Abstract
In arid regions, according to self-organization models, the spatial structure of periodic vegetations depends on local processes (facilitation or competition). The current study verified the importance of these processes and the role of Combretum micranthum G. Don through a detailed description of the spatial distribution of ligneous species in gapped bush vegetation of southwest Niger, using Ripley’s K function. Spatial organization of shrubs reveals a two-scale structure: (i) aggregative (average radius, 9–15 m) and (ii) repulsive (at about 35 m), corresponding to the general periodic pattern. Only C. micranthum presents a similar distribution, which suggests that it could be the ‘‘keystone species’’, determiningthe vegetation structure. A local aggregation of secondary species around C. micranthum strongly suggests facilitation at ashort distance (1–3 m), proportional to the crown’s average radius of mature shrubs, in accordance with model predictionsand empirical studies of soil hydrodynamics. The absence of spatial relations between C. micranthum and the other speciesof the family Combretaceae, which tend to cluster together, suggests the existence of a three-component structure. Theseelements open up new perspectives for field research and for the improvement of models, which are essential for the man-agement of Sahelian vegetations against the exploitation and aridity constraints.