Ring-formation in a Namibian resurrection grass and its relation to climate, neighborhood competition and soil microbes

Ring-formation in plants is observed worldwide in drylands. The mechanisms underlying the shift from spot-like tussocks to open rings with a central dieback can be multi-facetted and are often not clearly discernable. We therefore applied a holistic approach to study a wide-spread ring-forming species in Namibia – the resurrection grass Eragrostis nindensis.We mapped 200 individual rings in 43 locations and drone-mapped more than 1000 rings to infer potential neighborhood competition from the patterns of rings. We measured the volumetric soil-water content within rings and outside, and we analyzed the microbial composition in these soils.Aridity had an effect because rings formed primarily when mean annual precipitation (MAP) was below 250 mm. However, the extent of the central dieback at a certain location was mainly determined by edaphic conditions such as the stoniness of soils, rather than by the climatic aridity of an area. Drone-mapped rings did not show strongly regular patterns, indicating the absence of strong intra-specific neighborhood competition. However, competition between neighboring tillers at the scale of individual rings was probably strong because there was significantly lower soil moisture inside the rings as compared to the outside. We identified no potentially phyto-pathogenic bacteria or fungi inside the rings...