Hyper-arid Environment Research Articles

Unexpectedly low δ 13C in leaves, branches, stems and roots of three acacia species growing in hyper-arid environments

AbstractAimsIn plant eco-physiology, less negative (enriched) carbon 13 (13C) in the leaves indicates conditions of reducing leaf gas exchange through stomata, e.g. under drought. In addition, 13C is expected to be less negative in non-photosynthetic tissues as compared with leaves. However, these relationships in δ 13C from leaves (photosynthetic organs) to branches, stems and roots (non-photosynthetic organs) are rarely tested across multiple closely related tree species, multiple compartments, or in trees growing under extreme heat and drought.MethodsWe measured leaf-to-root 13C in three closely related desert acacia species (Acacia tortilis, A. raddiana and A. pachyceras). We measured δ 13C in leaf tissues from mature trees in southern Israel. In parallel, a 7-year irrigation experiment with 0.5, 1.0 or 4.0 L day−1 was conducted in an experimental orchard. At the end of the experiment, growth parameters and δ 13C were measured in leaves, branches, stems and roots.Important FindingsThe δ 13C in leaf tissues sampled from mature trees was ca. −27‰, far more depleted than expected from a desert tree growing in one of the Earth’s driest and hottest environments. Across acacia species and compartments, δ 13C was not enriched at all irrigation levels (−28‰ to ca. −27‰), confirming our measurements in the mature trees. Among compartments, leaf δ 13C was unexpectedly similar to branch and root δ 13C, and surprisingly, even less negative than stem δ 13C. The highly depleted leaf δ 13C suggests that these trees have high stomatal gas exchange, despite growing in extremely dry habitats. The lack of δ 13C enrichment in non-photosynthetic tissues might be related to the seasonal coupling of growth of leaves and heterotrophic tissues.

On the analysis of a summertime convective event in a hyperarid environment

AbstractA summertime convective event that developed on 5 September 2017 over the United Arab Emirates (UAE) is investigated in this study. Atmospheric profiles from a ground‐based microwave radiometer along with satellite observations and in situ data from three weather stations in the UAE were used. The event was simulated using the Weather Research and Forecasting (WRF) model, forced with four input datasets: Global Forecast System (GFS), Climate Forecast System Reanalysis (CFSR), and the European Centre for Medium‐Range Weather Forecasts ERA‐5 and ERA‐Interim reanalyses. The afternoon and evening convection was triggered by the intrusion of a mid‐level trough from midlatitudes and was favoured by the convergence over land of thermally forced maritime air masses. Near‐surface observations at a weather station revealed a 7 °C drop in air temperature, a doubling of the wind speed to 9 m·s−1 in 30 min, and a shift in wind direction from easterly, to southerly and then westerly in about 45 min, associated with the passage of the cold pool emanating from a Mesoscale Convective System (MCS). At the location where the microwave radiometer was deployed, the pre‐squall low and wake low signatures are also captured, with a 5 m·s−1 increase in the wind speed in just 5 min. The observed features of the studied MCS were found to compare with those reported for MCSs in the Tropics. The four experiments gave a similar performance, although the GFS simulation generally generated higher skill scores. The investigated MCS event was not captured by WRF, which was attributed to a misrepresentation of soil moisture in the model. This study highlights the difficulties regional models like WRF may have in reproducing MCSs over arid/hyperarid regions, which may result in a misrepresentation of their impacts in climate projection studies.

Vegetation growth and landscape genetics of Tillandsia lomas at their dry limits in the Atacama Desert show fine-scale response to environmental parameters.

Ecosystem dry limits have been studied in the context of species biology, fitness, and interactions with biotic and abiotic parameters, but the interactive effects of these parameters remain underexplored. Therefore, information on the putative effects of global climate change on these ecosystems is often lacking.We analyzed the interplay between fine‐scale landscape genetics and biotic and abiotic factors of terrestrial Tillandsia lomas in the hyperarid Atacama Desert, characterized by a fog‐dependent vegetation type almost entirely dominated by one single vascular plant species.We showed that metapopulations of Tillandsia landbeckii are genetically connected over many hundreds of square kilometers, and despite having a large potential for clonal propagation, genetic diversity is regionally and locally structured. At the landscape level, genetic diversity correlates well with fitness parameters such as growth, flowering, and vegetation density. We also observed fine‐scale correlation with a 3‐D landscape model indicating a positive feedback with seasonal fog occurrence and availability. The various interactions of biotic and abiotic factors resulted in regular linear banding patterns of vegetation arranged orthogonally toward the landscape slope. Ex situ growth experiments indicated that T. landbeckii grows at optimal rates in this extreme hyperarid environment, and we can extrapolate mean biomass production for this ecosystem. Synthesis. Our results suggest that the unique ecosystem of terrestrial Tillandsia lomas in the hyperarid Atacama Desert is an evolutionarily balanced and fine‐scaled system. The vegetation itself is composed of long‐lived and persistent modules. We developed a descriptive model of the various interacting factors, thereby also highlighting the severe threat caused by global climate change potentially associated with fog disturbance patterns along the Chilean Pacific coast.

Revealing hidden plant diversity in arid environments

Estimating total plant diversity in extreme or hyperarid environments can be challenging, as adaptations to pronounced climate variability include evading prolonged stress periods through seeds or specialized underground organs. Short‐term surveys of these ecosystems are thus likely poor estimators of actual diversity. Here we develop a multimethod strategy to obtain a more complete understanding of plant diversity from a community in the Atacama Desert. We explicitly test environmental DNA‐based techniques (eDNA) to see if they can reveal the observed and ‘hidden' (dormant or locally rare) species.To estimate total plant diversity, we performed long‐term traditional surveys during eight consecutive years, including El Niño and La Niña events, we then analyzed eDNA from soil samples using high‐throughput sequencing. We further used soil pollen analysis and soil seed bank germination assays to identify ‘hidden' species. Each approach offers different subsets of current biodiversity at different taxonomic, spatial and temporal resolution, with a total of 92 taxa identified along the transect. Traditional field surveys identified 77 plant species over eight consecutive years. Observed community composition greatly varies interannually, with only 22 species seen every year. eDNA analysis revealed 37 taxa, eight of which were ‘hidden' in our field surveys. Soil samples contain a viable seed bank of 21 taxa. Soil pollen (27 taxa) and eDNA analysis show affinities with vegetation at the landscape scale but a weak relationship to local plot diversity.Multimethod approaches (including eDNA) in deserts are valuable tools that add to a comprehensive assessment of biodiversity in such extreme environments, where using a single method or observations over a few years is insufficient. Our results can also explain the resilience of Atacama plant communities as ‘hidden' taxa may have been active in the recent past or could even emerge in the future as accelerated global environmental change continues unabated.

Is salinity the main ecological factor that influences foliar nutrient resorption of desert plants in a hyper-arid environment?

BackgroundSoil salinity is a major abiotic constraint to plant growth and development in the arid and semi-arid regions of the world. However, the influence of soil salinity on the process of nutrient resorption is not well known. We measured the pools of both mature and senesced leaf nitrogen (N), phosphorus (P), potassium (K), and sodium (Na) of desert plants from two types of habitats with contrasting degrees of soil salinity in a hyper-arid environment of northwest China.ResultsN, P, K revealed strict resorption, whereas Na accumulated in senesced leaves. The resorption efficiencies of N, P, and K were positively correlated with each other but not with Na accumulation. The degree of leaf succulence drives both intra-and interspecific variation in leaf Na concentration rather than soil salinity. Both community- and species-level leaf nutrient resorption efficiencies (N, P, K) did not differ between the different habitats, suggesting that soil salinity played a weak role in influencing foliar nutrients resorption.ConclusionsOur results suggest that plants in hyper-arid saline environments exhibit strict salt ion regulation strategies to cope with drought and ion toxicity and meanwhile ensure the process of nutrient resorption is not affected by salinity.

Physiological responses of the xerohalophyte Suaeda vermiculata to salinity in its hyper-arid environment

Few plants can survive and grow equally well in salty and salt-free substrates (i.e., habitat-indifferent halophytes). Such plants provide a good opportunity to understand physiological and biochemical mechanisms underlying salinity tolerance. In this study, we investigated the environmental salinity impacts on several physiological and biochemical features of Suaeda vermiculata, a habitat-indifferent halophyte. Samples of different organs were collected from S. vermiculata from both a highly saline marsh habitat (HSMH) and non-salty gravel plain (NSGP) for the determination of the following physiological and bio-chemical features: chlorophyll and carotenoids, proline, malondialdehyde (MDA), and hydrogen peroxide (H2O2), antioxidant enzymes (Catalase, CAT; guaiacol peroxidase, GPX; Ascorbate peroxidase, APX) activities. Elemental compositions in soil and plant samples from both habitats were also assessed. Results showed that plants from HSMH had significantly lower values of chlorophyll a, b, carotenoids and leaf biomass, compared to those from NSGP. Roots from HSMH attained higher levels of antioxidant enzymes (CAT, GPX, APX) and lower values of reactive oxygen species (MDA and H2O2), indicating that the enzymes are more likely scavenging the reactive oxygen species (ROS). The enzyme activities and ROS levels were much lower in the shoots of both HSMH and NSGP than in roots. Accumulation of sodium was higher in leaves and shoots than roots of S. vermiculata. This study indicates that Suaeda vermiculata is a salt tolerant plant with adaptations to different environments through down-regulation of different biochemical and physiological features to avoid oxidative stress.

Towards agro-environmentally sustainable irrigation with treated produced water in hyper-arid environments

Produced water (PW) is the main waste stream generated from oil and gas extraction. Nowadays, half of the global PW volume is managed through environmentally controversial and expensive disposal practices, such as re-injection through deep wells. In dry areas such as in the Arabian Peninsula, PW could be reused to irrigate crops, creating environmental, economic and social value. However, the quality of most PWs remains challenging as their high salinity, sodicity and alkalinity can degrade soil fertility and crop yield. Mitigating these negative impacts is costly as it requires specific PW treatment and irrigation management. Thus, the environmental sustainability and the cost of irrigation with PW are uncertain. The aims of this paper was to assess the agro-environmental sustainability of irrigating crops with PW in hot and hyper-arid climate and to estimate the operating cost of reusing PW for irrigation in order to compare this PW management approach to PW disposal in terms of environmental impacts and financial cost. To this end, a soil-water model was used to simulate irrigation of jojoba (Simmondsia chinensis) with oilfield-PW. Different irrigation strategies combining over-irrigation, PW blending and desalination were tested to preserve the soil structural stability and the crop yield. The operational costs of identified sustainable scenarios were estimated using a cost analysis. In this case study, the simulations showed that using an irrigation volume up to ∼390 % of the crop water needs with a blend composed of raw and desalinated PW in a 2:1 ratio could preserve the soil structural stability and the crop yield. However, for irrigation, the least-cost option was to mix raw and desalinated PW in a 1:4 ratio and to reduce the irrigation amount to just meet the crop water needs. Although the cost of managing PW in irrigation remains up to 2.5 times higher than PW disposal, this practice might be competitive considering the crop value generated and the increasing need for sustainable alternatives to PW disposal.

Unraveling biogeochemical phosphorus dynamics in hyperarid Mars‐analogue soils using stable oxygen isotopes in phosphate

With annual precipitation less than 20mm and extreme UV intensity, the Atacama Desert in northern Chile has long been utilized as an analogue for recent Mars. In these hyperarid environments, water and biomass are extremely limited, and thus, it becomes difficult to generate a full picture of biogeochemical phosphate-water dynamics. To address this problem, we sampled soils from five Atacama study sites and conducted three main analyses-stable oxygen isotopes in phosphate, enzyme pathway predictions, and cell culture experiments. We found that high sedimentation rates decrease the relative size of the organic phosphorus pool, which appears to hinder extremophiles. Phosphoenzyme and pathway prediction analyses imply that inorganic pyrophosphatase is the most likely catalytic agent to cycle P in these environments, and this process will rapidly overtake other P utilization strategies. In these soils, the biogenic δ18 O signatures of the soil phosphate (δ18 OPO4 ) can slowly overprint lithogenic δ18 OPO4 values over a timescale of tens to hundreds of millions of years when annual precipitation is more than 10mm. The δ18 OPO4 of calcium-bound phosphate minerals seems to preserve the δ18 O signature of the water used for biogeochemical P cycling, pointing toward sporadic rainfall and gypsum hydration water as key moisture sources. Where precipitation is less than 2mm, biological cycling is restricted and bedrock δ18 OPO4 values are preserved. This study demonstrates the utility of δ18 OPO4 values as indicative of biogeochemical cycling and hydrodynamics in an extremely dry Mars-analogue environment.

Burrowing crabs in arid mangrove forests on the southwestern Arabian Gulf: Ecological and biogeographical considerations

Many species of mangrove crabs are essential in the formation of mangrove ecosystems, and the bioturbation resulting from the excavation of their underground chambers is directly correlated with substrate oxygenation and carbon and nitrogen cycling within the ecosystem. This study examines the ecological and biogeographical characteristics of bioengineering crab assemblages that inhabit the environmentally challenging arid mangroves in the southwestern Persian-Arabian Gulf. Results demonstrated that a hyper-arid environment could directly impact crab diversity, reducing it to only one species per niche or coastal zone. A biogeographic comparison revealed an exclusive divergence of species composition in each Gulf region. Variations in seasonal temperatures were found to have a direct influence on the abundance of species and how the zonation is driven by sediment composition and trophic relations. Two crabs – Nasima dotilliformis in the salt march (supratidal zone) and Macrophthalmus (Mareotis) depressus in the intertidal – were considered to be the most influential functional species for this hyper-arid ecosystem due to their abundance, size, bioturbation potential and niche specificity for the intertidal zone. The limited number of species within the arid mangrove system emphasised the biogeographic pressure and inherent importance of the few active bioengineering species present and adapted to the harsh conditions, indicating that the dominant crabs within the biotope are directly related to the successful evolutionary establishment of these hyperarid mangroves. The study highlights important implications for future forest management strategies of the only forest ecosystem that tolerates the extremes of the southwestern Persian-Arabian Gulf area.

On the performance of microlysimeters to measure non-rainfall water input in a hyper-arid environment with focus on fog contribution

The measurement of non-rainfall atmospheric water input (NRWI) in arid environments requires instruments that are capable to detect even smallest amounts of total daily water input of less than 0.1 mm. Microlysimeters yield robust and high precision data of such low NRWI. We provide a technical description of a self-constructed microlysimeter and demonstrate its excellent performance regarding the analysis of NRWI in the Central Namib Desert. Three stations of the FogNet measurement network have been equipped with microlysimeters in order to measure fog deposition. NRWI and evaporation for days/nights without fog shows a persistent diurnal course. Deviations from this baseline define the amount of fog deposition, intensity and duration of a fog events. A more detailed analysis of a five-day period reveals the complex nature and variation between individual fog events with respect to the different patterns of fog deposition and fog precipitation and the contribution of adsorption, dew and fog to NRWI. The relation between fog precipitation and fog deposition is not straightforward and a simple parameterization of the processes that quantifies the amount of the water sampled by fog collectors and its connection to NRWI is still lacking.

How does an alien amphibian expand its distribution in an hyper-arid environment?: African common toad (Sclerophrys regularis) in Qatar

Qatar is one of the few countries where there is no permanent natural surface freshwater source, and yet toads are known to occur, recently identified as the African common toad, Sclerophrys regularis. However, little information is available concerning the geographical distribution of toads in Qatar, as well as how they have expanded their distribution. We conducted the first-ever nationwide field survey of toads in Qatar. Toads were present in sites where water was supplied from outside sources by irrigation pipes and/or water transportation vehicles statistically significantly more frequently than in sites where water was provided exclusively from inside sources (wells). The distribution pattern of toads in Qatar may suggest that toads were initially introduced to eastern parts of the country, and have dispersed from there through water transportation networks.

Survival of the Qaidam mega-lake system under mid-Pliocene climates and its restoration under future climates

Abstract. The Qaidam Basin in the north of the Tibetan Plateau has undergone drastic environmental changes during the last millions of years. During the Pliocene, the Qaidam Basin contained a freshwater mega-lake system although the surrounding regions showed increasingly arid climates. With the onset of the Pleistocene glaciations, lakes began to shrink and finally disappeared almost completely. Today, hyperarid climate conditions prevail in the low-altitude parts of the Qaidam Basin. The question of how the mega-lake system was able to withstand the regional trend of aridification for millions of years has remained enigmatic so far. This study reveals that the mean annual water balance, i.e. the mean annual change in terrestrial water storage in the Qaidam Basin, is nearly zero under present climate conditions due to positive values of net precipitation in the high mountain ranges and shows positive annual values during warmer, less dry years. This finding provides a physically based explanation for how mid-Pliocene climates could have sustained the mega-lake system and that near-future climates not much different from present conditions could cause water storage in reservoirs, raising lake levels and expanding lake areas, and may even result in restoration of the mega-lake system over geological timescales. The study reveals that a region discussed as being an analogue to Mars due to its hyperarid environments is at a threshold under present climate conditions and may switch from negative values of long-term mean annual water balance that have prevailed during the last 2.6 million years to positive ones in the near future.

Emerging and Historical Contaminants Detected in Desert Rodents Collected Near a Low-Level Radioactive Waste Site.

In an effort to determine contaminant presence, concentrations, and movement from a low-level radioactive waste (LLRW) burial disposal site to ecosystems in the surrounding area, a study was developed to assess concentrations of per- and polyfluoroalkyl substances (PFAS), polychlorinated biphenyls (PCBs), and tritium. To complete this assessment small mammals, vegetation, soil, and insect samples were collected from areas within and adjacent to the Beatty, Nevada, LLRW site and from a reference area located approximately 3 km south of the LLRW site. Samples underwent analysis via liquid chromatography tandem mass spectrometry, gas chromatography mass spectrometry, or scintillation spectroscopy depending on the analyte of interest. Small mammal tissues showed maximum concentrations of over 1700 ng/g for PFAS, 1600 ng/g for PCBs, and 10 000 Bq/kg for tritium. The primary contaminants found in soil samples were PCBs, with maximum concentrations exceeding 25 ng/g. Trace amounts of PFAS were also detected in soils and insects. Only qualitative data were obtained from vegetation samples because of the complex matrix of the dominant plant species (creosote bush; Larrea tridentata [Sessé & Moc. ex DC.] Coville). Overall, these data indicate the presence of various anthropogenic contaminants in the ecosystem surrounding the LLRW area, but additional analyses are necessary to confirm the sources and migration pathways of PFAS and PCBs in this hyperarid environment. Environ Toxicol Chem 2021;40:727-734. © 2020 SETAC.

Early-to-mid Miocene erosion rates inferred from pre-Dead Sea rift Hazeva River fluvial chert pebbles using cosmogenic <sup>21</sup>Ne

Abstract. In this work, we utilize a novel application of cosmogenic 21Ne measurements in chert to compare exposure times measured in eroding surfaces in the central Jordanian Plateau with exposure times from chert pebbles transported by the Miocene Hazeva River. The Miocene Hazeva River was a large fluvial system (estimated catchment size > 100 000 km2) that drained the Arabian Plateau and Sinai Peninsula into the Mediterranean Sea during the early-to-mid Miocene. It was established after the rifting of the Red Sea uplifted the Arabian Plateau during the Oligocene. Following late-Miocene-to-early-Pliocene subsidence along the Dead Sea rift, the Hazeva drainage system was abandoned and dissected, resulting in new drainage divides on either side of the rift. We find modern erosion rates derived from cosmogenic 21Ne, 26Al, and 10Be in exposed in situ chert nodules to be extremely slow (between 2–4 mm kyr−1). Comparison between modern and paleo-erosion rates, measured in chert pebbles, is not straightforward, as cosmogenic 21Ne was acquired partly during bedrock erosion and partly during transport of these pebbles in the Hazeva River. However, 21Ne exposure times calculated in Miocene cherts are generally shorter (ranging between 0-0+59 and 242±113 kyr) compared to exposure times calculated in the currently eroding chert nodules presented here (269±49 and 378±76 kyr) and other chert surfaces currently eroding in hyperarid environments. Miocene exposure times are shorter even when considering that they account for bedrock erosion in addition to maintained transport along this large river. Shorter exposure times in Miocene cherts correspond to faster paleo-erosion rates, which we attribute to a combination of continuous surface uplift and significantly wetter climatic conditions during the early-to-mid Miocene.

On the Analysis of the Performance of WRF and NICAM in a Hyperarid Environment

Abstract The Weather Research and Forecasting (WRF) Model and the Nonhydrostatic Icosahedral Atmospheric Model (NICAM) are forced with the Global Forecast System (GFS) data and run over the United Arab Emirates (UAE) for two 4-day periods: one in the cold season (16–18 December 2017) and another in the warm season (13–15 April 2018). The models’ performance is evaluated against four observational datasets: weather station observations, eddy-covariance flux measurements at Al Ain, microwave radiometer–derived temperature profile, and twice-daily radiosonde measurements at Abu Dhabi. An overestimation of the daily mean air temperature by 1°–3°C is noticed for both models and periods. This warm bias is attributed to the reduced cloud cover and resulting increased surface downward shortwave radiation flux. A comparison with the eddy-covariance data suggested that both models also underestimate the observed albedo. However, when the models predict heavier amounts of precipitation, they tend to be colder than observations, typically by 2°–3°C. NICAM and WRF overpredict the strength of the near-surface wind speed at all weather stations by roughly 1–3 m s−1, which has been attributed to a poor representation of its subgrid-scale fluctuations and surface drag parameterization. WRF tends to be wetter and NICAM drier than the station observations, possibly because of differences in the cloud microphysics schemes. While the performance of both models for the near-surface fields is comparable, NICAM outperforms WRF in the simulation of vertical profiles of temperature, relative humidity, and wind speed, being able to partially correct some of the biases in the GFS data.

Assessing Subwavelength VHF Radar Scattering Losses in Hyperarid Carbonate Formations

Subsurface scattering losses associated with very high frequency (VHF) subwavelength heterogeneities are crucial for assessing the total radar signal attenuation in fractured and heterogeneous environments such as carbonate formations, which represent one of the most predominant aquifer types in hyperarid areas. Yet, the resulting signal losses are largely unquantified due to the difficulties in constraining their sources, amplitude, and frequency dependence on the VHF spectrum, hence, compromising the ability to perform large-scale characterization of shallow aquifers using radar probing in these areas. To address this deficiency, we present an experimental model quantifying volumetric scattering losses in heterogeneous carbonate formations accounting for wavelength-sized structural elements and subwavelength-sized heterogeneities ranging from 1 mm to 2 cm for the VHF radar signals. In particular, we use an analytical model that estimates the overall scattering effects produced by the interaction of an electromagnetic plane wave with randomly distributed vugs scatterers. We then compare our analysis with field radar measurements collected in the karst limestone aquiferous formation in the Qatar Peninsula at 80 MHz (10-dB bandwidth (BW): 30-150 MHz). Our results estimate that the total losses in karstic environments range between ~0.6 and 1.4 dB/m, of which 20%-65% is due to volume scattering. Furthermore, we find that despite being usually underestimated, subwavelength volumetric scattering accounts for a considerable portion (~40%) of the overall radar attenuation: between ~0.045 and 0.35 dB/m. The results of our analysis can constrain the degree of karstification associated with vertical artesian movements in fossil aquifer systems in hyperarid environments.

Vegetation traits are accurate indicators of how do plants beat the heat in drylands: Diversity and functional traits of vegetation associated with water towers in the Sahara Desert

Abstract Water is the key determinant factor of ecosystem balance, especially in hot desert regions facing multiple climatic and edaphic constraints. In the Sahara Desert, vegetation associated to man-made mesic environments such as open-air irrigation channels and oases is little studied. In order to understand the vegetation–water relationship in xeric environments, the present study focused on estimating diversity and determining functional traits of plant communities established around water points developed near water towers in the region of Adrar (Sahara Desert of Algeria). Based on data collected from 31 phytoecological releves, spontaneous vegetation was analyzed using diversity parameters (viz. species richness, diversity, richness estimation and similarity), and plant functional traits ‘PFT’ (viz. life forms, morphological, phytogeographical and dispersal types, Grime’s and Noy-Meir’s strategies). A total of 4695 individuals belonging to 53 plant species and 23 families were collected. Rarefaction and extrapolation curves applied for all the surveys carried out indicated that the estimated species richness is projected to be 77.2 ± 10 species from 31 survey plots and it is predicted to reach up to 116 ± 30 species from 200 survey plots, which represents an increase of 119% of the observed species richness. According to the real spectrum (abundance-based data) of PFT, plant species are mainly hemicryptophytes (54.12%) and arid-actives (69.97%) that adopt various dispersal strategies including anemochores (57.68%) and barochores (36.68%). Grime’s strategies showed the predominance of competitive and stress-tolerant species (52.76%). The phytogeographical analysis revealed the dominance in the strict sense of the Tropical-Mediterranean-Saharo-Arabian element (36.59%) which adapts well to the Saharan climate. This study showed the role of water in shaping the functional characteristics of vegetation in hyper-arid regions. The functional diversity of Saharan vegetation reflects species strategies for a better exploitation of the water and guaranteeing their coexistence in hyper-arid environments.

A phenology-based approach to the analysis of conifers intra-annual xylem anatomy in water-limited environments

Xylem structure and cambial phenology (i.e. onset and cessation of cambial cell division) of conifers growing under severe water-limitations can change dramatically in relation to moisture availability. In hyperarid environments, analytical tools commonly used to investigate intra-annual variability of xylem anatomy (i.e. tracheidograms), may fail to capture the complexity of tree phenological responses to environmental conditions. This greatly limits our ability to accurately date the onset of intra-annual density variations, including the transition between earlywood and latewood. I present a new approach for developing phenological tracheidograms (“pheno-tracheidograms”) calibrated to account for the seasonal variations in cell division rates. Pheno-tracheidograms were developed for a population of Pinus ponderosa in the Mojave Desert (Nevada, USA) during the period 2015–2016 in order 1) to determine the onset date of latewood formation and 2) to investigate relationships between environmental conditions, lumen diameter, and cell wall thickness targeting specific climatic windows for each tracheid. Pheno-tracheidograms were standardized at the tree-level, showing more flexibility compared to tracheidograms standardized according to a pre-determined number of cells. By displaying cellular parameters with respect to the date of formation of the tracheid to which each measurement is associated, pheno-tracheidograms allowed to determine the beginning of latewood formation with daily resolution. Lumen diameter was significantly correlated with the onset date of cellular enlargement, while cell wall thickness showed a weaker relationship with the beginning of secondary wall deposition. Soil moisture positively affected the duration of cell enlargement and tracheid lumen diameter, particularly in the earlywood, while cell wall thickness was not significantly influenced by environmental conditions. Pheno-tracheidograms represent an empirical, yet effective way to date intra-annual xylem structures and to investigate high-resolution climate-anatomy relationships in conifer species from arid environments characterized by high phenological plasticity.

Further Analysis of the Development of Vadose Water Profiles Over Deep Aquifers With Minimal Recharge

AbstractUnderstanding transport water from deep aquifers to the atmosphere in hyperarid environments is needed to evaluate potential waste storage facilities and reveals fundamental features of evolving scales of hydrologic processes on Earth. We seek a physical framework for the process and scales at which water and salt movement take place where there is no net recharge over long periods of time. We consider this extreme case as an end‐member of the driest possible situation, with homogeneous media. The analysis of Ross (1984, https://doi.org/10.1029/WR020i011p01627) and Selker (2017, https://doi.org/10.1002/2017WR021014) is expanded upon to provide a simple algebraic expression, derived from the governing physical processes, predicting the fluxes and the vertical extent of such systems. In much of such an unsaturated zone a counterflux region of downward liquid flow counter to upward vapor flow results from the geothermal gradient. Net upward transport is about 0.01 mm/year from the water table, which is augmented by the depletion of the residual water in the drained soil, resulting in surface fluxes of 1.0, 0.1, and 0.02 mm/year after 100 year, 10 kyr, and 150 kyr. Ten to thirty million years without recharge would be required for soil to dry to depths of 200 m. It is found that the influence of capillary flow to the upward transport of water is only influential within less than 5 m of the water table and so is insignificant to the overall flux but does imply a convergence line where vapor loss is predicted to leave behind salts delivered from above and below to the top of the capillary zone.

On the analysis of ground-based microwave radiometer data during fog conditions

Abstract The goal of this study is twofold. First, we verify the retrievals of temperature and humidity profiles from a ground-based microwave radiometer (MWR) using radiosonde soundings. Second, we assess the potential of the MWR to nowcast fog formation and dissipation in a hyper-arid environment through the analysis of its profiles. The MWR is installed and operated at Masdar Institute in Abu Dhabi, United Arab Emirates (UAE), adjacent to Abu Dhabi's International Airport. MWR observations collected from 18 February 2017 to 31 March 2019. A comparison with airport radiosonde data reveals a good agreement in particular for temperature, with the two measurements generally within 1 K from the surface to 10 km. For the specific humidity, the biases can reach 5 g kg−1 in the PBL. Biases can be attributed to errors in the radiosonde measurements and a non-optimized retrieval algorithm for the MWR. A total of 14 fog events are identified during the study period. A strong near-surface inversion with lapse rates in the lowest 250 m of up to 30 K km−1 is needed for fog to form. Subsidence warming and drying associated with the subtropical anticyclone can descend down to 500 m, aiding the formation of fog at night. Cloud base height retrievals from the infrared (IR) camera capture fog onset and dissipation and complement the MWR observations. A synergy of different instruments could allow for more accurate fog detection.