Impact of various soil hydraulic properties and water source-sink relations on plant rings formation and sizes under arid conditions

In semi-arid and arid environments, clonal plants occasionally appear in ring patterns with a barren soil centre. There is a general agreement that this pattern forms when ramets grow radially, leaving a dead centre where the parent plant once was. Nevertheless, there is still some controversy over the nature of water source-sink relations in and around the rings, and the mechanism that drive the ring formation. We studied Asphodelus ramosus rings in two sites with different soil types (sand and loess) but comparable climate, in order to understand whether differences in soil hydraulic properties create different water source-sink relations and mechanisms that drive ring formation. We characterised soil hydraulic properties and dynamics along the rainy season and during the dry season, accompanied by measurements of soil texture and of belowground storage root biomass. We found that the nature of source-sink relations varies with soil type and properties. In sandy soils, water supply to ring perimeters is mainly from their centre. In loessial soils, water supply to ring perimeters is mainly from the surrounding matrix. Consequently, rings are larger in sandy soils than in loessial soils, in each case in order to optimise water source area. Therefore, studying the formation of rings and other vegetation spatial patterns should consider local soil properties and the possibility that similar patterns may emerge through various mechanisms.