The floristic composition, ecological and coenotic structure in paludified secondary birch forests (peat capacity up to 30 cm) of the southern taiga have been studied. Also, the growing conditions were evaluated based on the indicator values of plants and soil chemical properties. The study plots were laid on transects directed from upland forests through peripheral wetland forests to bogs, which are located in relief depressions, in the North-East edge of the Great Vasyugan mire. The sites of the transition forest-to-bog zone to different extent transformed under the influence of the bog water regime are considered as successive stages of paludification. Because of the high carbonation of the soil-forming rocks and also the birch as a forest formator, raw grass forests without a developed moss cover on rich peat-humus soils form here at the initial stages of hydromorphic transformation, unlike typical plant successions developing under the influence of acid bog waters on non-carbonate sediments. Reed grass (Calamagrostis phragmitoides) dominates in the ground cover with the participation of sedges (Carex canescens, C. vaginata, C. disperma). The richness with elements of mineral nutrition of organo-accumulative dark humus soils determines the high floristic diversity of paludified birch forests, comparable with the diversity of upland communities. A significant part of the ground cover of these forests (25% of the species composition, up to 50% of the projective cover) is nutrient-demanding forest- and meadow-swamp plant species, which are usual for eutrophic boreal forested swamps of West Siberia with groundwater input. These species are absent in the mesotrophic paludified forests of taiga. At a later stage of hydromorphic transformation, pine-birch dwarf shrub-moss forests form on peat-humus-gley and peat-gley soils. As the nutrient-availability of soils decline, the number of plant species decreases, among them the less nutrient-demanding begin to predominate. This leads to the restructing of the ecological structure of wet communities and brings them closer to paludified forests on non-carbonate sediments; peat accumulation begins with the deposition of mesotrophic peat. Thus, in comparison with the typical mesotrophic paludification of dark-coniferous forests on loams, the more substantional change of the ecological structure of plant communities occurs on high-carbonate clays in the course of their hydromorphic transformation.