| Summary: | Humus profile formation in southern Vasyugan plain is closely related to its parent rocks ranging from carbonaceous clays and heavy loams. Carbonates ensure slowing down of restoration cycles of the indigenous vegetation at the stage of grass deciduous forests (for more than one generation), because well developed grass cover dominated by bunch grasses impedes restoration of dark coniferous species. This gives an insular pattern to some features of climatic zonality due to different soil-vegetation relationship between zonal and successional derivative communities. Plant and soil cover determine local conditions for soil drainage and leaching. In contrast to zonal forest communities with tree storey composed of dark conifers, growing on sod podsolic soils with residual humus horizons, under revegetating deciduous forests soil formation takes place under higher soil biological activity due to differing quantity and quality of plant matter input. The well drained areas under aspen and birch forest with understorey dominated by herbs and grasses (the most abundant species being Betula pubescens, Populus tremula, Carex macroura, Aegopodium podagraria, Calamagrostis obtusata, Equisetum pratense, Rubus saxatilis) are occupied by organic matter accumulating soddy podzolized soils with carbonate-containing layers close to the surface. In semihydromorphic and hydromorphic biotopes under smallreed aspen-birch forests (the most abundant species being Betula pubescens, Populus tremula, Calamagrostis phragmitoides Carex globularis, Equisetum sylvaticum, Filipendula ulmaria) organic matter accumulating soils with varying thickness of the humus/peat horizons are common. The species composition of such plant communities corresponds to the one typical for the southern taiga, but their production characteristics are closer to those common for the more southern communities of the northern forest-steppe with 3–5 t/ha (dry mass) of phytomass. Phytomass mineralization rate is higher, and leaves and grass decomposition products are ...
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