Alvar grasslands in Estonia: variation in species composition and community structure

. In order to understand the variation of Estonian calcareous thin‐soil grasslands on Ordovician or Silurian limestone (alvars), 58 stands, distributed over the whole alvar region of Estonia, were described and clustered numerically using TABORD. Alvars are characterized by a high species richness. These mainly semi‐natural communities have mostly developed after clear‐cutting of forests. Grazing by domestic animals and removal of woody plants is needed for their maintenance. Primary (natural) alvar grasslands are found in a few places in coastal regions or in areas with thin‐soil on monolithic bedrock. In data processing a whole stand, described by several small relevés, was used in the classification as one description, including species frequencies as a quantitative measure. Seven clusters were separated and their configuration checked in a PCA‐ordination. The resulting classification agreed with the previous ‘rough’ classification. Both geographical and ecological factors were related with the clustering and the ordination. The broad classification units, suggested for Estonian alvar grasslands, were quite similar to the ones described earlier for alvar vegetation on Öland, Sweden. Clusters differed in their species richness, environmental conditions and life‐form spectra. Differences in species richness were defined by regional species pools. Compared to earlier surveys, only a small fraction of alvars still remains in Estonia. Active conservation, i.e. suitable management is needed to protect the still surviving valuable plant communities. The present study can provide guidelines on how to protect and manage different types of alvars in Estonia.