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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]

In a novel study, Getzin et al. (2022) have excavated 500 grasses at four regions of the Namib to systematically investigate the temporal process of how the young grasses die in fairy circles. Based on measurements of the root lengths, statistical testing, and comparative photo documentations the authors showed that sand termite herbivory did not cause the death of the freshly germinated grasses within fairy circles (FCs). Roots of those dead grasses were initially undamaged and even longer than those of the living grasses outside in the vegetation matrix, which is contrary to termite herbivory. The dying annual grasses within FCs had significantly higher root-to-shoot ratios than the vital grasses in the matrix, both of which can be attributed to the same grass-triggering rain event. This indicates that they died from water stress because the desiccating grasses invested biomass resources into roots, trying to reach the deeper soil layers with more moisture, but they failed. Jürgens and Gröngröft (2023) commented on our research findings. Here, we shed light on their statements by investigating the existing data evidence on the Namib fairy circles, which includes a thorough literature review about the proposed termite-feeding mechanism, as well as describing the properties of soil water within and around the FCs. Our review shows that there is no single study to date that has demonstrated with systematic field evidence in the form of root measurements and data from several regions of the Namib that the green germinating grasses within the FCs would be killed by root herbivory of sand termites. We emphasize that the top 10 cm of soil in the FCs is very susceptible to drying out. In this topsoil layer, the freshly germinated grasses with their 10 cm long roots die quickly after rainfall due to lack of water, because these small plants cannot reach and utilize the higher soil moisture content, which is only found in deeper soil layers below the dry topsoil. Based on 400 measurements of soil moisture during the rainy season 2024, we show that the topsoil in the FCs is significantly drier than in the matrix outside. Finally, we show that the soil physical conditions allow a very high hydraulic conductivity that supports the "uptake-diffusion feedback" during the first weeks after grass-triggering rainfall. During the first two weeks, the soil moisture at 20 cm depth ranged for several rainfall events between 9% and 18% within the FCs, hence way above the 6–8% threshold below which the hydraulic conductivity strongly declines. Even 20 days after rainfall, soil moisture was still above 8%. During this biologically active period, new grasses germinate after about five days, the large perennial grasses along the FC edge resprout and strongly draw water with their established root system at 20–30 cm depth, and the freshly germinated grasses in the FCs desiccate and die within 10–20 days. With our continuous soil moisture measurements, we argue that the quickly greening and competitively superior grasses on the FC edge, as well as the vital matrix grasses, draw soil water from the FCs. This rapid depletion of soil water and drying out of the topsoil leads to the death of the new grasses in the fairy circles. • The sand termite theory and proposed mechanism of root feeding on green germinated grasses lacks fundamental field evidence. • During the rainy season, the topsoil inside the fairy circles is significantly drier than the topsoil in the matrix outside. • The freshly germinated grasses with their 10 cm long roots die in fairy circles due to lack of water in the upper topsoil. • The hydraulic conductivity is high up to 20 days after rainfall. The uptake-diffusion feedback is supported by field data. [ABSTRACT FROM AUTHOR]

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]

The fairy circles of Namibia form a remarkable gap pattern in arid grassland along the Namib Desert. The origin of the fairy circles is subject to an ongoing debate. Solving the mystery of the fairy circles (FCs) requires the right timing in fieldwork after rainfall, as the newly appearing grasses complete their life cycle within only a few weeks. Here we followed the rains along the Namib between 2020 and 2022 and assessed the cause of the grass death within FCs at different time intervals after grass-triggering rainfall. To assess whether termite herbivory was the cause, we used grass excavations and observations on the roots and shoots. To test if edaphic differences may explain the grass death in FCs, we undertook infiltration measurements in 10 FC-hotspot regions. Finally, we used continuous soil-moisture measurements from the dry into the rainy seasons to examine how the newly emerging grasses affect the soil-water content in space and time. Generally, in study plots that received grass-triggering rainfall most recently, the roots of the dead grasses in FCs were in 100 % of the cases undamaged, root-shoot ratios were significantly greater, and the roots were as long or even longer as those of the surrounding matrix grasses outside of the FCs. This indicates that drought stress caused grasses in the FCs to invest resources into roots to reach the percolating water. The results also show that the cause of the grass death in fairy circles was not induced by termite herbivory. Also, we found no systematic differences in the rate of water infiltration between FCs and the matrix, hence the plant wilting cannot result from quicker percolation within FCs. However, the soil-moisture measurements indicate that the matrix grasses strongly depleted the upper soil water of the FCs after rainfall, which explains why most grasses cannot establish and quickly die in the FCs. The research shows that grass death in fairy circles occurs immediately after rainfall due to plant water stress but not due to termite activity. Our results conform with previous fieldwork, pattern analysis and theoretical modeling, suggesting that Namibia's fairy circles are a self-organized vegetation phenomenon induced by ecohydrological feedbacks. • Namibia's fairy circles (FCs) were investigated in 10 regions along the Namib Desert. • After grass-triggering rainfall, new grasses within FCs die instantly due to wilting. • Termite herbivory is not causing this grass death as the plants had undamaged roots. • Soil-moisture data indicate that the matrix grasses severely depleted the water in FCs. • Ecohydrological biomass-water feedbacks and plant self-organization likely cause the FCs. [ABSTRACT FROM AUTHOR]