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Nebkhas are distinctive biogeomorphological landforms prevalent in global drylands and coastal environments. They play a crucial role in supporting local biodiversity and preventing land desertification and often serve as an indicator of local environmental change. Despite their significance, the environmental factors that affect their geographical distribution and how they respond to climate change have not been fully explored. This study represents a novel application of machine learning models to quantifying the biogeomorphological niche of Nitraria nebkhas in northern China and simulating their geographical distribution under future climate change conditions. Findings underscore that climatic variables influence the growth of formative shrub species on nebkhas, whereas climate, soil and geomorphological conditions, along with their spatial configuration, determine the probability of nebkha occurrence. Predictions under medium and high greenhouse gas emission scenarios indicate a northward shift in the potential distribution of nebkhas in northern China by the end of the century, accompanied by a decrease in the south due to rising temperatures. Given the potential impact of nebkha field degradation on biodiversity and soil hydrological conditions, adaptive land use strategies should be designed to protect nebkhas and mitigate the impact of climate change. Our study not only provides valuable insights for informing policy‐making and conservation initiatives but also serves as an example for quantifying the niche of biogeomorphological landforms and simulating their dynamics by integrating machine learning approaches into empirical geomorphological studies. [ABSTRACT FROM AUTHOR]

Aims: The fairy circles along the Namib Desert in southern Africa are round grassland gaps that have puzzled scientists for about 50 years. With the discovery of fairy circles in Australia in 2016, the debate on the origin of the circles has been extended to a new continent. Research interest on the topic has since then risen strongly but so has the use of the term "fairy circle". This term has become more imprecise and, by analogy, has been applied to circular vegetation gaps or plant rings that are largely unrelated to fairy circles. For this reason, we define the concept of fairy circles by identifying their three main characteristics based on in situ field observations and soil excavations to larger‐scale spatial patterns, and regional‐scale distribution. Results: Following this approach, fairy circles are defined by: (a) being "empty gaps" in grassland without a central insect‐nest structure; (b) their ability to form spatially periodic patterns, which are regular hexagonal patterns with an extraordinary degree of spatial ordering; and (c) their strongly regional distribution confined within a narrow arid climatic envelope. In these combined traits, fairy circles differ from other common vegetation gaps which, for example, always have a central insect‐nest structure and may occur across broad climatic gradients on continents. Also plant rings have their own specific characteristics that largely differ from the combined attributes of genuine fairy circles. Conclusions: There are many other vegetation‐gap patterns in arid lands but if such gaps cannot jointly show the three characteristics defining the fairy circles, they should be carefully discussed on their own, rather than mixing them up with fairy circles. Our synthesis provides a new etymology for the different types of vegetation gaps and rings, aiming to guide the reader through various classes of circular plant patterns. [ABSTRACT FROM AUTHOR]

Livestock grazing is often perceived as being detrimental to the quality and functioning of dryland ecosystems. For example, a study in a semiarid Kenyan savanna proposed that cattle form bare spaces throughout the landscape, which indicate ecosystem degradation. Other studies, conducted in north‐eastern Spain, where climatic conditions range between semiarid and Mediterranean subhumid, reported that sheep and goat trails have increased the emergence of rill erosion processes. Sometimes, this negative perception is extended to include wild, large ungulate herbivores as well. Here, we challenge this perception by highlighting the generally nonadverse and even ameliorative impacts of moderate animal rate on geoecosystem functioning of hilly drylands. Specifically, trampling routes (also known as treading paths, livestock terracettes, cattle trails, migration tracks, cowtours, etc.) formed across hillslopes by grazing animals—being either domesticated livestock or native large herbivores—transform the original two‐phase vegetation mosaic of shrubby patches and interpatch spaces into a three‐phase mosaic. The animal routes increase the complexity of ecosystem, by strengthening the spatial redistribution of water and soil resources at the patch scale and decreasing hydrological connectivity at the hillslope scale. As a consequence, the animal routes improve functioning of hilly drylands and increase their resilience to long‐term droughts and climatic change. Therefore, instead of viewing the animal routes as degraded spots, they should be perceived at a wider perspective that allows to properly understand their overall role in sustaining dryland geoecosystems. [ABSTRACT FROM AUTHOR]

Biological soil crusts (biocrusts) play important roles in ecosystem functioning and are considered ecosystem engineers: stabilizing the soil surface, preventing soil erosion, changing soil surface hydrology, and enriching the soil with carbon and nitrogen. The link between geodiversity and biodiversity incorporates physical aspects from geology, geomorphology and pedology. We aimed to assess the effects of hillslope geodiversity on the cover, composition and biophysiological properties of biological soil crusts (biocrusts) in a dryland ecosystem. The geodiversity of the hillslope is determined by the stone cover on the ground surface, the content, stones in the soil profile, and the soil depth. It is known from the scientific literature that geodiversity affects biodiversity; thus, we hypothesized that the hillslope type may affect biocrust properties that develop on these hillslopes. The results indicate that the mean covers of biocrust types in the homogeneous (low geodiversity) hillslopes were 60.6% cyanobacterial crusts and 39.4% biocrusts composed of cyanobacteria, moss and lichens, whereas those of the heterogeneous (high geodiversity) hillslopes were 81.6% cyanobacterial crusts and 18.4% biocrusts composed of cyanobacteria, moss and lichens. It was concluded that geodiversity enhances the growth of biocrust species that require higher soil moisture. The results imply that wherever restoration activities are planned, and especially in drylands, the geodiversity of the area should be taken into account. Highlights: Geodiversity expressed by the degree of stoniness impacts ecosystem functioning and soil propertiesGeodiversity impacts the biocrust's cover and compositionGeodiversity enhances the growth of biocrusts that require high soil moistureChlorophyll a, b and a + b in biocrusts are increased in heterogeneous hillslopes [ABSTRACT FROM AUTHOR]

On Mars, large aeolian ripples with wavelengths typically 1–3 m but lacking very coarse sand at crests have been encountered by rovers and observed from orbit. These bedforms have no terrestrial counterpart and several hypotheses for origins have been proposed. This work reports results of Computational Fluid Dynamics (CFD) experiments with ANSYS Fluent under terrestrial and Martian boundary layer conditions, using the k − ω SST turbulence model to evaluate shear stress along a topographic profile of large Martian ripples at different boundary layer wind speeds. Results indicate that, compared with Earth conditions: (1) boundary‐layer flow along large ripples under Martian conditions is less turbulent due to higher kinematic viscosity; (2) reverse‐flow vortex regions from crests at ripple lee flanks are larger; and (3) shear stresses at crests of large ripples are relatively low, so ripple flattening is less likely at high wind speeds. These results indicate Martian ripples formed by the saltation impact splash mechanism should be less constrained by shear stress effects limiting growth of exposed ripple crests, because the low‐density Martian atmosphere applies relatively low wind‐related shear stress to ripple surfaces. Other origins for the large Martian ripples are not excluded, however. On Earth, very large ripples with crests unprotected by very coarse grains do not develop due to higher wind‐related shear stresses. Plain Language Summary: Ripples of wind‐blown sand are ubiquitous in terrestrial sandy deserts, and also on the arid surface of Mars. On Earth, aeolian ripples in typical dune sands have wavelengths <30 cm and develop when surface grains are disturbed by numerous rebounding impacts of high‐speed, wind‐driven grains that bounce rapidly downwind. On Mars, rovers have observed aeolian ripples in similar sands but of much greater size, including numerous examples with wavelengths >2 m. There is no consensus explanation why much larger ripples develop on Mars than on Earth. Work reported here describes Computational Fluid Dynamics (CFD) numerical flow experiments using a ground profile modeled after large Martian ripples. Experiments with this modeled surface were conducted in Earth as well as Mars conditions to compare levels of turbulence and shear stress applied by wind to the ripple surfaces. Results indicate that (1) wind‐related stresses would be much weaker on Mars than on Earth and (2) wind flow is less turbulent compared with terrestrial conditions. These results indicate that aeolian impact ripples could grow larger on Mars because lower wind stresses should allow ripple crests to reach greater heights (with corresponding greater wavelengths) than on Earth. Key Points: Computer Fluid Dynamics (CFD) numerical simulations of wind over profiles of large ripples were conducted under Earth and Mars conditionsCFD results indicate larger reverse‐flow vortex regions from crests at ripple lee flanks as well as less turbulence on Mars, compared with EarthCFD results indicate lower shear stresses along ripple surfaces under Mars conditions than Earth conditions [ABSTRACT FROM AUTHOR]

So‐called fairy circles (FCs) comprise a spatially periodic gap pattern in arid grasslands of Namibia and north‐west Western Australia. This pattern has been explained with scale‐dependent ecohydrological feedbacks and the reaction‐diffusion, or Turing mechanism, used in process‐based models that are rooted in physics and pattern‐formation theory. However, a detailed ecological test of the validity of the modelled processes is still lacking.Here, we test in a spinifex‐grassland ecosystem of Western Australia the presence of spatial feedbacks at multiple scales. Drone‐based multispectral analysis and spatially explicit statistics were used to test if grass vitality within five 1‐ha plots depends on the pattern of FCs that are thought to be a critical extra source of water for the surrounding matrix vegetation. We then examined if high‐ and low‐vitality grasses show scale‐dependent feedbacks being indicative of facilitation or competition. Additionally, we assessed facilitation of grass plants for different successional stages after fire at fine scales in 1‐m2 quadrats. Finally, we placed soil moisture sensors under bare soil inside the FC gap and under plants at increasing distances from the FC to test if there is evidence for the 'infiltration feedback' as used in theoretical modelling.We found that high‐vitality grasses were systematically more strongly associated with FCs than low‐vitality grasses. High‐vitality grasses also had highly aggregated patterns at short scales being evidence of positive feedbacks while negative feedbacks occurred at larger scales. Within 1‐m2 quadrats, grass cover and mutual facilitation of plants was greater near the FC edge than further away in the matrix. Soil moisture after rainfall was lowest inside the FC with its weathered surface crust but highest under grass at the gap edge, and then declined towards the matrix, which confirms the infiltration feedback.Synthesis. The study shows that FCs are a critical extra source of water for the dryland vegetation, as predicted by theoretical modelling. The grasses act as 'ecosystem engineers' that modify their hostile, abiotic environment, leading to vegetation self‐organization. Overall, our ecological findings highlight the validity of the scale‐dependent feedbacks that are central to explain this emergent grassland pattern via the reaction‐diffusion or Turing‐instability mechanism. [ABSTRACT FROM AUTHOR]

Numerical analysis of spatial pattern is widely used in ecology to describe the characteristics of floral and faunal distributions. These methods allow attribution of pattern to causal mechanisms by uncovering the specific signatures of patterns and causal agents. For example, grassland‐gap patterns called fairy circles (FCs) in Namibia and Australia are characterized by highly regular and homogenous distributions across landscapes that show spatially periodic ordering. These FCs have been suggested to be caused by both social insects and competitive plant interactions. We compared eight Namibian and Australian FC patterns and also modeled FCs to 16 patterns of social insect nests in Africa, Australia, and America that include the most regular termite mound patterns known. For pattern‐process inference, we used spatial statistics based on both nearest‐neighbor analysis and neighborhood‐density functions. None of the analyzed insect‐nest distributions attain the spatially periodic ordering that is typical of FCs. The inherently more variable patterns of termite and ant nests are commonly attributable to well documented aspects of the faunal life‐history. Our quantitative evidence from drylands shows that the more variable insect‐nest distributions in water‐limited environments cannot explain the characteristic spatial signature of FCs. The analysis demonstrates the interpretation of scale‐dependent neighborhood‐density functions and that it is the identification of unique spatial signatures in regular patterns that need to be linked to process. While our results cannot verify a specific hypothesis, they support the hypothesis that FCs in these drylands are more likely an emergent vegetation pattern caused by strong plant competition for water. Key Points: Regular spatial patterns are common in ecology, but highly ordered patterns with spatial periodicity like fairy circles (FCs) are rareBased on 25 data sets worldwide, we show that typical termite and ant nest patterns in drylands are systematically less ordered than the FCsWe identify termites in drylands as an unlikely cause for periodic FC patterns and discuss plant self‐organization as an alternative cause [ABSTRACT FROM AUTHOR]

Fairy circles (FCs) are extremely ordered round patches of bare soil within arid grasslands of southwestern Africa and northwestern Australia. Their origin is disputed because biotic factors such as insects or abiotic factors such as edaphic and eco-hydrological feedback mechanisms have been suggested to be causal. In this research, we used a multi-scale approach to shed light on the origin of Australian FCs. At a local scale, we investigated the potential cause of FCs using analyses of soil compaction and texture within FCs, the surrounding matrix vegetation, and in nearby large bare-soil areas. We found that soil hardness and clay content were similarly higher inside the FCs and in the large bare-soil areas. When compared to the matrix soils with protective grass cover, the 2.6–2.8 times higher clay content in FCs and large bare-soil areas is likely sourced via multiple abiotic weathering processes. Intense rainfall events, particle dispersion, surface heat, evaporation, and mechanical crust building inhibit plant growth in both areas. At the landscape scale, a systematic survey of 154 soil excavations within FCs was undertaken to evaluate the presence of pavement termitaria that could inhibit plant growth. We show that in up to 100% and most of the excavations per plot, no hard pavement termitaria were present in the FCs. This fact is substantiated by the observation that small, newly forming FCs are initiated on soft sand without evidence of termite activity. At the regional scale, we investigated the spatial properties of FCs and common termite-created gaps in Western Australia, using spatially explicit statistics. We mapped three 25-ha FC plots with a drone and compared them with three aerial images of typical vegetation gaps created by harvester and spinifex termites. We demonstrate that the small diameters, the lower ordering, and the heterogeneous patterns of these common termite gaps strongly differ from the unique spatial signature of FCs. Our multi-scale approach emphasizes that FCs are not trivial termite gaps and that partial correlation with termites at some sites does not imply causation. Instead, we highlight the need to study the edaphic and eco-hydrological drivers of vegetation-pattern formation in water-limited environments. [ABSTRACT FROM AUTHOR]

Abstract: Mass mortality of shrubs, especially the Noaea mucronata species, has been observed in the semi‐arid Negev of Israel since the early twenty‐first century. This has followed a long‐term drought episode, and suggests a hysteresis‐like effect. However, recent studies have revealed that the mortality has been varied across the region. Therefore, we assessed the depth and stoniness of the soil profile, in homogeneous and heterogeneous hillslopes. Then, we studied the volumetric moisture content during two consecutive growing seasons, in the topsoil of shrubby patches and of inter‐shrub spaces in these hillslopes. The study shows that geodiversity – characterized by shallow soil, a high content of stones in the soil, and a high cover of rock fragment on the surface – reduced shrub mortality. This was attributed to the soil moisture content, which was considerably greater in the heterogeneous hillslopes, than that in the homogeneous hillslopes. It is proposed that the shallow soil halted the growth of herbaceous vegetation in the inter‐shrub spaces of the heterogeneous hillslopes. Therefore, under rainstorms, this hillslope configuration results in considerable generation of overland water flow in the inter‐shrub spaces. The water accumulates in the shrubby patches, allowing them to thrive, even during long‐term dry episodes. In hillslopes with a deep soil layer, no stoniness in the soil profile, and no cover of rock fragments, the herbaceous vegetation is well developed, covering a considerable share of the inter‐shrub spaces. This negates runoff formation and source–sink relations, limiting water availability for the shrubs, and resulting in their mass mortality. Despite no direct pastoral value for livestock, the shrubs play an important role in overall ecosystem functioning. This is due to their capacity to transect hydraulic connectivity, and negate ecosystem collapse. We propose a conceptual model for demonstrating the role played by geodiversity in alleviating drought stress in drylands. Copyright © 2018 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Geodiversity has recently been attracting increasing attention as a measure of diversity for the physical components of natural environments. It has shown positive relations with biodiversity, as well as with several ecosystem services. Yet, so far, geodiversity studies have focused on relatively large spatial scales, ranging between hillslope, basin, and landform scales. It is proposed that either natural‐induced or anthropogenic‐induced, small‐scale (centimeter‐scale to few decimeter‐scale) geodiversity has a large impact on the hydrological connectivity and overall functioning of semi‐arid rangelands and other shrubby and woody drylands. It is further proposed that greater small‐scale geodiversity increases the on‐site retention of water and soil resources, decreasing the vulnerability of rangelands to prolonged droughts and climatic changes. Particularly, positive impact of moderate grazing intensity on rangelands functioning is demonstrated by the formation of livestock trampling routes, which transect hillslopes, increase ecosystem geodiversity, and modify the spatial redistribution of scarce water and soil resources at the patch scale. Numerical simulations of a mathematical model for vegetation patterns in water‐limited systems show that the trampling routes increase the survivability of vegetation patches under prolonged droughts. In practical terms, the concept of small‐scale geodiversity is relevant for the determination, monitoring, and assessment of land degradation, as well as for restoration projects of eroded lands and degraded ecosystems. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Vegetation structure and patchiness are central controllers of ecohydrological function in semiarid regions. The feedback interactions between vegetation patchiness and water redistribution make semiarid ecosystems sensitive to state-shifts, where nonlinearities appear in the structure-function correlations. Hydrological connectivity of runoff sources is functionally important for source-sink interactions over a range of spatial scales and plays a key role in ecosystem state-shifts. Accordingly, the study of the functional responses of ecosystems to changes in connectivity is important for assessing the system's resilience in response to drivers of degradation. We used runoff data collected over 18 years in experimentally manipulated plots to study both the primary functional response to the manipulations and the changes in both structure and function over two decades. By comparing simultaneous changes in woody and herbaceous cover, biocrust cover and connectivity, and runoff yield, we examined the interactions among the different cover classes and assessed the functional consequences of these interactions. The manipulated changes in vegetation and biocrust cover caused large differences in runoff yields, with positive correlation between biocrust cover and runoff. However, changes in vegetation patterns reduced these differences, as the spread of herbaceous plant cover, at the expense of biocrust and woody cover, caused a shift in the cover-runoff relationship. The landscape was resilient to degradation due to rapid shrub growth in locations of high biocrust cover. On the other hand, a positive feedback of herbaceous plant cover replacing shrub cover caused a state-shift, likely driven by a combination of drought recurrence and cessation of grazing. [ABSTRACT FROM AUTHOR]

Question: Ecosystem engineer patch dynamics can drive biodiversity patterns by controlling habitat heterogeneity. We studied changes to composition and diversity of shrub patch understorey plant communities to patch stage at a small scale, and to patch dynamics at larger scales. These patch dynamics were driven by widespread mortality of the facilitating engineer shrub Noaea mucronata, following a drought period. Location: Park Shaked LTER station, semi-arid northern Negev Desert, Israel. Methods: We used diversity partitioning and similarity-based metrics to relate annual plant diversity to shrub state at the scale of individual patches, and to the ecosystem engineer shrub mortality rates at larger scales. We separately tested changes to species diversity within patches and to assemblage variation among patches. Results: (1) At the patch scale, annual plant richness and diversity decreased following the loss of shrub canopy. (2) Community similarity among patches increased following shrub mortality, with a significant increase in the abundance of one dominant grass species and a decrease of most other species. (3) Taken together, these factors led to a plot-scale loss of diversity under high rates of shrub mortality. (4) Species richness and evenness metrics were significantly correlated with the shrub mortality gradient. (5) Overall, high mortality rates resulted in landscapes that were more homogeneous in both habitat structure and community composition. Conclusions: Ecosystem engineer population dynamics and the dynamics of their patches can drive community-level processes. In our case, biodiversity patterns are strongly driven by shrub patch dynamics at all scales examined. In other ecosystems where the engineer patches are well studied and easily recognized, they can be used as indicators of ecosystemand community state. [ABSTRACT FROM AUTHOR]

In this article, the author discusses vegetation self-organisation and spatial patterns for the fairy circles of Namibia. The author highlights statistic of spatial pattern analysis, homogeneity and ecohydrological feedbacks. The author mentions vegetation interaction with abiotic aeolian or erosion effects.

The mysterious 'fairy circles' are vegetation-free discs that cover vast areas along the pro-Namib Desert. Despite 30 yr of research their origin remains unknown. Here we adopt a novel approach that focuses on analysis of the spatial patterns of fairy circles obtained from representative 25-ha aerial images of north-west Namibia. We use spatial point pattern analysis to quantify different features of their spatial structures and then critically inspect existing hypotheses with respect to their ability to generate the observed circle patterns. Our working hypothesis is that fairy circles are a self-organized vegetation pattern. Finally, we test if an existing partial-differential-equation model, that was designed to describe vegetation pattern formation, is able to reproduce the characteristic features of the observed fairy circle patterns. The model is based on key-processes in arid areas such as plant competition for water and local resource-biomass feedbacks. The fairy circles showed at all three study areas the same regular spatial distribution patterns, characterized by Voronoi cells with mostly six corners, negative correlations in their size up to a distance of 13 m, and remarkable homogeneity over large spatial scales. These results cast doubts on abiotic gas-leakage along geological lines or social insects as causal agents of their origin. However, our mathematical model was able to generate spatial patterns that agreed quantitatively in all of these features with the observed patterns. This supports the hypothesis that fairy circles are self-organized vegetation patterns that emerge from positive biomass-water feedbacks involving water transport by extended root systems and soil-water diffusion. Future research should search for mechanisms that explain how the different hypotheses can generate the patterns observed here and test the ability of self-organization to match the birth- and death dynamics of fairy circles and their regional patterns in the density and size with respect to environmental gradients. [ABSTRACT FROM AUTHOR]

In this article, we develop a unifying framework for the understanding of spatial vegetation patterns in heterogeneous landscapes. While much recent research has focused on self-organised vegetation the prevailing view is still that biological patchiness is mostly due to top-down control by the physical landscape template, disturbances or predators. We suggest that vegetation patchiness in real landscapes is controlled both by the physical template and by self-organisation simultaneously, and introduce a conceptual model for the relative roles of the two mechanisms. The model considers four factors that control whether vegetation patchiness is emerged or imposed: soil patch size, plant size, resource input and resource availability. The last three factors determine the plant-patch size, and the plant-to-soil patch size ratio determines the impact of self-organisation, which becomes important when this ratio is sufficiently small. A field study and numerical simulations of a mathematical model support the conceptual model and give further insight by providing examples of self-organised and template-controlled vegetation patterns co-occurring in the same landscape. We conclude that real landscapes are generally mixtures of template-induced and self-organised patchiness. Patchiness variability increases due to source-sink resource relations, and decreases for species of larger patch sizes. [ABSTRACT FROM AUTHOR]

Vegetation rings are a unique vegetation pattern found in drylands. Most examples are found in clonal plants growing in sandy soils with confined root zones. Using field measurements and numerical simulations, we found that water overland flow is a predominant mechanism that drives ring formation in the clonal species Asphodelus ramosus. In these rings, an infiltration contrast develops due to aeolian feedback between vegetation and wind‐induced particle transport. Fine particles settle at the patch's centre, reducing water infiltrability compared with that of its perimeter. In turn, this encourages the development of biological soil crust in the ring's centre, further decreasing water infiltration in this micro‐environment. These processes cause the development of surface run‐off source–sink relations between the ring's centre and perimeter. The outcome of this process is the formation of three different micro‐environments: the centre of the patch, characterized by low soil‐water content; its perimeter, characterized by higher soil‐water content; and the matrix, also characterized by higher soil‐water content. Competition for the water resource between the ramets at the perimeter and the ones at the centre leads to dieback, resulting in ring formation. Measurements of soil‐water content in rings of A. ramosus in the semiarid Negev of Israel throughout 5 years showed that the soil‐water content in the ring's centre is lower than that at its perimeter and matrix. These results are in accord with numerical simulations of a mathematical model for plant species with confined roots grown under highly seasonal rainfalls in water‐limited environments. [ABSTRACT FROM AUTHOR]