Nizzana Research Site Research Articles

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.

The contrasting effect of biocrusts on shallow‐rooted perennial plants (hemicryptophytes): Increasing mortality (through evaporation) or survival (through runoff)

AbstractThe effect of biocrusts on plants is ambiguous. At the Nizzana research site (Negev Desert, Israel), frequent droughts in the last 2 decades resulted in high shrub mortality, which has been explained by the negative effect of the low‐albedo crusts on soil water evaporation. Nevertheless, shallow‐rooted perennial plants, such as Stipagrostis plumosa and Cyperus sp., exhibit relatively little mortality. We hypothesized that these hemicryptophytes (HCs) grow at or adjacent to small 2‐ to 4‐m2 depressions (termed IDDEP) in the interdune (ID), which are scattered within the undulating terrain (termed IDINC) of ID, and as previously found, serve as sink for short‐distance runoff from IDINC to IDDEP. To test this, we randomly demarcated 50 5 × 5 m plots in the ID and semistable uncrusted dune crest (CR), and the cover of live HCs in the three habitats CR, IDINC, and IDDEP were monitored. In addition, the available water content (AWC) for the 0‐ to 80‐cm depth was measured during 2012–2014. Although AWC exhibited a clear pattern with IDDEP > CR > IDINC, HCs were almost absent from IDINC (above ground biomass of 0.3 kg/ha) but relatively high in IDDEP (55.3 kg/ha) and CR (113.0 kg/ha). The high biomass at CR and IDDEP is explained by higher AWC due to lower evaporation or runoff addition, respectively. The findings highlight the contrasting role played by the biocrust: Although responsible for the mortality of most perennials at IDINC due to increased evaporation, biocrusts assist the survival of relatively shallow‐rooted perennials at IDDEP through runoff generation.

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.

Linking surface and subsurface properties of biocrusted and non-biocrusted habitats of fine-grained fluvial sediments (playas) from the Negev Desert

Abstract With biocrusts playing a cardinal role in C and N fixation in arid zones, information regarding the factors that determine their limits of growth is of uttermost importance for the study of ecosystem structure and function. This is also the case in the western Negev dunefields, where although abundant on the sandy surfaces, biocrusts are scarce on fine-grained (mainly loessial) sediments, termed playas. In the Nizzana research site (NRS), visibly distinct surfaces, with and without biocrusts were noted within a single playa. In an attempt to characterize these distinct surfaces, a set of random measurements were carried out, which included measurements of crack density, microrelief and chlorophyll content of the upper 0–1 cm. Following a cluster analysis, four distinct types of surfaces (hereafter habitats) were defined, one with substantial amount of chlorophyll content which can be regarded as biocrust (P4), and three non-crusted surfaces (P1–P3). Within each type, two 50 cm-deep pits were dug and the pH, electrical conductivity (EC) and fine (silt and clay) content (FC) of samples collected at 1–5, 5–10, 10–20, 20–30, 30–40 and 40–50 cm-depth were analyzed. In addition, periodical moisture measurements were carried out (in pairs) to a depth of 0–20 cm at each surface type during 2013/14. All non-crusted habitats (P1–P3) were characterized by loessial subsurface sediments. Conversely, P4 was either characterized by loessial subsurface sediments (and in this case it was characterized by a slightly concave surface) or having a sandy subsurface (at ~5–10 cm depth). While the non-crusted surfaces exhibited low moisture content, P4 exhibited deeper and higher moisture content explained either by the more sandy sediments or by lower water loss through runoff. The findings point to the close link between surface and subsurface properties and indicate that water availability may explain biocrust establishment and growth also at the loessial playa surfaces. Biocrusts may thus serve as bioindicators for habitats with high moisture content.

Vertical Divergence of Cultured Microfungal Communities Through the Depth in Different Soil Formations at Nahal Nizzana, Western Negev Desert, Israel

ABSTRACTWe examined the depth-wise distribution of microfungi through 0–50-cm crusted sandy and playa profiles at the Nizzana research site, the Negev desert, Israel. A total of 188 species from 77 genera was isolated using the soil dilution plate method. Density of microfungal isolates sharply decreased with depth highly positively correlating with organic matter content at the sandy profiles. High load of solar radiation (at the surface) as well as strongly limited aeration, water infiltration, and increased salinity (at the playa depth) led to dominance of melanin-containing species with large, multicellular conidia both in the topsoil of all profiles and in the deep playa layers, while at 1–30 cm, species producing light-colored small one-celled conidia mostly prevailed. In that way, the above vertical variations resemble differences in the composition of topsoil microfungal communities found between the sites located in the Negev desert and in the Mediterranean region of Israel.

Wind speed determines the transition from biocrust-stabilized to active dunes

Abstract We examine the hypothesis that above a certain height, crusted, stabilized dunes become non-crusted with a mobile crest. Toward this end, twelve plots, 10 × 10 m, were demarcated along a 1 km-long transect in the Nizzana research site (NRS), western Negev, Israel, extending along a ridge of a dune from the crusted interdune up to a height of 22 m above the interdune, characterized by a non-crusted mobile crest. Within each plot, a 4 × 4 m subplot was established where the upper 3 cm of all surfaces was removed. Surface stability was monitored using six erosion pins from March 2010 to February 2012. In addition, data from a nearby meteorological station were analyzed. The data indicated that drift potential (DP) was the highest during winter and spring. A good correlation (with r2 = 0.73) was found between the monthly DP and the absolute change in pin height. Also, a good correlation (with r2 = 0.85) was found between altitude and the absolute change in pin height. A monthly change in pin height of ∼0.3 cm marked the threshold between the crusted and the non crusted sections of the dune, which corresponds to 8 m above the interdune. The findings imply that as long as the absolute monthly change in pin height is

Under-canopy microclimate within sand dunes in the Negev Desert

Summary Scattered shrubs are a common phenomenon in many arid landscapes. Once established, shrubs are known to create “islands of fertility”, i.e., preferential habitats for annuals and animals. In an attempt to characterize the physical conditions prevailing under the shrub, radiation, temperatures and soil moisture (0–40 cm) following rain were measured during 1993–1995 at the shaded under-canopy (UC) and at the exposed inter-shrub habitat (EXP) of two pairs of shrubs located at the north- and south-facing slopes of dunes in the Nizzana research site, western Negev Desert, Israel. In addition, the soil organic matter (SOM) and the fine (silt and clay) content (FC) were also measured. Whereas the differences in the amounts of SOM and FC were small, daylight temperatures at UC were substantially lower (6–15 °C), subsequently resulting in extended time during which the UC habitat remained wet. Moisture was retained for up to 10.5–42.6% longer at UC in comparison to EXP, mainly explained by the shading effect. SOM was found to explain only 8.6–19.6% of the results. By shading, shrubs in the Negev Desert may thus provide relatively wetter conditions for annuals, rendering them an advantage over inter-shrub habitats at this harsh arid environment.

The effect of shrub canopy upon surface temperatures and evaporation in the Negev Desert

AbstractAlthough known as ‘islands of fertility’ or ‘resource islands’, information regarding the effect of shrubs upon microclimate in deserts is scarce. Here we report on measurements of evaporation and temperatures that were carried out in and around a pair of shrubs at the Nizzana research site in the western Negev Desert during 1993–94 and during the growing season (November–March) of 1994–95 and 1996–1997. Whereas evaporation was measured monthly using mini‐atmometers (10 cm diameter and 10 cm tall) at an exposed site and under and around the shrub (at the eastern, southern, western and northern aspects), temperature was measured under a shrub canopy, at its northern aspect, and at an exposed habitat. Evaporation was aspect dependent with increasing rates in the following order: exposed > south‐facing > west‐facing ≈ east‐facing > north‐facing > under canopy. Except from the northern aspect, the under‐canopy habitat showed substantially lower rates of evaporation in comparison with all other habitats. The differences between the under‐canopy and the exposed habitat were larger during wintertime (with the under‐canopy habitat having 0·53 times the evaporation rate than that of the exposed habitat) although higher differences in temperatures characterized both habitats in summertime (up to 14·4 °C in summer as compared with 6·9 °C only in winter). The results were explained by extended surface wetness that characterized the under‐canopy habitat following rainstorms. While already being dried out at the exposed habitat, surface wetness at the under canopy habitat persisted for several days afterwards, resulting, following one rainstorm, in vapour pressure of 2·15–2·39 kPa in comparison with only 0·82–0·83 kPa of the exposed habitat. The substantially lower evaporation rates that characterize the under‐canopy habitat may thus play a pivotal role in providing preferential conditions for lush under‐canopy annual growth. Copyright © 2008 John Wiley and Sons, Ltd.

The limited role of localized convective storms in runoff production in the western Negev Desert

High-scale floods in the Negev Desert are usually linked to rains originating from the Red Sea Trough (RST), characterized by high-intensity rainspells. While the effect of the RST storms was assessed in the southern, central and eastern parts of the Negev, almost no information exists as to their possible impact in the western parts of the Negev.Rain characteristics and its role in runoff production were studied during the years 1989–1994 in the Nizzana research site in the western Negev Desert. Only 7 out of 49 rainstorms (≥1mm) originated from the RST, and only two of these seven storms produced runoff. While the RST-induced runoff accounted for only 3.9% of the total runoff in Nizzana, it accounted for 83.5–100% of the total runoff of four major ephemeral streams in the eastern part of the Negev. It is suggested that the limited role of the RST in the western Negev and its major role in flood formation in the eastern Negev may contribute to stream capture of formerly westward flowing streams eastward into the Dead Sea. A positive feedback mechanism is thus suggested. Whereas the topography may enhance atmospheric instability and hence high convective rainstorms, the resultant floods may enhance rift widening which in turn will foster atmospheric instability.

The vegetation of linear desert dunes in the north-western Negev, Israel

Summary In 1992 the flora and vegetation of an area of linear sand dunes at Nizzana, western Negev desert, Israel, was investigated. Seven different plant communities were found and a vegetation map of the research area was produced. The plant communities were distinguished by only the dominating perennial species. Nevertheless, the results of the study indicate, that also annual species may be suitable as a tool to distinguish between desert plant communities The results suggest, that the most important environmental factor determining patterns of plant distribution in the Nizzana research site is the structure and dynamics of the soil-surface. Vegetation cover was lowest in highly stable areas with hard surface crust or a high amount of fines in the soil (playas) and on the mobile dune tops. Highest plant cover was found at the dune bases and in areas which had been used for cultivation in former times. Mean species richness per unit area was lowest on the dune tops and highest in the interdune areas with more or less stable surface crust. Absolute number of species found was highest for the playas and lowest on the dune-tops. As most of the plant communities described for the research site are restricted to areas which are characterized by a certain surface stability it is discussed to regard some of the vegetation units as succession stages in a process of continuing stabilization of the dunes.