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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]

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]

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]

• In arid environments, Stipagrostis obtusa occasionally appears in rings. • The soil water content is higher in the inter-ring spaces than in the ring's center. • The penetration resistance is higher in the inter-ring spaces than in the ring's center. • The rings form small mounds due to sand and dust deposition. • Formation of S. obtusa rings is mainly attributed to the infiltration feedback. In drylands, particularly in sandy environments, clonal plants occasionally develop into ring patterns that slowly expand radially, leaving bare soil in the ring's center. Several mechanisms have been suggested to explain ring formation, and yet it is very difficult to find out the dominant driving mechanism for each specific ecosystem. This study is the first to assess the rings of Stipagrostis obtusa in the north-eastern Negev of Israel, an arid region with mean annual precipitation of ≈65 mm. Our results propose that S. obtusa rings form in response to the infiltration feedback, which is enhanced by microtopography; the rings developed on small mounds due to continuous deposition of sand and dust. The infiltration feedback is caused by the sharp contrast in water infiltrability between the rings and the inter-ring spaces, where infiltration is lower. In addition, during heavy rainstorms, runoff generated in the inter-ring spaces partially percolates in the rings' periphery and enhances radial expansion. These source–sink relations increase water availability for the S. obtusa plants, increasing their resilience to the harsh dryland conditions. [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]

Vegetation gap patterns in arid grasslands, such as the "fairy circles" of Namibia, are one of nature's greatest mysteries and subject to a lively debate on their origin. They are characterized by small-scale hexagonal ordering of circular bare-soil gaps that persists uniformly in the landscape scale to form a homogeneous distribution. Pattern-formation theory predicts that such highly ordered gap patterns should be found also in other water-limited systems across the globe, even if the mechanisms of their formation are different. Here we report that so far unknown fairy circles with the same spatial structure exist 10,000 km away from Namibia in the remote outback of Australia. Combining fieldwork, remote sensing, spatial pattern analysis, and process-based mathematical modeling, we demonstrate that these patterns emerge by self-organization, with no correlation with termite activity; the driving mechanism is a positive biomass- water feedback associated with water runoff and biomass-dependent infiltration rates. The remarkable match between the patterns of Australian and Namibian fairy circles and model results indicate that both patterns emerge from a nonuniform stationary instability, supporting a central universality principle of pattern-formation theory. Applied to the context of dryland vegetation, this principle predicts that different systems that go through the same instability type will show similar vegetation patterns even if the feedback mechanisms and resulting soil-water distributions are different, as we indeed found by comparing the Australian and the Namibian fairy-circle ecosystems. These results suggest that biomass-water feedbacks and resultant vegetation gap patterns are likely more common in remote drylands than is currently known. [ABSTRACT FROM AUTHOR]

Vegetation rings are a common pattern in water-limited environments and mostly occur in clonal plants. This study presents, for the first time, rings of the geophyte species Urginea maritima. The rings, typically 40–90 cm in diameter, are abundant in the sandy environment of Little Petra and Wadi Rum, in the southern Jordanian drylands. Soil properties were studied in the rings' center, periphery, and matrix. Soil-water volumetric content was significantly higher in the rings' periphery than in the center and matrix. The soil organic carbon was highest in the periphery, intermediate in the center, and lowest in the matrix. At the same time, the soil texture, hydraulic conductivity, and gravimetric moisture content at the hygroscopic level were similar in the three microenvironments. According to the results, a possible ring formation mechanism is the soil-water uptake mechanism, which results in competition between the plants at the periphery and those in the center and is generally attributed to plants with large lateral root zones. Numerical simulations of a mathematical model implemented in this study support the soil-water uptake mechanism. A second possible mechanism is negative plant-soil feedback due to the accumulation of dead biomass and its consequent decomposition, with the resultant release of autotoxic compounds. It is possible that several mechanisms occur simultaneously and synergistically affect the formation of U. maritima rings. [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]

Nebkhas, also known as nebkha dunes or coppice dunes, are a unique biogeomorphological aeolian landform that is common in arid and semi-arid regions. They are often regarded as a signal of regional desertification and could be potential dust sources due to their relatively large content of fine sediments, but they also serve as "fertile islands" that enhance biodiversity at a micro-habitat. How the interactions between plant growth and aeolian sand deposition affect the formation and development of nebkhas has not been well documented, though. In this study, the chronology and morphological features of nebkhas in the Mu Us dune field, north-central China were studied using radionuclide (137Cs) dating and unmanned aerial vehicle (UAV) photogrammetric survey. The results show that the nebkhas are mainly distributed around lake basins and dry river valleys with relatively flat topography. Most nebkhas have heights around 0.5–1.5 m, and according to a new integrated age-height model, they mostly formed in the last fifty years. Widespread nebkha growth over that time is interpreted as a response to both desiccation of lake beds and river valleys and a pronounced decline in strong wind frequency. The regular rounded shape of the nebkhas in the study area may reflect the growth form of prostrate, extensively branched Nitraria tangutorum , the dominant formative shrub, and response to the low-energy wind environment. A nebkha's height and horizontal length increase synchronously in the early stage of the development, while the increase in horizontal length and projected area accelerates when the dunes become larger. Feedbacks involving both aeolian sand transport and shrub growth can explain a decline in growth rate with increasing height and an apparent height limit of about 4 m. This study also implies that nebkhas are important biogeomorphological features demonstrating how biotic and abiotic interactions can shape landforms and influence ecological functioning in arid and semi-arid environments. • Nebkhas are a unique biogeomorphological landform. • Nebkhas can have rounded or elongated shape under different wind regimes. • Most nebkhas in the Mu Us dune field formed during the last fifty years. • The growth limit of nebkhas is controlled by plant-aeolian sand feedbacks. [ABSTRACT FROM AUTHOR]

Geodiversity effects on ecosystem productivity were investigated using remotely sensed data and a mathematical model, by introducing a spatially non-uniform surface-water infiltration. Two hillslope types, with different geodiversity levels, as determined by the soil depth and stoniness, were selected for this study. Low-geodiversity hillslopes ('homogenous hillslopes') were identified by their dense herbaceous vegetation cover, and lack of rock fragments cover. High-geodiversity hillslopes ('heterogeneous hillslopes') are characterized by a more sparse cover of herbaceous vegetation, and relatively high rock fragment cover (>20%). Calculating Soil Water Index (SWI) from Landsat-5 TM, Landsat-7 ETM+ (SLC-ON) and Landsat-8 satellite images for the years 1994–2017 shows that during most of these years, SWI was higher at the heterogeneous hillslopes. This was particularly prominent after 2009 when mass mortality of shrubs took place on the homogenous hillslopes but not in the heterogeneous ones. Numerical simulations show that after a prolonged drought, the vegetation in the homogenous system dried out, and the system did not recover even after the yearly rainfall amounts returned to normal. In the heterogeneous system, the vegetation patches survived the drought episode and then recovered. This demonstrates the crucial role played by hillslope geodiversity in determining the durability of vegetation under prolonged droughts in semiarid regions. • The study area includes two hillslope types that differ by their geodiversity. • Soil Water Index (SWI) was calculated from satellite images throughout 1994–2017. • During most of these years, SWI was higher at the heterogeneous hillslopes. • Geodiversity, expressed by stoniness, increases water availability for shrubs. • Numerical simulations of vegetation patches subjected to droughts support this view. [ABSTRACT FROM AUTHOR]