Plant-microbe interactions as a cause of ring formation in Bouteloua gracilis

Patterned vegetation growth such as grass rings is found in many arid ecosystems, yet the mechanisms behind their formation are often unknown and have been minimally tested in the field. One explanation is pathogen accumulation in the center of a long-lived plant, which could cause central dieback and the formation of a ring as the plant grows toward pathogen-free soil. We tested this mechanism by comparing the growth of blue grama grass (Bouteloua gracilis) in live and sterilized soils from inside or outside naturally occurring grass rings. Field-collected roots from the inner edge of grass rings had higher fungal colonization than roots from the outer edge, suggesting the potential for intensified pathogen interactions on the inside of rings. However, while plants grown in live soils performed worse than those in sterile soils, this pathogenic effect did not differ between soils collected from inside versus outside of grass rings. Further work on the spatial distribution of plant-microbe interactions is needed to confirm their direct role in ring formation. Our findings suggest that soil and root microbes, in addition to known mechanisms such as soil hydrology, potentially promote ring formation of a widespread North American grass species.