Colorado Desert Research Articles

Population genomics of flat-tailed horned lizards (Phrynosoma mcallii) informs conservation and management across a fragmented Colorado Desert landscape.

Phrynosoma mcallii (flat-tailed horned lizards) is a species of conservation concern in the Colorado Desert of the United States and Mexico. We analysed ddRADseq data from 45 lizards to estimate population structure, infer phylogeny, identify migration barriers, map genetic diversity hotspots, and model demography. We identified the Colorado River as the main geographic feature contributing to population structure, with the populations west of this barrier further subdivided by the Salton Sea. Phylogenetic analysis confirms that northwestern populations are nested within southeastern populations. The best-fit demographic model indicates Pleistocene divergence across the Colorado River, with significant bidirectional gene flow, and a severe Holocene population bottleneck. These patterns suggest that management strategies should focus on maintaining genetic diversity on both sides of the Colorado River and the Salton Sea. We recommend additional lands in the United States and Mexico that should be considered for similar conservation goals as those in the Rangewide Management Strategy. We also recommend periodic rangewide genomic sampling to monitor ongoing attrition of diversity, hybridization, and changing structure due to habitat fragmentation, climate change, and other long-term impacts.

Whole community shotgun metagenomes of two biological soil crust types from the Mojave Desert.

We present six whole community shotgun metagenomic sequencing data sets of two types of biological soil crusts sampled at the ecotone of the Mojave Desert and Colorado Desert in California. These data will help us understand the diversity and function of biocrust microbial communities, which are essential for desert ecosystems.

Wetting-induced soil CO2 emission pulses are driven by interactions among soil temperature, carbon, and nitrogen limitation in the Colorado Desert.

Warming-induced changes in precipitation regimes, coupled with anthropogenically enhanced nitrogen (N) deposition, are likely to increase the prevalence, duration, and magnitude of soil respiration pulses following wetting via interactions among temperature and carbon (C) and N availability. Quantifying the importance of these interactive controls on soil respiration is a key challenge as pulses can be large terrestrial sources of atmospheric carbon dioxide (CO2 ) over comparatively short timescales. Using an automated sensor system, we measured soil CO2 flux dynamics in the Colorado Desert-a system characterized by pronounced transitions from dry-to-wet soil conditions-through a multi-year series of experimental wetting campaigns. Experimental manipulations included combinations of C and N additions across a range of ambient temperatures and across five sites varying in atmospheric N deposition. We found soil CO2 pulses following wetting were highly predictable from peak instantaneous CO2 flux measurements. CO2 pulses consistently increased with temperature, and temperature at time of wetting positively correlated to CO2 pulse magnitude. Experimentally adding N along the N deposition gradient generated contrasting pulse responses: adding N increased CO2 pulses in low N deposition sites, whereas adding N decreased CO2 pulses in high N deposition sites. At a low N deposition site, simultaneous additions of C and N during wetting led to the highest observed soil CO2 fluxes reported globally at 299.5μmol CO2 m-2 s-1 . Our results suggest that soils have the capacity to emit high amounts of CO2 within small timeframes following infrequent wetting, and pulse sizes reflect a non-linear combination of soil resource and temperature interactions. Importantly, the largest soil CO2 emissions occurred when multiple resources were amended simultaneously in historically resource-limited desert soils, pointing to regions experiencing simultaneous effects of desertification and urbanization as key locations in future global C balance.

RANGE EXTENSIONS AND POPULATION DECLINE OF THE ENDEMIC DESERT PERENNIAL OROCOPIA SAGE (SALVIA GREATAE [LAMIACEAE]) IN THE MECCA HILLS AND OROCOPIA MOUNTAINS, CALIFORNIA

Orocopia Sage, Salvia greatae Brandegee (Lamiaceae), is an endemic shrub restricted to the Orocopia and Chocolate Mountains within the Colorado Desert of Riverside and Imperial Counties, California. Very little has been published on this species' demographics, distribution, range, ecology, or threats. Generally, habitat for S. greatae has been described as alluvial fans, slopes, and washes between 30–450 m elevation. Our study was designed to focus specifically on documenting threats and describing the demographic patterns of S. greatae populations, as well as establishing a set of baseline data for long-term monitoring. Our surveys, performed in fall and winter of 2019–2020, revealed patterns that differed from the previously documented distribution for this species; we found denser, more expansive populations, and lower mortality rates at even higher elevations, up to 1011 m on steep slopes and rugged terrain. Populations at mid-elevations, 200 to 500 m, had higher mortality rates than high elevation sites. For populations previously considered to be within ‘core habitat’ at elevations below 200 m and within bajadas and alluvial fans, we relocated very few of the populations recorded in the early 1900s, and observed high mortality within those that persisted. Over the gradient studied, we found that the condition of plants significantly increased with elevation, with the highest proportion of vigorous individuals found at higher elevations. Our data indicate that either this species is shifting to higher elevations, or the previously described habitat of S. greatae was biased toward the lower, easily accessible populations, and may have been composed of waifs, as opposed to stable upland populations. Declines noted in mid- and lower-elevation populations are consistent with the effects of drying due to anthropogenic climate change.

Wintering Artemisiospiza sparrows: patterns of segregation between Sagebrush Sparrow ( A. nevadensis ) and Mojave Bell's Sparrow ( A. belli canescens ) across Lower Colorado Desert vegetation assemblages, with evidence for differential migration in Mojave Bell's Sparrow

Wintering Artemisiospiza sparrows: patterns of segregation between Sagebrush Sparrow ( A. nevadensis ) and Mojave Bell's Sparrow ( A. belli canescens ) across Lower Colorado Desert vegetation assemblages, with evidence for differential migration in Mojave Bell's Sparrow

Review: The Settler Sea: California’s Salton Sea and the Consequences of Colonialism, by Traci Brynne Voyles

In Traci Brynne Voyles’s new work on the largest body of water in California, paradoxes abound. The Salton Sea, she shows in this profound and compelling book, is a difficult entity to define and narrativize. Is it a man-made entity or part of a centuries-old ecological rhythm? An environmental disaster, or the heart of a thrumming ecosystem? A consequence of settler colonialism’s disastrous designs, or evidence of its pervasive failures? In The Settler Sea, the answer to all of the above, and more, is both.The Settler Sea builds on the familiar narrative of the Salton Sea, a body of water created in 1905 in the middle of the desert when rain-swollen Colorado River water jumped a canal headgate and flowed unimpeded into the below-sea-level basin for two years. This resulted in a highly saline lake that has served as an irrigation runoff catch, a short-lived resort and boating hub, an avian nesting ground along the Pacific flyway, a bomb testing site, a center of massive bird and fish die-offs, and a cause of toxic pollution.Voyles adds to the already well-documented history of the Sea by combining approaches from settler colonial studies and environmental history. This method is particularly potent not only because the Salton Sea is a shared epicenter of white settlers’ totalizing dreams (familiar to all students of settler colonialism) but also because of the manifold unintended consequences of their environmental tinkering (a common theme among environmental historians). Following from these fields, The Settler Sea largely focuses its analysis on the relationship between social inequality and environmental problems. “The functions of social power built into settler colonial processes—racism, sexism, classism, heteropatriarchy, ableism,” Voyles argues, “are themselves environmentally ruinous” (p. 7). In showing how social and environmental issues intertwine in the settler process, the narrative centers the experiences of desert Indigenous groups, especially the Cahuilla and Kumeyaay peoples. It is, however, no mere tale of destruction, but a complex weave of survival, paradox, and change within the wider settler project.Like floodwater carving its way through the sand of the Colorado Desert, Voyles’s narrative is anything but straightforward. The book’s three sections respectively deal with the Salton Sea’s creation, its rise to political, cultural, and economic prominence, and its role in various environmental disasters. But with titles ranging from Birds to Bodies to Bombs, its chapters meander and twist across time, space, and subject, all ultimately returning to the sea at its heart. While this method yields powerful results in tracking the wide scope of histories tied to the Salton Sea, it sometimes obscures the more basic flow of causality and narrative, especially for readers unfamiliar with the history of the region.The Settler Sea is, nonetheless, delightfully written. So often dismissed as a wasteland, the desert blooms under Voyles’s pen. In capturing the many stories of the Salton Sea’s precariousness—“the way it sits uncomfortably between worlds, existing always in the interstices of human and natural influences” (p. 267)—this book is an important new entry, useful to scholars of settler colonialism, environmental history, Indigenous studies, and the American West writ large.

Morphological and Molecular Identification of Culicoides (Diptera: Ceratopogonidae) Species of the Southern California Desert.

Culicoides Latreille (Diptera: Ceratopogonidae) biting midges are vectors of important animal pathogens including bluetongue virus (BTV) and epizootic hemorrhagic disease virus (EHDV). While some Culicoides species present in the southern California desert are implicated in the transmission of these viruses to ruminant animals, these species have not been extensively studied due in part to the challenge of identifying Culicoides to species and to the lack of published gene sequences for these species to support their molecular identification. In this study, Culicoides were captured using suction traps baited with either carbon dioxide or UV light from transitional habitat between the southern California peninsular mountain ranges and the Colorado desert of southeastern California. Captured midges were initially identified using traditional morphological methods, with species identification subsequently confirmed by sequence analysis of COI and 28S rDNA genes. Phylogenetic analyses support that some Culicoides subgenera are not monophyletic. Two recognized species (C. sitiens Wirth and Hubert and C. bakeri Vargas) shared the same COI and 28S sequences. An additional cryptic species may be present within C. sitiens. Two additional recognized species (C. cacticola Wirth and Hubert and C. torridus Wirth and Hubert) may be conspecific or cryptic to each other. In total, 19 Culicoides species (or species aggregate) were collected in this study, with genetic sequences published for the first time for 16 of them. Published genetic sequences will support future research on these species, including studies on the ecology and habits of their immature stages which are often tedious to identify using morphology.

Correction to: Agriculture induces isotopic shifts and niche contraction in Horned Larks (Eremophila alpestris) of the Colorado Desert

Correction to: Agriculture induces isotopic shifts and niche contraction in Horned Larks (Eremophila alpestris) of the Colorado Desert

Trichotorquatus gen. nov. - a new genus of soil cyanobacteria discovered from American drylands1.

Cyanobacteria are crucial ecosystem components in dryland soils. Advances in describing α-level taxonomy are needed to understand what drives their abundance and distribution. We describe Trichotorquatus gen. nov. (Oculatellaceae, Synechococcales, Cyanobacteria) based on four new species isolated from dryland soils including the coastal sage scrub near San Diego, California (USA), the Mojave and Colorado Deserts with sites at Joshua Tree National Park and Mojave National Preserve, California (USA), and the Atacama Desert (Chile). The genus is morphologically characterized by having thin trichomes (<4.5 μm wide), cells both shorter and longer than wide, rarely occurring single and double false branching, necridia appearing singly or in rows, and sheaths with a distinctive collar-like fraying and widening mid-filament, the feature for which the genus is named. The genus is morphologically nearly identical with Leptolyngbya sensu stricto but is phylogenetically quite distant from that genus. It is consequently a cryptic genus that will likely be differentiated in future studies based on 16S rRNA sequence data. The type species, T. maritimus sp. nov. is morphologically distinct from the other three species, T. coquimbo sp. nov., T. andrei sp. nov. and T. ladouxae sp. nov. However, these latter three species are morphologically very close and are considered by the authors to be cryptic species. All species are separated phylogenetically based on sequence of the 16S-23S ITS region. Three distinct ribosomal operons were recovered from the genus, lending difficulty to recognizing further diversity in this morphologically cryptic genus.

Agriculture induces isotopic shifts and niche contraction in Horned Larks (Eremophila alpestris) of the Colorado Desert

Agriculture can dramatically alter community composition, food webs, and resource availability, yet the impacts of agriculture on many ecosystems and their constituents remain unstudied. Here, we examined the effect of agriculture on the isotopic ecology of Horned Larks (Eremophila alpestris) in the Colorado Desert. Over the last century, the Imperial Valley in southeastern California has transformed from desert scrub into agricultural land, while breeding Horned Larks have persisted in the area. We compared the isotopic composition of nitrogen (δ15N), carbon (δ13C), and hydrogen (δ2H) from feathers of lark specimens from two time periods in the Imperial Valley: a historical series collected between 1916 and 1923 and more recent vouchers collected between 1984 and 2014. We also quantified δ15N and δ13C values from plants and soil samples in contemporary agricultural land and adjacent desert scrub to estimate isotopic baseline changes associated with agricultural intensification. Contemporary larks had lower δ15N and δ13C ratios compared to historical larks, while plant and soil δ15N ratios were higher at agricultural sites compared to adjacent desert sites. These opposing trends suggest temporal change in isotope values that cannot be explained by shifting baselines alone. Furthermore, isotopic niche breadth (‰2; δ15N and δ13C) was lower among contemporary larks. Thus, contemporary larks in the Imperial Valley may capitalize on seeds and phytophagous insects that are abundant among crops, possibly representing an opportunistic change in diet over time in response to agriculture. Finally, δ2H values did not differ between historical and contemporary larks, but contemporary larks had δ2H values that differed from historical ground water, suggesting intake of water introduced by irrigational canals. Collectively, these findings highlight the power of stable isotope analyses combined with natural history collections to examine ecological change in the Anthropocene.

Evaluating Drought Impact on Postfire Recovery of Chaparral Across Southern California.

Chaparral shrubs in southern California may be vulnerable to frequent fire and severe drought. Drought may diminish postfire recovery or worsen impact of short-interval fires. Field-based studies have not shown the extent and magnitude of drought effects on recovery, which may vary among chaparral types and climatic zones. We tracked regional patterns of shrub cover based on June-solstice Landsat Normalized Difference Vegetation Index series, compared between the periods 1984-1989 and 2014-2018. High spatial resolution ortho-imagery was used to map shrub cover in distributed sample plots, to empirically constrain the Landsat-based estimates of mature-stage lateral canopy recovery. We evaluated precipitation, climatic water deficit (CWD), and Palmer Drought Severity Index in summer and wet seasons preceding and following fire, as regional predictors of recovery in 982 locations between the Pacific Coast and inland deserts. Wet-season CWD was the strongest drought-metric predictor of recovery, contributing 34-43 % of explanatory power in multivariate regressions (R 2 =0.16-0.42). Limited recovery linked to drought was most prevalent in transmontane chamise chaparral; impacts were minor in montane areas, and in mixed and montane chaparral types. Elevation was correlated negatively to recovery of transmontane chamise; this may imply acute drought sensitivity in resprouts which predominate seedlings at higher elevations. Landsat Visible Atmospherically Resistant Index (sensitive to live-fuel moisture) was evaluated as a landscape-scale predictor of recovery and explained the greatest amount of variance in a multivariate regression (R 2 = 0.53). We find that drought severity was more closely related to recovery differences among twice-burned sites than was fire-return interval. Summarily, drought has a major role in long-term shrub cover reduction within xeric chaparral ecotones bounding the Mojave Desert and Colorado Desert, likely in tandem with other global change stressors.

Using movement to inform conservation corridor design for Mojave desert tortoise

BackgroundPreserving corridors for movement and gene flow among populations can assist in the recovery of threatened and endangered species. As human activity continues to fragment habitats, characterizing natural corridors is important in establishing and maintaining connectivity corridors within the anthropogenic development matrix. The Mojave desert tortoise (Gopherus agassizii) is a threatened species occupying a variety of habitats in the Mojave and Colorado Deserts. Desert tortoises have been referred to as corridor-dwellers, and understanding how they move within suitable habitat can be crucial to defining corridors that will sustain sufficient gene flow to maintain connections among populations amidst the increases in human development.MethodsTo elucidate how tortoises traverse available habitat and interact with potentially inhospitable terrain and human infrastructure, we used GPS dataloggers to document fine-scale movement of individuals and estimate home ranges at ten study sites along the California/Nevada border. Our sites encompass a variety of habitats, including mountain passes that serve as important natural corridors connecting neighboring valleys, and are impacted by a variety of linear anthropogenic features. We used path selection functions to quantify tortoise movements and develop resistance surfaces based on landscape characteristics including natural features, anthropogenic alterations, and estimated home ranges with autocorrelated kernel density methods. Using the best supported path selection models and estimated home ranges, we determined characteristics of known natural corridors and compared them to mitigation corridors (remnant habitat patches) that have been integrated into land management decisions in the Ivanpah Valley.ResultsTortoises avoided areas of high slope and low perennial vegetation cover, avoided moving near low-density roads, and traveled along linear barriers (fences and flood control berms).ConclusionsWe found that mitigation corridors designated between solar facilities should be wide enough to retain home ranges and maintain function. Differences in home range size and movement resistance between our two natural mountain pass corridors align with differences in genetic connectivity, suggesting that not all natural corridors provide the same functionality. Furthermore, creation of mitigation corridors with fences may have unintended consequences and may function differently than natural corridors. Understanding characteristics of corridors with different functionality will help future managers ensure that connectivity is maintained among Mojave desert tortoise populations.

Refining genetic boundaries for Agassiz’s desert tortoise (Gopherus agassizii) in the western Sonoran Desert: the influence of the Coachella Valley on gene flow among populations in southern California

Understanding the influence of geographic features on the evolutionary history and population structure of a species can assist wildlife managers in delimiting genetic units (GUs) for conservation and management. Landscape features including mountains, low elevation depressions, and even roads can influence connectivity and gene flow among Agassiz’s desert tortoise (Gopherus agassizii) populations. Substantial changes in the landscape of the American Southwest occurred during the last six million years (including the formation of the Gulf of California and the lower Colorado River), which shaped the distribution and genetic structuring of tortoise populations. The area northwest of the Gulf of California is occupied by the Salton Trough, including the Coachella Valley at its northern end. Much of this area is below sea level and unsuitable as tortoise habitat, thus forming a potential barrier for gene flow. We assessed genetic relationships among three tortoise populations separated by the Coachella Valley. Two adjacent populations were on the east side of the valley in the foothills of the Cottonwood and Orocopia mountains separated by Interstate 10. The third population, Mesa, was located about 87 km away in the foothills of the San Bernardino Mountains at the far northwestern tip of the valley. The Cottonwood and Orocopia localities showed genetic affiliation with the adjacent Colorado Desert GU immediately to the east, and the Mesa population exhibited affiliation with both the Southern Mojave and Colorado Desert GUs, despite having a greater geographic distance (0.5x–1.5x greater) to the Colorado Desert GU. The genetic affiliation with the Colorado Desert GU suggests that the boundary for that GU needs to be substantially extended to the west to include the desert tortoise populations around the Coachella Valley. Their inclusion in the Colorado Desert GU may benefit these often overlooked populations when recovery actions are considered.

Responding to increased aridity: Evidence for range shifts in lizards across a 50-year time span in Joshua Tree National Park

North American deserts are warming and becoming more arid at rates that exceed global averages. Desert lizard populations are typically dependent on plant and arthropod food resources catalyzed by variations in aridity. Shifts in those lizard distributions coincident with increasing aridity therefore present an opportunity to document responses to modern climate change as it is happening. Here, we analyzed observation frequencies across an elevation gradient for seven lizard species, comparing a historical dataset (1958–1972) to recent observations (2014–2018) within Joshua Tree National Park, a landscape straddling the Mojave and Colorado Deserts in southern California. Phrynosoma platyrhinos, Aspidoscelis tigris, and Callisaurus draconoides, now appear to occupy new, higher elevation habitats compared to the historical baseline. Except for Sceloporus occidentalis, observation records for each species indicated shifts toward the upper elevations of their ranges. Observations for Uta stansburiana, C. draconoides and A. tigris each shifted their mean elevation upslope ≥10%. Phrynosoma platyrhinos and A. tigris shifted their maximum elevations by ≥15%. To assess whether those elevation shifts could then be corroborated with independent data, we compared the historic-recent observation data with plot-based surveys where we measured lizard densities varying with elevation and rainfall levels from 2014 to 2019. Those historical-recent shifts paralleled changes in lizard densities over the same elevation gradient, with statistical support for the upslope shifts identified for U. stansburiana, A. tigris and, S. magister. Additionally, despite increases in warming and aridity, we found species-specific elevations where local conditions are supporting higher population densities and that may represent climate refugia.

Impact of the Temperature on the Phenology of Diaphorina citri (Hemiptera: Liviidae) and on the Establishment of Tamarixia radiata (Hymenoptera: Eulophidae) in Urban Areas in the Lower Colorado Desert in Arizona.

The invasive pest Diaphorina citri Kuwayama was first detected in Arizona in 2009. Since late 2013, the parasitoid Tamarixia radiata (Waterson), the main biocontrol agent of D. citri, has been released as part of a biological control program on citrus grown in urban areas of two western Arizona counties in the lower Colorado desert environment. Here we report a 3-yr survey aimed at evaluation of T. radiata releases on D. citri populations and assess the impact of the climate conditions on the phenology of D. citri and on the establishment success of T. radiata. We also monitored the phenology of D. citri as part of this assessment on different citrus host species. We show that the high summer temperatures in the Arizona desert halt the development of D. citri for about 3 mo every year which appears to have limited the establishment and impact of T. radiata. At survey sites distant from release areas the parasitism rates over the season ranged from 0 to 75% and on average peaked around 50% in 2016 but it was low or absent in 2015 and 2017, respectively. We discuss the consequences of this phenology of D. citri in the desert areas for the prospects of long-term establishment of T. radiata and the management of this key citrus pest.

The Status of Triorophus punctatus LeConte (Coleoptera: Tenebrionidae: Pimeliinae: Edrotini).

While working on a revision of Triorophus LeConte, 1851 we examined type material for the genus. One species, Triorophus punctatus LeConte, 1851 (Figs. 1a-1e), was described from a dead, partial specimen (Type 4475, Museum of Comparative Zoology, Harvard University) collected at "Vallecitas," California in October or November of 1850 (LeConte, 1851). The locality likely refers to Vallecito, a contemporary United States Army depot and stage station (Roberts, 1988) located on the western edge of the Colorado Desert in eastern San Diego County (32.9755°, -116.35°).

Multiyear monitoring of survival following mitigation-driven translocation of a long-lived threatened reptile.

Translocation is used by managers to mitigate the negative impacts of development on species. Moving individuals to a new location is challenging, and many translocation attempts have failed. Robust, posttranslocation monitoring is therefore important for evaluating effects of translocation on target species. We evaluated the efficacy of a translocation designed to mitigate the effects of a utility-scale solar energy project on the U.S. federally listed Mojave desert tortoise (Gopherus agassizii). The species is a long-lived reptile threatened by a variety of factors, including habitat loss due to renewable energy development in the Mojave Desert and portions of the Colorado Desert in southern California (southwestern United States). We translocated 58 individual tortoises away from the project's construction site and intensively monitored them over 5 years (2012-2017). We monitored these individuals and tortoises located in the translocation release area (resident tortoises; n = 112) and control tortoises (n = 149) in a nearby location. We used our tortoise encounter data and known-fate survival models to estimate annual and cumulative survival. Translocated tortoises in each of 2 size classes (120-160 mm, >160 mm) did not survive at lower rates than resident and control tortoises over the study period. For models with different sets of biotic and abiotic covariates, annual and cumulative estimates of survival were always >0.87 and >0.56, respectively. Larger tortoises tended to have higher survival, but translocated tortoises were not differentially affected by the covariates used to model variation in survival. Based on these findings, our translocation design and study protocols could inform other translocation projects for desert species. Our case study highlights the benefits of combining rigorous scientific monitoring with well-designed, mitigation-driven management actions to reduce the negative effects of development on species of conservation concern.

Microhabitat characteristics and impacts of invasive species for the triple-ribbed milkvetch (Astragalus tricarinatus [Fabaceae]) in the San Bernardino Mountains, California.

Astragalus tricarinatus A.Gray is a federally-listed short lived perennial herb endemic to southern California. Its primary habitat is along the ecotone of the Mojave and Colorado Deserts in the San Bernardino and Little San Bernardino Mountains between 390 and 1525 m. Discovery of this species' upland habitat has prompted research into the ecology of populations across its range, as the life history and threats may be different from those of the previously-known wash populations. Our study sought to further document A. tricarinatus distribution and abundance within its range, as well as quantify impacts to its growth and reproductive efforts associated with co-occurring native and invasive plant species. We mapped all known localities in our study area on aerial imagery then used this to visually recognize its typical edaphic habitat and identify new potential sites. Where A. tricarinatus was found, we established plots in which plant size, the number of reproductive structures, and microhabitat characteristics were recorded. Two new occurrences (defined here as >1 km away from known localities) were recorded within its existing range as well as a previously undocumented locality between two known occurrences containing an estimated 2600 plants. Most plants were found on south and east aspects, and on slopes ranging from 30–45°. We found that increases in invasive cover correlated with a decrease in production of reproductive structures. As a federally-listed species occurring in steep, remote habitats with few threats, these findings suggesting an association between invasive species presence and reduced reproductive structures should be further investigated experimentally, as they affect population dynamics and conservation of this species.

Controls on Land Surface Temperature in Deserts of Southern California Derived from MODIS Satellite Time Series Analysis, 2000 to 2018

The land surface temperature (LST) in arid regions is a primary controller of many ecological processes. Consequently, we have developed a framework for detection of LST change on a regional scale using data sets covering all deserts of southern California from the Moderate-Resolution Imaging Spectroradiometer (MODIS) satellite sensor. The Breaks for Additive Season and Trend (BFAST) methodology was applied to MODIS 1-km monthly LST data from the years 2000 to 2018 to estimate significant time series shifts (breakpoints) and gradual trends. Area-wide results showed five times more positive LST breakpoints (abrupt temperature warming events) than negative (surface cooling) breakpoints. Cross-correlations with high rainfall periods around Mojave dry lake playas, and comparison with timing of wildfire burns for breakpoint patterns, showed that abrupt shifts in LST had the strongest response to these controllers. We detected negative LST (abrupt cooling) breakpoints as consistently associated with the construction of new solar energy facilities. Over the majority of the study area, BFAST results showed warming LST trends between the years 2000 and 2018. The western-most margins of the study area showed consistent widespread warming trends, whereas the eastern portions of the Mojave and Lower Colorado Deserts showed a mix of positive and neutral LST trends. Long-term cooling LST trends were detected only in some of the largest dry lake formations in the Antelope Valley, Death Valley, and Bristol, Cadiz, and Danby playas.

Field Measurements and Satellite Remote Sensing of Daily Soil Surface Temperature Variations in the Lower Colorado Desert of California

The purpose of this study was to better understand the relationships between diurnal variations of air temperature measured hourly at the soil surface, compared with the thermal infra-red (TIR) emission properties of soil surfaces located in the Lower Colorado Desert of California, eastern Riverside County. Fifty air temperature loggers were deployed in January of 2017 on wooden stakes that were driven into the sandy or rocky desert soils at both Ford Dry Lake and the southern McCoy Mountains wash. The land surface temperature (LST) derived from Landsat satellite images was compared to measured air temperatures at 1 m and at the soil surface on 14 separate dates, until mid-September, 2017. Results showed that it is feasible to derive estimated temperatures at the soil surface from hourly air temperatures, recorded at 1 m above the surface (ambient). The study further correlated Landsat LST closely with site measurements of air and surface temperatures in these solar energy development zones of southern California, allowing inter-conversion with ground-based measurements for use in ecosystem change and animal population biology studies.