The pine forests on shallow calcareous rendzina-like soils have a limited distribution in the northern Europe. Due to the peculiar ecological conditions, these forests include different floroelements together with red-listed species and have high value for biodiversity conservation. The calcareous pine forests are well represented in two biggest islands of the Baltic Sea – in Saaremaa and Gotland. The aim of the paper was to compare the floristic composition and environment conditions of calcareous pine forests on these islands, to elucidate their typological similarities and divergence, and to estimate the characteristic/indicator species in the established forest types. In Gotland 34 stands and in Saaremaa 35 stands were studied on 0.1 ha sample plots. For data processing the detrended correspondence analysis, cluster analysis, the multi-response permutation procedure, the indicator species analysis and the multiple regression analysis were used. The soil humus horizon chemistry characteristics (pH, nitrogen and carbon content, and their ratio) in the considered forests vary within quite narrow limits; the difference of soils is mainly expressed by the humus horizon thickness, from which depends the soil water holding capacity. Climatically Saaremaa has a lower average temperatures and more precipitation than Gotland. For calcareous pine forests on both islands numerous significant indicator species are characteristic, whereby this list is two times longer for Gotland. The total number of species in Saaremaa depends significantly (negatively) only on the dominating pine trees abundance, in the forests of Gotland the shrub layer coverage has cardinal importance. On the number of shrub layer species on both islands the shrub layer coverage has a significant impact but in Gotland also abundance of spruce has considerable effect. On the communities level the disagreement between the compared islands is even more conspicuous than on the species level: from nine established community types three are connected exclusively with one island, with Saaremaa or Gotland; on both islands only communities of two types are represented more or less evenly.

Key words: Alvar forests, classification structure, human impact, indicator species, Rendzinas, species diversity.