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Context: Vegetation patterns in hummock grasslands of Australia's arid interior can be very complex. Additionally, the grasslands are interspersed with variable amounts of trees and shrubs. Objectives: To better understand the spatial arrangement of this vegetation structure, and in particular the unvegetated bare-soil gaps, we analyzed the scale-dependent patterns of gaps, trees, and shrubs. Methods: We focused on two size categories of grassland gaps, large gaps ≥ 4 m2 known as fairy circles (FCs) and small gaps 1 to < 4 m2, and on trees and shrubs. We mapped four 200 m × 200 m study plots located east of the town of Newman in Western Australia, using drone-based aerial images and LiDAR. The RGB images were converted into binary images and the gaps and woody plants were automatically segmented. The spatial patterns of the four vegetation components were analyzed, as well as the shape properties of the vegetation gaps. Results: The most striking result was that small gaps appeared consistently at about 5 m distance away from the FCs, which are known as the most water-depleted locations in the grassland. The FCs were also rounder than the small gaps and this symmetry underlines their function as an extra source of water for the surrounding matrix vegetation. Trees and shrubs had spatial patterns that were unrelated to FCs, which likely results from their water uptake in deeper sub-soil layers. Conclusions: The consistent distance of small gaps to FCs is further support that the Australian fairy circles are a self-organized vegetation pattern that results from ecohydrological feedbacks. [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]

Drylands are pattern-forming systems showing self-organized vegetation patchiness, multiplicity of stable states and fronts separating domains of alternative stable states. Pattern dynamics, induced by droughts or disturbances, can result in desertification shifts from patterned vegetation to bare soil. Pattern formation theory suggests various scenarios for such dynamics: an abrupt global shift involving a fast collapse to bare soil, a gradual global shift involving the expansion and coalescence of bare-soil domains and an incipient shift to a hybrid state consisting of stationary bare-soil domains in an otherwise periodic pattern. Using models of dryland vegetation, we address the question of which of these scenarios can be realized. We found that the models can be split into two groups: models that exhibit multiplicity of periodic-pattern and bare-soil states, and models that exhibit, in addition, multiplicity of hybrid states. Furthermore, in all models, we could not identify parameter regimes in which bare-soil domains expand into vegetated domains. The significance of these findings is that, while models belonging to the first group can only exhibit abrupt shifts, models belonging to the second group can also exhibit gradual and incipient shifts. A discussion of open problems concludes the paper. [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]

A letter to the editor is presented in response to the article "Fairy circles or ghosts of termitaria? Pavement termites as alternative causes of circular patterns in vegetation of desert Australia," by F. J. Walsh and colleagues in the 2016 issue.

• In semi-arid and arid environments, clonal plants occasionally appear in rings. • We studied rings in two sites with different soils but comparable climate. • Water source areas for rings differ between sandy and loessial soils. • Ring sizes differ between sites, optimising water source area. • Rings are formed by various mechanisms. In semi-arid and arid environments, clonal plants occasionally appear in ring patterns. There is a general agreement that this pattern forms when ramets grow radially, leaving a dead centre where the parent plant once was. Nevertheless, there is still some controversy over the actual causes of this dieback and how water source–sink relations in and around the rings are involved in their formation. We studied Asphodelus ramosus rings in two sites with different soil textures (sand and loess) but comparable climate, in order to understand whether differences in soil hydraulic properties create different water source–sink relations and mechanisms that drive ring formation. We characterised soil hydraulic properties and dynamics, accompanied by measurements of soil texture and of belowground storage root biomass. We found that the nature of source–sink relations varies with soil texture and properties. In sandy soils, water supply to ring perimeters is mainly from their centres. In loessial soils, water supply to ring perimeters is mainly from the surrounding matrix. Consequently, rings are larger in sandy soils than in loessial soils. This suggests that rings in both sites are formed by different mechanisms: fine particle deposition in the centres of ring in the sandy soil but competition over water in rings in the loessial soil. Studying the formation of rings and other vegetation spatial patterns should consider local soil properties and the possibility that similar patterns may emerge through various processes. [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]