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We report on a combined theoretical-experimental study of ring formation by plant species in water-limited systems. We explored, under controlled laboratory conditions, the relationships between water regime and ring formation using a small perennial grass, Poa bulbosa L., as a model species. Our experimental studies show that ring-shaped patches are formed as a result of differential biomass growth across the patches, in spite of uniform water supply. The theoretical studies were based on a mathematical model for vegetation pattern formation that captures feedback processes between biomass growth and water exploitation. Our model studies reproduce the experimental findings and predict that spots destabilize to form rings in species whose lateral root augmentation per unit biomass growth is sufficiently small. This finding explains why rings in nature are formed by dense clonal plants. (C) 2007 Elsevier B.V. All rights reserved.