Edaphic properties enable facilitative and competitive interactions resulting in fairy circle formation

Millions of generally regularly spaced, roughly circular barren patches called fairy circles occur in a narrow band ca 100 km inland of the south‐west African coast. These generally have conspicuously taller peripheral grasses in a shorter grass matrix. The origins of these fairy circles are controversial, but one possibility is that they are self‐organizing emergent vegetation patterns that are the consequence of interplay between positive (facilitative) and negative (competitive) interactions between grasses. We hypothesized that the coarse textured sand on which fairy circles occur creates a hydraulically and nutritionally connected landscape, in which neighbouring fairy circles competitively influence each other over several metres, while providing opportunity for focusing of resources around the peripheral grasses. To test our hypotheses we conducted three main groups of analyses: 1) we measured grass biomass to assess facilitative and competitive effects of the component grasses; 2) across a region with fairy circles we measured the size and density of fairy circles and correlated that with water infiltration rates into soil; 3) we measured the capacity of soil to conduct water pulses and 15 N tracers. We found evidence of facilitative interactions in the periphery of the fairy circles and competitive suppression of the matrix grass proximal to the periphery. Across the region, fairy circle size was positively correlated with soil infiltration rates and negatively with precipitation. This suggests that fairy circles emerge in soils with high capacity for water flux that enables landscape hydraulic connectivity. Water‐ and 15 N‐pulse experiments showed that edaphic resources were highly mobile, moving up to 7.5 m over a period of 1–3 weeks. We concluded that the evidence is consistent with an emergent vegetation pattern explanation for the origins of fairy circles and that the circles are more closely associated with a highly connective edaphic environment, rather than with particular biota.