Vegetation ring formation by water overland flow in water-limited environments: Field measurements and mathematical modelling

Vegetation rings are a unique vegetation pattern found in drylands. Most examples are found in clonal plants growing in sandy soils with confined root zones. Using field measurements and numerical simulations, we found that water overland flow is a predominant mechanism that drives ring formation in the clonal species Asphodelus ramosus. In these rings, an infiltration contrast develops due to aeolian feedback between vegetation and wind-induced particle transport. Fine particles settle at the patch's centre, reducing water infiltrability compared with that of its perimeter. In turn, this encourages the development of biological soil crust in the ring's centre, further decreasing water infiltration in this micro-environment. These processes cause the development of surface run-off source-sink relations between the ring's centre and perimeter. The outcome of this process is the formation of three different micro-environments: the centre of the patch, characterized by low soil-water content; its perimeter, characterized by higher soil-water content; and the matrix, also characterized by higher soil-water content. Competition for the water resource between the ramets at the perimeter and the ones at the centre leads to dieback, resulting in ring formation. Measurements of soil-water content in rings of A. ramosus in the semiarid Negev of Israel throughout 5 years showed that the soil-water content in the ring's centre is lower than that at its perimeter and matrix. These results are in accord with numerical simulations of a mathematical model for plant species with confined roots grown under highly seasonal rainfalls in water-limited environments.