Abstract
Sixteen stands of dry alvar grassland in Estonia, representing the same vegetation type in similar ecological conditions, were investigated in order to study a considerable variation in species richness within and between stands. Alpha diversity and within stand beta diversity (spatial turnover of species) were negatively correlated. The distribution of species frequencies across the stands was clearly U-shaped, which made it possible to distinguish core species (occurring in more than 75% of stands) and satellite species (occurring in less than 25% of stands), including most of species from the total species pool. Species richness showed significant nestedness-scarce species occurred more often in species-richer stands. Both mean alpha diversity and the size of the community species pool were significantly positively correlated with the number of core species found in a stand, while beta diversity was correlated with the number of satellite species. Species frequencies across the stands were negatively correlated with species average height, seed mass and requirement for soil fertility. These differences were also evident in comparison of core and satellite species by 14 Phylogenetically Independent Contrasts. Also, satellite species had a significantly higher variation in average height and seed size than core species. It was concluded that the community matrix is made up of core species, which are better suited to dry oligotrophic conditions. Beta diversity is largely caused by satellite species, the optimum conditions of which may be found in other neighbouring communities. Further studies have to show whether stands with higher richness and thus a higher number of satellite species are those which were or are surrounded by communities which may potentially act as sources of satellite species.