The causes of moss rarity were analysed in three neighbouring countries. Estonia, Latvia and Lithuania, situated along the coast of the Baltic Sea, cover together 175,000km2. They belong to the same vegetation zone, and have similar economic and natural history, but there are still certain differences in climate, bedrock, soil and land use. Three frequency groups of mosses, altogether 184 species, were compared in this study: (1) mosses present in all three countries and frequent in every country, (2) mosses present in all three countries, but rare in every country and (3) mosses present in only one country and rare in this country, i.e. very rare mosses. The distribution of moss species according to families, life history characters (sexuality, spore size and life span) and ecological characters (substrate and community type preferences) were compared among these three groups. There were differences in the distribution of species according to families between the three frequency groups. Frequent and rare moss species groups were also characterised by different life historical (life span) and ecological characteristics (substrate pH and community type). Comparison of ecological indexes in the group of country specific mosses revealed that species preferences regarding soil alkalinity follow the pattern of the distribution of alkaline soils in the three countries. Species occurring in the most southern country - Lithuania - are characterised by highest temperature indexes. On the basis of the studied species we can conclude that taxonomical and life historical trends explain one part of rarity in the studied region, while local ecological conditions in connection with land use are responsible for the other part.
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