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Semiarid gypsum karstlands in north central Mexico are characterized by a mosaic of shrubland and endemic gypsophile grassland community types. Proximal physical factors affecting community patterns on a local scale were investigated at an undisturbed site south of Matehuala, San Luis Potosi. Soil surface characteristics associated with depth of calcareous alluvium overlying gypsum base material were strongly correlated with distribution of desert shrub and endemic grass species. Muhlenbergia purpusii was dominant on bare gypsum soils with indurated crusts, while Bouteloua chasei was dominant where a thin veneer of alluvium permitted development of a cryptogamic crust. A high-diversity mixed shrub-succulent community occupied gypsum overlain by 5 - 20 cm of alluvium, whereas Larrea tridentata was the principal species on gypsum overlain by > 20 cm of alluvium. Open sink walls were occupied mostly by species of the Muhlenbergia grassland, while closed sinks supported more mesophytic mixed shrubland vegetation.

Physical factors operating mostly at the establishment stage are probably more important in mediating local species patterns on gypsum than factors associated with mineral nutrition. Gypsum material was much less fertile than alluvial material but did not exclude widely distributed species unless exposed at the surface.