We studied the spatial pattern of species richness in the mountain grasslands of central Argentina, where three distinct vegetation types occur: shortgrass in shallow soils, tallgrass in deeper soils and wet turf in moist depressions. The shortgrass community had the highest vascular plant species richness on different scales, the maximal number of vascular plant species per m2 was 89. The richness within the shortgrass vegetation was not related to spatial location or soil depth. Small-scale species richness was significally positively related to the size of the community species pool. Within shortgrass and tallgrass communities, the floristic similarity between stands was higher when they were located closer to each other. This pattern was interpreted as showing the internal diaspore limitation. The number of species in shortgrass vegetation was significantly positively dependent on the proximity of tallgrass stands. Also, floristic similarity between shortgrass and other communities (tallgrass and wet turf) was significantly higher when stands were located closer in space. This pattern was interpreted as showing the external diaspore limitation, called also spatial mass effect - other communities act as seed donors and increase the number of species coexisting in the shortgrass community. The mean height of the predominating species was lower in the shortgrass than in the tallgrass or wet turf communities. In communities with a tall canopy, species composition and richness were not dependent on surrounding communities, which may be explained by the relatively more significant competition for light there. In the shortgrass community, where the canopy is lower, the pattern of species richness was related to the vicinity of potential diaspore sources, which indicates the relative significance of internal and external diaspore limitation. The proportion of exotic species in the community was not related to the richness of native species. The highest proportion of exotics was observed in the wet turf community, which was also the one most heavily grazed.