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Plant species distribution along a gradient of precipitation was analysed in western Patagonia. First, the affinity of 53 vegetation releves was tested by principal component analysis of the matrix of n x n binary correlations among species cover values. Further, the relationship between plant cover (Festuca pallescens, shrubs, and Stipa spp.) and the environmental factors (precipitation, soil texture, soil depth, altitude, landscape form, aspect, and disturbance) was analysed by stepwise regression analysis. Five homogeneous vegetation units could be identified at several different regions of the gradient. Soil characteristics associated to precipitation levels (i.e. texture, profile depth), but not precipitation per se, were strongly related to the cover of F. pallescens and shrub species. A model of transitions in the composition of vegetation is introduced which includes soil variables, site conditions and the spatial distribution of grazing herds.