Due to originating from various mire or paludified forests and consequently developing after drainage under different growth con - ditions, the drained forests are very heterogeneous and complex. In the official Estonian forest typology, the old-drained stands are divided into Myrtillus and Oxalis site types, but recently the validity of the autonomous Dryopteris (expansa) forest site type was again asserted. The aims of the current study were to (i) elucidate the main factors determining the structure and variation of the Estonian old-drained for- ests, (ii) elaborate the typology of these forests at the community level and, (iii) establish the indicator species of the established community types. 118 forest stands drained not less than 35–40 years ago were analysed. According to multivariate data analyses (cluster, ordination and variance analyses, multi-response permutation procedures, indicator species analyses) it appeared that the soil reaction, nutrients, and moisture content, assessed by the Ellenberg ecological indicator values for habitats are much more significant factors for plant growth and community structure than the thickness of soil/peat horizons. Nevertheless, the litter and peat horizons in soils of drained Dryopteris site type forests are significantly thinner than in Oxalis and Myrtillus site type stands. The Dryopteris site type forests can be divided into six, the Oxalis site type forests into three, and the Myrtillus site type forests into two types of communities. Each of the 11 established community types differ significantly (p < 0.05) from each other and have their own dominant and significant indicator species. When comparing the Estonian old-drained forests with analogous stands in neighbouring countries (Latvia, Finland, Sweden, northwestern Russia), we can find rather large similarities; the typological differences result mainly from the methodological approaches and geographical scope of countries.

Keywords: community types, drainage impact, Dryopteris forest site type, fern-rich forests, indicator species, Ellenberg indicator values, nutrition gradient