Giora J. Kidron

Physical properties of the psammophile cryptogamic crust and their consequences to the water regime of sandy soils, north-western Negev Desert, Israel

The water regime in the sandy dunal area of Nizzana, north-westem Negev Desert, Israel, is highly dependent on a fragile cryptogamic crust only several millimetres thick. This crust develops due to the presence of Cyanobacteria which agglomerate the sand grains and trap aeolian dust particles. Not only does this semi-permeable crust increase runoff but the water which does infiltrate the soil is protected from excessive evaporation. This study presents quantitative measurements of the physical properties of the crust which are important to the water regime in desert dunal areas—the granulometry, porosity and water retention capacity. The grain size distribution shows a concentration of silt and clay in the crustcompared to the sands just beneath the crust. The microporosity (measured using a mercury pore sizer) shows that approximately 40% of the access pores can be blocked by the swelling of Cyanobacteria trichomes as they absorb water, which limits rainwater infiltration. These observations concur with rain simulation experiments made in the field. An evaporation phase was simulated in the laboratory in order to quantify the water retention capacity of the crust and compare it with that of other sediments, in which the algal mat is not intact, or absent. At the end of the cycle, the crust was found to contain approximately ten times more water than the other samples.

Rainfall–runoff relationship over encrusted dune surfaces, Nizzana, Western Negev, Israel

A thin microbial crust covers the lower part of longitudinal dunes in the western Negev, where average annual rainfall is 95 mm. In order to study the effect of the microbial crust on rainfall–runoff relationships under natural rainfall conditions, runoff plots equipped with pressure gauges were established on opposite north- and south-facing slopes that differ in their vegetal cover and crust properties. The study covered four years (1990–94). The first two years were wet and the following two years relatively dry. One to five flow events were recorded per year. No correlation was found between runoff yield, rain amount and rain intensity. Unlike many microbial crusts reported in the literature, the microbial crust in the western Negev is not hydrophobic. Infiltration rate is high under dry surface conditions and of the order of 9–12 mm h−1 when the crust is saturated. The high final infiltration rate is explained by the occurrence of large pores that do not seal when the crust is saturated. Typical hydrographs have very steep rising and falling limbs, pointing at a limited contributing area. Most flows last less than 10 min and runoff volumes collected are, on the whole, very small. Owing to differences in crust properties, runoff is higher on north- than on south-facing slopes.

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.

Spectral Characteristics of Cyanobacteria Soil Crust in Semiarid Environments

Abstract Large areas of sand fields in arid and semiarid regions are covered by cyanobacteria soil crusts. The objective of this article is to analyze (systematically throughout the VIS, NIR, and the SWIR regions of the spectrum) the unique spectral features of cyanobacteria crust relative to bare sands and under different moisture conditions. It was found that: 1) When biogenic soil crusts are wet, their NDVI value can reach 0.30 units due to their photosynthetic activity; 2) the closer the red edge inflection point is to the longer wavelengths, the higher the relative abundance and distribution of the microphytic community; 3) the phycobilin pigments, which are unique to cyanobactria, contribute to higher reflectance in the blue region relative to the sand substrate; 4) a crust index based on this uncommon spectral feature can be useful for detecting and mapping, from remote sensing imagery, different lithologic/morphologic units; 5) although most dune sand areas are generally made of quartz, other notable features appear on their spectra. In the study area, there are absorption features representing minerals (iron oxides at 860 nm and clay minerals at 2200 nm) and biogenic crusts (chlorophyll at 670 nm and organic matter at 1720 nm, 2180 nm, and 2309 nm).

Analysis of dew precipitation in three habitats within a small arid drainage basin, Negev Highlands, Israel

Abstract Water availability is the most important limiting factor in arid lands. Any additional source of water, such as dew and fog, may have a positive impact upon the ecosystem. Dew and fog precipitation are largely dictated by weather conditions and habitat. Dew and fog measurements were carried out for 29 days in the fall of 1987–1989 at three distinctive habitats within a single drainage basin in the Negev Highlands, Israel. The habitats were a sun- and wind-exposed habitat, at two hilltops, a sun-shaded habitat, at the north- and west-facing footslopes, and a wind-protected habitat, at two wadi beds. Morning weather conditions (cloudiness, wind speed) were also monitored. An analysis of the dew and fog quantities and duration was performed. Clear mornings and a single foggy morning recorded were characterized by high dew and fog amounts and duration, whereas lower values were recorded during cloudy and especially windy mornings. The hilltop stations and especially the sun-shaded footslope stations obtained significantly higher values of dew and fog for a significantly longer duration than the wadi bed stations. Whereas the results did not support the hypothesis that advective condensation is responsible for the high dew amounts at the sun-shaded habitat, the data showed a continuous dew condensation even after sunrise. This continuous condensation, averaging up to 1.1 h following sunrise, was especially pronounced at the sun-shaded habitat and may explain the higher dew values and longer time duration obtained at this habitat. Since maximal dew values may not necessarily be obtained at sunrise and may change according to habitat, dew collection time should be carefully considered.

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

Approximately 20% of the worlds 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.

Reproductive strategies of Bryum dunense in three microhabitats in the Negev desert

Abstract The role of dew, fog, and rain in the development of reproductive organs in the moss Bryum dunense A.J.E. Smith & H. Whitehouse (B. bicolor complex) was examined in three different microhabitats within a dune field in the Negev Desert. Growth and reproduction were compared prior to the first rainfall of the winter and subsequently twice during the winter. The microhabitats, differing in their exposure to radiation and precipitation, were 1) an exposed site, 2) a shaded (under a shrub canopy) site, and 3) a partially shaded microhabitat at the foot of a north-facing dune slope. We hypothesized that both microhabitat and time of year would impact the reproductive strategies of the species. In all three microhabitats, reproductive organs were initiated prior to the first winter rain. The source of bryophyte wetness was atmospheric humidity from dew and fog. The type and the extent of development of the reproductive organs was habitat dependent. Prior to the rains, antheridia and archegonia were scarce in the exposed microhabitat, in small numbers in the shaded microhabitat, and most abundant in the partially shaded microhabitat. Subsequently, sporophytes were formed more often in the partially shaded microhabitat following the winter rains. Asexual propagation by bulbils was prevalent at the partially shaded and the exposed microhabitats, whereas regeneration by secondary protonemata was especially abundant at the shaded microhabitat. Bryum dunense is highly adapted to dew and fog utilization and is able to develop reproductive organs prior to the first rains. In this way, plants are prepared for dispersal by bulbils and ready for fertilization with the onset of the first rain. This early initiation of reproductive development is especially advantageous for dioicous species in unpredictable desert environments enabling the completion of the life cycle. The partially shaded microhabitat formed by the shrub canopy was found to be the most suitable microhabitat, yielding the highest amounts of sexual (antheridia and archegonia) and asexual (bulbils) organs. The shrub may thus serve as an ‘island of fertility’ for B. dunense.

Dew moisture regime of endolithic and epilithic lichens inhabiting limestone cobbles and rock outcrops, Negev Highlands, Israel

Summary Endolithic and epilithic lichens proliferate on calcareous cobbles and rock outcrops in the Negev Highlands, Israel. Whereas epilithic lichens predominate in shaded mesohabitats, extensively covering rock outcrops, endolithic lichens proliferate on loose cobbles. Endolithic lichens were thought to predominate in habitats having a poor dew regime. Dew measurements were carried out at habitats of endolithic and epilithic lichens. The measurements took place on loose and partially embedded cobbles with 90–100 % of endolithic lichen cover, and on rock outcrops inhabited by epilithic lichens (75-90 % cover). In addition, independent dew measurements were carried out with the Plate Cloth Method (CPM). A total of 60 days of measurements was performed. Average daily dew amount as obtained by the CPM was 0.20 mm, as compared to 0.18 mm obtained on the loose cobbles, 0.09 mm obtained by the partially embedded cobbles and 0.04–0.08 mm obtained on the bedrock surfaces. The dew amounts obtained on the cobbles were significantly higher than those obtained on the bedrock surfaces (paired t-test, p

An Investigation of the Role of Water (Rain and Dew) in Controlling the Growth Form of Lichens on Cobbles in the Negev Desert

Dew and rain were measured along the north facing slope (NFS, with epilithic lichens predominating) and the south-facing slope (SFS, with endolithic lichens predominating) in the Negev (P = 95 mm) in order to evaluate the relationships between the abiotic conditions and the lichen growth form. Although insignificant differences characterize the rain and dew amounts with the epilithic growth form, high correlation was obtained between dew duration (r2 = 0.73) and the combined duration of dew and rain (r2 = 0.79). The data imply that daylight wetness duration rather than the amount of precipitation may explain the predominance of epilithic lichens at the shaded NFS.

The Effect of Dew and Fog on Lithic Lichens Along an Altitudinal Gradient in the Negev Desert

The effect of dew upon lichen communities was studied in three sites along an altitudinal gradient in the Negev Desert. The sites [Nizzana, NIZ (250m a.s.l), Sede Boqer, SB (530m a.s.l.) and Har Harif, HH (990m a.s.l.)] have similar rain precipitation (∼100 mm) but different dew precipitation of 0.11, 0.21 and 0.32 mm, respectively. A clear gradient was found for the chlorophyll content with NIZ (38.6 mg m−2)<SB (52.6 mg m−2)<HH (71.3 mg m−2), and the epilithic growth form with NIZ (1.4%) < SB (4.3%) ≪ HH (53.6%) highlighting the fundamental role played by dew in dictating the structure of lichen communities. The findings may assist us in identifying lichens that may serve as biomarkers for dew availability and in anticipating changes in the lichen communities following global warming.