Is small-scale species richness limited by seed availability or microsite availability?

To evaluate the relative roles of seed availability and competitive interac tions in creating within-community patterns of species richness in unproduc tive grassland, we conducted a sowing experiment in a dry calcareous (alvar ) grassland, where both the number of arriving seeds and the number of arri ving species were approximately doubled compared to the natural seed rain. Also, in half of the plots, 36% of the vegetation and bryophyte cover was r emoved to simulate disturbance. Sowing significantly increased species rich ness and the number of seedlings in plots. Disturbance increased the number of seedlings but had no significant effect on species richness. In the fir st year, the highest number of seedlings was found in disturbed and sown pl ots. The dynamics of seedling numbers differed among species. Of the 15 sow n species, seedlings of nine species were found in some plots. The number o f seedlings of two species were not dependent on treatments, those of three species depended only on sowing, and for four species there was a signific ant positive interaction between sowing and disturbance. The establishment of sown species was not dependent on initial species richness or number of adult ramets in experimental plots. It was concluded that, though the behav ior of individual species may differ, the local deficiency of seeds may be an important force generating small-scale community patterns of calcareous grasslands.