Is species richness dependent on the neighbouring stands? An analysis of the community patterns in mountain grasslands of central Argentina

We studied the spatial pattern of species richness in the mountain grasslan ds of central Argentina, where three distinct vegetation types occur: short grass in shallow soils. tallgrass in deeper soils and Het turf in moist dep ressions. The shortgrass community had the highest vascular plant species r ichness on different scales. the maximal number of vascular plant species p er m(2) was 89. The richness within the shortgrass vegetation was not relat ed to spatial location or soil depth. Small-scale species richness was sign ifically positively related to the size of the community species pool. With in shortgrass and tallgrass communities, the floristic similarity between s tands 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 short grass and other communities (tallgrass and wet turf) was significantly high er when stands were located closer in space. This pattern was interpreted a s 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 predominatin g species was lower in the shortgrass than in the tallgrass or wet turf com munities. In communities with a tall canopy, species composition and richne ss were not dependent on surrounding communities, which may he explained by the relatively more significant competition for light there. In the shortg rass 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 richn ess of native species. The highest proportion of exotics was observed in th e wet turf community. which was also the one most heavily grazed.