Ring formation in clonal plants

Ring shaped patches of clonal plants fascinated plant ecologists since long time. In this work we review the reports on the occurrence of ring pattern in different environmental conditions, the growth forms of ring-forming plants, the mechanisms underlying ring formation, and the consequences for species diversity at community scale. Rings formed by 83 species of clonal vascular plants have been found in grasslands, deserts, bare substrates of lava flow, harvested peat lands, salt marshes, and sand dunes. Four causal hypotheses have been proposed for the emergence of ring patterns: i. occurrence of architectural constraints for ramets development; ii. induction by fire, drought, trampling or overgrazing; iii. nutrient and water depletion by competition inside the ring; and iv. onset of species-specific negative plant-soil feedback in the inner zone of the clone. Since almost all the available studies are observations of ring structure or modelling exercises, none of the putative mechanisms for ring formation emerged from the literature as either generally applicable or suitable for rejection. Therefore, long-term field experiments are needed to investigate the relative prevalence of different mechanisms in different environments. Ring formation bears important consequences at community scale, because ring forming plants often act as “nurses”, enhancing the recruitment and development of different plant species. In fact, ring establishment modifies above- and below-ground environmental conditions, providing specialized safe sites for beneficiaries in the inner zone of the clones. Such interspecific facilitation by ring forming plants, particularly in chronically stressed environments, contributes to increase plant species richness and can locally promote the successional dynamics.