Small-scale plant species richness in calcareous grasslands determined by the species pool, community age and shoot density

Variation in small-scale plant species richness was examined between twenty discrete sites of calcareous alvar grasslands in the western Estonian coas tal area. This community type is very species rich ar the small-scale. The pattern of diversity has often been explained by variation in ecological co nditions which makes it possible to avoid competitive exclusion. Here we te st an alternative species pool hypothesis. The species pool was defined as the set of species present in a community, but excluding all those species whose presence may be connected with exceptional environmental conditions f or this community. Monte Carlo modelling was used to determine the strength s of the relationships between the non-independent variables. richness vs s pecies pool and average shoot density vs richness. Site age was characteriz ed by its elevation above sea level, since there is a continuous land uplif t in the study area. Species richness was positively correlated to the size of the species pool, community age and vegetation density. We conclude tha t historical processes on both the regional and local levels, which determi ne the arrival of anp particular species to a target community, and not env ironmental heterogeneity, are responsible for the variations in species ric hness between communities of this type.