Patterns of vegetation biomass are typical of (semi-)arid regions where the potential evapotranspiration substantially exceeds the mean annual precipitation. This hydric deficit impedes the development of individual plants and, at the community level, promotes clustering behaviors which, via a modulational instability, take shape even if the topography is isotropic.

(a): Sub-Sahelian gapped landscape dominated by the shrub species Combretum micranthum G. Don in South-West Niger. Shrub crown radius and bare spots distance are approximately 1.75 m and 50 m (photography: N. Barbier). (b): Typical example of tiger bush in Niger. Width of vegetated bands: approximately 50 m; width of bare soil bands: approximately 50 m (photography: courtesy of C. Valentin). (c): Example of fairy circle in the pro-Namib zone of the west coast of southern Africa (photography: courtesy of J. Vergeer).

The adaptation of root systems (rhizospheres) to water scarcity is a determinant factor of this dynamics. Confronted to drought, plants strive to maintain their water uptakes by seizing hold of more territory. To this end, they increase their roots spread which, amazingly, may overshoot the aerial structure (crown) by one order of magnitude. This adaptation however increases plant-to-plant competition and, correlatively, the importance of counterbalancing feedbacks, called facilitation, which the biomass exerts on its own growth. Understanding how such local adaptation and interactions allow for the formation of patterns at the landscape scale is a fundamental ecological question.