Determinants of fine-scale plant diversity in dry calcareous grasslands within the Baltic Sea region

► Extensive dataset was used to study plant diversity in alvar grasslands. ► Species richness was the highest under intermediate environmental conditions. ► Richness and phylogenetic diversity were influenced by different processes. ► Areas where species' immigration routes merge were characterized by high richness. ► High phylogenetic diversity was connected to long history of continuous openness.