Hallamish Dune Field Research Articles

Effects of subsurface flow on dunefields: Doubling the amount of soil moisture and substantially increasing the biomass of annual plants

Plants in deserts exhibit heterogeneous distribution, which may stem from water redistribution following runoff. However, heterogeneous plant distribution is also common in dunefields regardless of runoff. In the Hallamish dune field in the hyper-arid Negev Desert, dense vegetation belts are noted in loci where an abrupt change in slope angles of uncrusted slopes (interface, IF) occurs, attributed to subsurface flow (SSF). While SSF was previously noted in dunefields, quantification of SSF is problematic, especially in arid regions where precipitation is low, evaporation is high, and the amount of SSF is not sufficiently large to facilitate direct measurement (in water volume). Here indirect measurements of SSF are reported based on near weekly measurements of available water content (AWC) during 2012/13, 2013/14 and 2015/16 at five slope positions along two dune catenas and the adjacent interdunes: flat semi-stable top (TOPs), flat crusted interdune (IDc), semi-stable (NFs) and crusted (NFc) north-facing slope, and IF. In addition, the hydrological rain and monthly surface stability were also measured, and additionally plant characteristics (cover, number of species, density, and biomass) were used as indicators for AWC. Higher AWC characterized the uncrusted slope positions (especially at IF) in comparison with crusted slope positions of similar slope angles. Although receiving the lowest amount of rain, AWC at IF was ∼ 2-fold higher than at most other slope positions, resulting in ∼ 3- to 5-fold higher cover and biomass, serving therefore as 'islands of fertility'. As verified by the Generalized Linear Regression Model, plant performance was mainly dictated by AWC and to a lesser degree by surface stability. Our findings may have broader implications for water redistribution in other dunefields, where a unique distribution of water has at times been attributed to factors such as the effect of biocrusts on infiltration rather than to SSF.

Biocrust research: A critical view on eight common hydrological‐related paradigms and dubious theses

AbstractAlthough being relatively young, research on biocrusts has already yielded numerous paradigms and/or dubious theses. The very existence of these “truths” may lead to erroneous conclusions and/or hinder research on these topics. In the current summary, eight paradigms and dubious theses are examined: (a) Biocrusts require physical crusts for establishment; (b) plant establishment is necessary for biocrust establishment on dunes; (c) developmental stages of biocrusts are often regarded as successional stages; (d) climate‐driven crust morphology determines crust hydrology; (e) infiltration (and subsequently run‐off) over biocrusts is determined by soil pores; (f) run‐off in dryland may stem from hydrophobicity; (g) exopolysaccharides trigger infiltration; and (h) mosses impede infiltration and subsequently the growth of perennial plants. During this summary, reasons that lie behind these paradigms and dubious theses are examined using data and/or rationale for a critical analysis. Experimental data, mostly gathered from the Nizzana research site of the Hallamish dune field (Negev Desert, Israel), and published material (mainly from the Tabernas and Tengger deserts) are used. It is argued that most of the paradigms were either based on improper techniques or never thoroughly investigated. The argument presented herein does not necessarily imply that none of the paradigms may take place under very specific conditions but that their acceptance as general “truth” is questionable.

Vertical Divergence of Cultured Microfungal Communities at the Hallamish Dune Field, Western Negev Desert, Israel

ABSTRACTWe examined the depthwise distribution of microfungi through 0–40 cm of sandy profiles under bare surface and different shrub species at the Nizzana research site (NRS) in the Negev Desert, Israel. A total of 125 species from 61 genera were isolated using the soil dilution plate method. At the topsoil of all profiles melanin-containing species with large multicellular conidia predominated. In the middle depths at the majority of the profiles, mesic group of microfungi, Penicillium spp. substantially increased in abundance. Density of microfungal isolates decreased with depth under shrubs and did not substantially vary in the nonshaded profiles, highly positively correlating with electrical conductivity and organic matter (OM) content. Substantial increase in isolate densities at the undercanopy habitats, especially in the interdune depression with higher substrate stability raising the accumulation of OM, highlights the role of shrubs as “islands of fertility” for microfungal communities at NRS.

Factors controlling the formation of coppice dunes (nebkhas) in the Negev Desert

AbstractDue to their role in increasing fertility, coppice dunes (nebkhas) are regarded by many researchers as important contributors to aridland ecosystems. Yet, despite their frequent occurrence, little information exists regarding the rate and factors that control their formation. The goal of the current study is to examine the formation rate and factors that determine the establishment of coppice dunes in the Hallamish dune field in the western Negev Desert. The rate in which sand and fines, hereafter aeolian input (AI) was trapped and its particle size distribution (PSD) were examined by means of the solidification of 2 m × 2 m plots using surface stabilizers, and by the installation of three pairs of artificial shrubs (SH), three pairs of artificial trees (TR), and a pair of control (CT) plots. Measurements were annually conducted during June 2004 and June 2008, with monthly collection during June 2004 and May 2006. The PSD was compared to coppice dunes located on the fine‐grained playa surface. AI was trapped at SH, while it was not trapped at TR and CT. The annual rate of AI accretion under the canopy was highly variable ranging between 1405 and 13 260 g m−2, with a four‐year average of 5676 g m−2, i.e. 3.8 mm a−1. It depended upon the wind power, with drift potential having a threshold velocity of Ut > 10 m s−1 yielding the higher correlations with the monthly AI (r2 = 0.59–0.84). No significant relations were obtained between the monthly AI and shrub height. Sand saltation, suspension and creep are seen responsible for mound formation, which based on the current rates of sand accretion are relatively fast with a 60 cm‐high coppice dune forming within ~150–160 years. The current data highlight the problematic design of some previous research using conventional traps and confining the measurements only to certain seasons. Copyright © 2015 John Wiley & Sons, Ltd.

Assessment of carbon gains from biocrusts inhabiting a dunefield in the Negev Desert

Biocrusts play an important role in carbon (C) assimilation in arid and semiarid regions. In order to assess C gain in the crusted Hallamish dune field, net photosynthesis (Pn) and respiration (R) were measured at five crust types (A–E) which show a gradual increase in their chlorophyll content (with A<B<C<D<E). Pn and R were calculated per chlorophyll a and b (hereafter a/b) content. Total C input was calculated based on a large database that includes data on the temporal change in the chlorophyll content during the winter growing season, annual daylight and nighttime wetness duration and the cover of each crust type. The data show high similarity of Pn and R per chlorophyll content for all crusts (0.9–1.1 and 0.5–0.6gC(Chla/b)−1h−1, respectively). Both Pn and R showed a gradual increase with chlorophyll content with A≈B<C<D<E, with Pn ranging between 0.0311 (for crust A) and 0.1784 (for crust E) gCm−1h−1. Total calculated C gain for crusts A–E assuming total crust cover ranged between 2.1–8.6gCm−2a−1 for crusts A–D and 50.90gCm−2a−1 for crust E, with total annual input into the crusted areas of the ecosystem and the entire ecosystem being 3.7 and 2.3gCm−2a−1, respectively. In comparison to net ecosystem C exchange (NEE) from arid regions, our findings highlight the important role played by biocrusts in CO2 sequestration and as C provider for an extreme arid ecosystem. Yet, although a conservative approach was adopted, the data should be regarded as rough estimation only. In-situ field measurements are still required in order to substantiate the crust role in the annual C gain.

Dune crests serve as preferential habitats for perennial plants during frequent drought years

Following frequent droughts in the last 19 years, high mortality of perennial plants was noted in the Hallamish dune field in the Negev Desert (P = 95 mm). To evaluate the vegetation change, vegetation cover and species composition were studied in 20 plots located at the semi-stable non-crusted dune crest (CR) and 20 plots located at the crusted interdune (ID) during the summer of 2012 and compared to data from 1994. In addition, periodical moisture measurements were carried out in a pair of plots, 50–100 m apart, at CR and ID between October 2009 and September 2012. The findings exhibit a substantial decrease in the living perennials (mainly consisting of shrubs), with ID exhibiting the highest reduction in living shrubs. The findings are in agreement with measurements of the available water content, AWC (i.e., above the wilting point) during 2010–2012. While being recorded at >60 cm at CR, AWC was not recorded at ID at 60–120 cm depth during the summer months. Lower evaporation rates due to higher albedo and especially subsurface flow from the adjacent highly active crest are mainly seen responsible for deeper infiltration and higher AWC at CR. It is concluded that while sufficient AWC was present at CR to sustain most of the perennial shrubs also during frequent drought years, insufficient moisture was retained at ID to sustain most perennials. Non-crusted dune crests may thus serve as preferential habitats (fertility belts) for perennial plants, providing refugia for shrubs during frequent drought years.

Sink plot for runoff measurements on semi-flat terrains: preliminary data and their potential hydrological and ecological implications

Abstract In arid and semiarid regions where water is the main limiting factor, water redistribution is regarded as an important hydrological process of great ecological value. By providing additional water to certain loci, moist pockets of great productivity are formed, characterized by high plant biomass and biological activity. These moist pockets are often a result of runon. Yet, although runoff may take place on semi-flat undulating surfaces, runoff measurements are thus far confined to slopes, where a sufficient gradient facilitates downslope water harvesting. On undulating surfaces of mounds and depressions, such as in interdunes, no quantification of the amount of water reaching depressions is feasible due to the fact that no reliable method for measuring the runoff amounts in semi-flat terrains is available. The current paper describes specific runoff plots, designed to measure runoff in depressions (sinks). These plots, termed sink plots (SPs), were operative in the Hallamish dunefield (Negev Desert, Israel). The paper presents measurements of runoff yield that were carried out between January 2013 and January 2014 on SPs and compared them to runoff obtained from crusted slope plots and fine-grained (playa) surfaces. The potential hydrological and ecological implications of water redistribution within semi-flat terrains for this and other arid ecosystems are discussed.

Contrasting hydrological response of coastal and desert biocrusts

Biocrusts abound in southern Israel, covering the Hallamish dune field near Nizzana (NIZ) in the Negev (mean annual precipitation of 95 mm) and the coast of Nizzanim (NIM) near Ashdod (mean annual precipitation of 500 mm). While the hydrological response of the NIZ crust to natural rain events was thoroughly investigated, no data is available on the hydrological response of the NIM crust. Runoff was monitored in runoff plots during the years 2005–2008, and in addition, sprinkling experiments were carried out on NIM and NIZ crusts. For the evaluation of the possible factors that may control runoff initiation, fine content of the parent material, crust thickness, compressional strength, hydrophobicity, surface microrelief, organic matter, biomass (chlorophyll a and total carbohydrates) and the crust's species composition of NIM were studied and compared to that of NIZ. The data showed that in comparison to the NIZ crust that readily generated runoff, no runoff was produced by the NIM crust. This was so despite the fact that (1) Microculeus vaginatus predominated in both crusts, (2) the substantially higher rain intensities in NIM, (3) the greater thickness and higher chlorophyll content and (4) the lower microrelief at NIM in comparison to NIZ. The lack of runoff in NIM was explained by its low amounts of exopolysaccharides that did not suffice to affectively clog the surface and in turn to facilitate runoff initiation. The absence of runoff and its consequences on the NIM ecosystem are discussed. Copyright © 2012 John Wiley & Sons, Ltd.

The use of microbiotic crusts as biomarkers for ponding, subsurface flow and soil moisture content and duration

Previous work at the Hallamish dunefield in the western Negev Desert defined and mapped 5 mature types of microbiotic crusts (MCs). Crust definition and sampling were based on 4×4m pixels. Yet within these pixels, microhabitats with different crust types could be noted. This was also the case within crust type A that extends over the south-facing slope and the sandy interdune. There, close examination mainly at <0.5×0.5m pixels revealed a high variability in crust types, as was also the case at fine-grained playa surfaces, scattered within the sandy interdune. These microhabitats belonged to two groups: habitats that received water from an additional source (runoff or subsurface flow), i.e., water-enriched habitats (WE) and habitats having a better water regime due to shading (SH), such as by shrubs.Daylight wetness duration and crust biomass were monitored during 1991–1994 in 12 habitats. WE habitats, which exhibited the highest biomass also exhibited longer wetness duration in comparison to SH habitats. Both groups exhibited higher biomass and wetness duration than the control. When the relation between daylight wetness duration and the chlorophyll content of the crust was compared, a positive linear relation with a good correlation (r2=0.74) was found. The findings indicate that crust biomass may be used as a biomarker for surface wetness duration and may thus point to habitats subjected to particular hydrological processes such as water ponding and subsurface flow. They may also point to soil patches having high moisture content and therefore long wetness duration. Higher moisture content that lasted for a longer duration was indeed recorded during preliminary measurements that were carried out at 0–40cm soil patches subjected to subsurface flow (as indicated by their MC) during the end of the winter season of 1994/95 and 1996/97. The findings may attest to differential rates of pedogenesis, thus highlighting the use of MCs as possible biomarkers for small soil habitats with variable pedogenesis.

The effects of heavy winter rains and rare summer rains on biological soil crusts in the Negev Desert

Abstract Biological soil crusts (BSCs) abound in the Hallamish dune field in the western Negev Desert, Israel. While their abundance may imply high adaptability to environmental change, such as fluctuations between wet and dry conditions following winter rains, summer rains, although rare, may also occur in the Hallamish dune field. The aim of the present paper is to examine crust responses to winter and summer rains, focusing particularly on its biomass components, chlorophyll and carbohydrate. In addition, species composition, during summer and winter was examined. Analysis took place during an exceptionally wet winter (1994/95 with 172 mm) and a summer rainstorm (12.5.93 with 9.7 mm). The data showed a 2–3 fold increase in chlorophyll a and total carbohydrates and a much richer species composition following the heavy winter rains of 1994/95. Yet, the data also showed ~ 15–30% decrease in the chlorophyll content of the crust (with no concomitant significant decrease in total carbohydrates) following the summer rainstorm. Intense weathering by the summer rain coupled with cell mortality may explain the decrease in the chlorophyll content following the summer rainstorm, suggesting possible changes in the BSC following a potential change in the precipitation regime due to global climate change.

Millimeter-scale microrelief affecting runoff yield over microbiotic crust in the Negev Desert

The negative effect of microrelief upon runoff yield, explained by the increase in water storage and the slowdown in flow velocity, is a known phenomenon. However, research concerning surface roughness hitherto focused on decimeter- or centimeter-scale microrelief. Owing to the heterogeneous character of many natural plots, the effect of centimeter-scale microrelief was mainly shown under controlled sprinkling experiments, whereas no data was yet made available concerning the possible effect of a millimeter-scale microrelief upon runoff yield under natural field conditions. When runoff yield was measured over crusted (i.e., surfaces covered by 1–3 mm of cyanobacterial crust) and scalped 2–6 m 2 plots at the Hallamish dune field (Negev Desert, Israel) during 5 winter seasons (1990–1995), no runoff was generated from the scalped plots during the first year following scalping while meagre quantities were received during the second year. Nevertheless, during the third season and onward, runoff yield at the scalped plots was consistently and significantly higher than that of the crusted (control) plots and this was so albeit the fact that the crust chlorophyll content of the scalped plots during the third season was as low as half the amount of the crusted plots. These findings appeared to contradict previous findings that showed the existence of a positive linear relationship between the crust chlorophyll content and runoff yield. The apparent contradiction was explained by the differences in the plot microrelief, indicating that minute differences, even of a millimeter-scale, may greatly affect runoff yield and thus may compensate for lower crust chlorophyll content. Surface smoothness, which characterizes many of the arid zones, may thus explain, at least partially, the high runoff yield obtained in deserts.

The role of dew as a moisture source for sand microbiotic crusts in the Negev Desert, Israel

Dew and fog were measured periodically during the years 1992–1995 at the Hallamish dune field, western Negev Desert (average annual rain precipitation ca. 95 mm). A major portion of the dunes are covered by cyanobacterial and moss-dominated microbiotic crusts. Chlorophyll- a measurements and microscopic examination were carried out in order to detect the possible changes in crust biomass and phenological changes of the mosses following the occurrence of dew and fog. Dew and/or fog were present in over 50% of the late summer and fall mornings with an average yield of 0·1 mm on cloth attached to glass plates. This corresponds to ca. 0·03 mm for the upper 0·5 cm of the crust, and is less than the 0·1 mm threshold which implies liquid water and which was also found necessary for net photosynthesis of the cyanobacterial crust. This threshold was met only during heavy dew or fog events, during which 10–40% of the crust surface area was moistened. It was estimated that dew and fog could account for upto 3·2–9·4% of the total annual time of crust wetness and consequently activity due to liquid water. Nevertheless, microscopic examination of the dominant moss species at Nizzana, Bryum dunense, disclosed phenological changes, mainly resulting in growth of vegetative reproduction organs (bulbils), and sexual organs, gametangia (antheridia and archegonia). B.dunense may thus be adapted to the utilization of dew and fog precipitation for the production of nearly mature sexual organs. The moss may thus be capable of already utilizing the first winter rains for fertilization and sporophyte formation.

Runoff‐induced sediment yield over dune slopes in the Negev Desert. 2: Texture, carbonate and organic matter

AbstractRunoff‐induced sediments were collected in the Hallamish dune field for four years (1990–1994). Runoff and consequently water‐transported sediments were generated on the dunes owing to the presence of a thin microbiotic crust. These sediments were analysed for their particle‐size distribution and carbonate content. In addition, the organic matter content was calculated by measuring the chlorophyll content within the runoff. The results were compared to the slope parent material, i.e. the crust and the underlying sand, as well as to playa sediments, which are scattered within the Hallamish interdunal areas, and which were previously hypothesized to originate from runoff‐induced sediments.Higher amounts of fines (silt and clay) and carbonate characterize the footslopes in comparison to the midslopes. Intermediate contents of fines (17 per cent) and carbonate (8 per cent) characterized the sediments in comparison to the fines (27 per cent) and carbonate (15 per cent) of the crust and to the fines (4 per cent) and carbonate (4 per cent) of the underlying sand. The runoff‐induced fines and carbonate contents were significantly different from those of the playas, suggesting that the playa flats do not originate from runoff‐induced sediments.The sediments were enriched with organic matter. Organic matter which originates from the crust amounted to 0·3–0·4 per cent as compared to ≤0·1 per cent in the bare sand. Nevertheless, the crust was found to be relatively resilient to water flow. Only 0·1–0·5 per cent of the crust was annually eroded off the slope by water, with south‐facing crusts showing higher resilience than north‐facing crusts. The data may thus assist in the evaluation of the crust's residence time. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Runoff‐induced sediment yield over dune slopes in the Negev Desert. 1: Quantity and variability

AbstractAlthough extensive data exist on runoff erosion and rates for non‐sandy hillslopes, data for arid dune slopes are scarce, owing to the widespread perception that the high infiltrability of sand will reduce runoff. However, runoff is generated on sandy dunes in the Hallamish dune field, western Negev Desert, Israel (P ≈ 95 mm) due to the presence of a thin (usually 1–3 mm) microbiotic crust. The runoff in turn produces erosion.Sediment yield was measured on ten plots (140–1640 m2) on the north‐ and south‐facing slopes of longitudinal dunes. Two plots facing north and two facing south were subdivided into three subplots. The subplots represented the crest of the active dune devoid of crust, the extensively crusted footslope of the dune, and the midslope section characterized by a patchy crust. The remaining plots extended the full length of the dune slope.No runoff and consequently no water‐eroded sediments were obtained from the crest subplots devoid of crust. However, runoff and sediment were obtained from the mid‐ and footslope crusted subplots. Sediment yield from the footslope subplots was much higher than from the midslopes, despite the higher sediment concentration that characterized the midslope subplots.The mean annual sediment yield at the Hallamish dune field was 432 g per metre width and was associated with high average annual concentrations of 32 g l−1. The data indicate that owing to the presence of a thin microbiotic crust, runoff and water erosion may occur even within arid sandy dune fields. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Microclimate control upon sand microbiotic crusts, western Negev Desert, Israel

Approximately 20% of the world's arid zones are covered by eolian sand. Parts of the sand are occupied by microbiotic crusts or have the potential of sustaining microbiotic crusts, which may have a great impact upon geomorphological processes and the ecology of the ecosystem. Any attempt to understand the distribution of microbiotic crusts inevitably necessitates knowledge concerning the microclimate within these ecosystems and in particular moisture, temperature, and eolian input. In the Hallamish dune field in the western Negev Desert, Israel, cyanobacterial crusts with a chlorophyll a content of 15–20 mg m −2 characterizes the south-facing footslopes, whereas a moss-dominated crust with a chlorophyll a content of 50–60 mg m −2 covers the north-facing footslopes. Since the entire dune field was re-stabilized concurrently, following the 1982 peace treaty with Egypt, it was hypothesized that physical conditions, rather than time duration, may account for the differences observed. Microclimatological data, which included temperature, rainfall, runoff, dew and fog, surface moistness and eolian input were monitored. The differences in crust type could not have been attributed to rainfall, dewfall, temperatures, or eolian input. Although lower amounts of incident rain, lower temperatures and lower eolian input characterized the moss-dominated north-facing footslope, moss-dominated crusts were also found in restricted areas of high temperatures and eolian input at the interface between the mobile and the encrusted dune section of the south-facing aspect. High variability in daytime moisture duration following rain was however monitored with surface moisture duration being approximately 2.5 times longer at the moss-dominated habitat. These differences are seen responsible for crust-type distribution within the ecosystem, which may affect, in turn, variable geomorphological processes.

TWO CAUSES FOR RUNOFF INITIATION ON MICROBIOTIC CRUSTS: HYDROPHOBICITY AND PORE CLOGGING

Hydrophobicity and pore clogging are suggested as two mechanisms responsible for generating runoff over microbiotic crusts overlying dune sand. Although natural microbiotic crusts in the Hallamish dune field (Negev Desert, Israel) did not show any hydrophobicity, that was not the case with natural crusts subjected to long periods of continuous wetness in the lab. Monoalgal crusts, grown in the lab, also showed high hydrophobicity when dry. The hydrophobicity vanished, however, once the surface was wetted. Runoff on monoalgal lab-grown crusts was obtained when (i) the dry crust exhibited hydrophobic properties and (ii) the wetted crust no longer showed water repellence. Although runoff generation caused by hydrophobicity is expected, it is suggested that runoff initiation when the crusts do not exhibit any hydrophobicity stems from the high water absorption and swelling of the exopolysaccharide cyanobacterial sheaths causing pore clogging. The multi-layered structure of the crust and filament migration to the surface may enhance pore clogging. The experiments and their interpretation are supported by reinterpretation of published data.