes an interdisciplinary study of the habitat requirements of threatened desert tortoises (Gopherus agassizii) on eight 225-ha study plots in a 14 000 ha study area near the southern boundary of the U.S. Army's National Training Center at Fort Irwin in the central Mojave Desert of southern California. The objective of the study was to produce an empirical, statistical, GIS-based model of desert tortoise habitat use based on a combination of field data and data derived from various spatial databases, including satellite imagery. A total of 11 primary and secondary data layers constitute the spatial database used for this project. Vegetation and tortoise relative density data were obtained from field surveys. Regression-tree methods were used to develop the statistical model. The tree has 11 terminal nodes and a residual mean deviance of 1.985. Out of 73 potential predictors in the model specification, only eight were selected by the algorithm to be used in construction of the tree. The mod...

Surface wave profiling of the lithosphere beneath the Mojave Desert using TERRAscope data

We have applied surface wave analysis to the Mojave Desert in southern California to extract the regional lithosphere velocity structure. Seismograms from teleseismic sources recorded by the TERRAscope stations, a recently established network of broadband instruments, were used to determine the phase velocity dispersion data of Rayleigh waves for wave paths crossing the Mojave Desert. An inversion analysis on the dispersion data for each path gave the shear wave velocity structure beneath the path. Inversion results show that there is a low‐velocity layer at the middle and lower crust, which is particularly pronounced for paths traversing the western part of the Mojave Desert. This low‐velocity layer at the midcrust may reflect the existence of a transition zone from the brittle to the ductile regimes, where there is an absence of earthquake occurrence.