Two-phase mosaics, or striped vegetation patterns (densely vegetated bands alternating regularly with bare areas), have been reported in arid and semiarid zones. They can occur provided (a) total rainfall is not sufficient t o maintain a dense cover; and (b) sufficient and uniform sheet flow can compensate, at least partly, for the lack of water. 2 Recent studies demonstrate that vegetation bands follow a successional model, in which bare areas are colonized by a pioneer front. Local studies in South West Niger suggest that transitions could occur between the various patterns observed within the same ecological region, but with varying average rainfall and surface features. 3 Because these transitions could not be followed in the-field without long term studies, a simple model has been elaborated to simulate the different structures observed. This model, based on cellular automata, is derived from the 'ganze of life' and depends only on two hypotheses which reflect conzpetition and synergy: the establishment, growth and survival of a given plant will be affected negatively by the influence of plants situated up-slope and positively by lateral and down-slope plants. 4 A matrix with 9 x 3 elements is applied to the same initial tree distribution grid, with three values of the a coefficient reflecting up-slope resource competition and two values of b reflecting lateral synergies. 5 The results demonstrate that almost all the structures observed in the field can be generated by this simple model, by varying only the two parameters a and 2, and the number of iterations. This result is independent of the initial tree density, showing that observed structures could equally well be derived from more or less dense vegetation patterns.

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