In many countries, including Estonia, the interpretation of forest vegetation-environment relationship and forest typology proceeds from A. K. Cajander’s site type concept. It presumes that the environmental conditions of habitats, the tree layer productivity of forests, and ground vegetation composition are well correlated, whereas the tree layer composition may develop rather independently. We studied (i) how uniform are the soil conditions within the limits of Estonian mesic Hepatica forest site type communities, (ii) how similar the species content of these forests is in different districts, (iii) how the species content similarity is expressed in the different layers of forest communities and, (iv) which environmental factors determine the species richness most essentially. One study area was situated in western Estonia on calcareous soils lying on bedrock limestone, two areas in southern Estonia on parent material with variable calcareousness. The soils of these areas differ remarkably by their morphology and by their chemical properties. The average species richness of vascular plants and bryophyte species in all three areas appeared to be rather similar. The diversity of herb layer species was positively affected only by the habitat lightness, while we did not discover any analysed environmental variable for the moss layer, which had significant effect on the species richness. The species composition in forests of the three study areas differs significantly for the moss, herb and bush layer. For every forest community layer its own system of relationships between the species as well as between the species and environmental factors is characteristic. The relative autonomy of plant community layers is in good accordance with Cajander’s theory of forest site types. In western Estonia the height of trees, the diameter of stems and the timber volume are essentially lower than in southern Estonia due to the dryer soils. Therefore, it seems relevant to divide the Estonian Hepatica site type forests into two subtypes: the first is confined to calcareous soils in the western and northern Estonia and has lower habitat productivity potential for tree layer in comparison with the forests of the second subtype that grow mostly in the eastern and southern Estonia on soils mainly depending on the topographical location and parent material calcareousness but having a higher tree layer productivity.

Key words:Cajander, classification, layers, management, productivity, site type, soils, species richness, stand age, tree growth