Exploring the causes and patterns of ring-formation in plants in an unknown diversity hotspot of the Namib Desert

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Ring formation in plants of the global drylands has attracted great interest in the research community, particularly regarding the mechanisms that generate these symmetric shapes. Here, plant rings include “tussock rings”, which result from slow vegetative growth processes of single tussock plants, and a new class of plant rings: “collective plant rings”, which were discovered in 2021 in the Namib Desert. Collective plant rings are composed of several individuals of annual grasses or annual forbs or a mixture of both, and they pop up spontaneously after rainfall events in the eastern Namib Desert. The function of these rings is currently unsolved and it is also unclear how their diameters, shapes, density, and spatial patterns change from rainy season to rainy season. Preliminary investigations suggest that the annual grasses and forbs form rings up to 1 m in diameter to collectively deplete the soil moisture within the rings. These rings could potentially be a consequence of vegetation self-organization whereby the individual plants on the ring periphery facilitate each other to increase their own biomass. However, in-depth research is necessary to better understand this new class of plant rings. Surprisingly, in 2021, altogether 10 different grass and forb species in the Namib desert were discovered to be ring-forming, making the Namib a genuine “diversity hotspot” of plant-ring formation. It is a major research gap that no information on the biogeographic diversity and range of plant rings exists for this ecosystem. Only the famous fairy circles have been widely studied in the Namib, but these vegetation gaps have much larger diameters than the plant rings, and are a different phenomenon. We will study the tremendous wealth of ring-forming plants in arid southwestern Africa. Our pioneering research will include a whole set of fieldwork methods. Using repeated drone monitoring during the rainy seasons, we will map both tussock rings and collective plant rings. From the drone data, we will derive digital terrain models and relate variables of topographical heterogeneity to the sizes, shapes, and spatial patterns of the rings. Potential aeolian feedback effects between soil and ring enlargement or ring shape will be investigated with in-situ measurements on infiltration rates, soil moisture, plant height and root length. Continuous soil-moisture measurements with data loggers will be used to test for biomass-water feedbacks and potential self-organization processes. Manipulative experiments on collective plant rings will investigate the cause of the rings, and all types of plant rings will be analyzed for microbial pathogens that may also contribute to their formation. Ultimately, our results will shed light on the basic processes of vegetation growth in water-stressed ecosystems that are increasingly prone to desertification under climate change.