The zone of temporary flooding of small rivers as an area of increased floristic diversity
AbstractDuring fluctuating water levels in the lower reaches of rivers, large areas of shallow water are exposed, forming a zone of temporary flooding (TFZ). The level regime of the water body determines the conditions for the growth of plants, the period available for vegetation of species and the path of seed migration. The seed bank is an integral part of a riparian biocenosis, experiencing the characteristics of environmental factors which influence its structure. Our study of the vegetation cover was carried out by the method of route survey of ecotopes in the estuaries of the Korozhechna and Ild rivers, tributaries of the Rybinsk Reservoir. Species composition of the flora is given in accordance with APG IV. The cenotic structure is characterized in terms of the Brown-Blanke school. To assess the species composition of the seed bank, a transect was placed along the width of the TFZ, on which cores were selected. Flora of the Korozhechny river is represented by 59 species from 24 families and 45 genera. Most numerous families: Poaceae (12%), Cyperaceae (10%), Polygonaceae (10%), Asteraceae, Ranunculaceae and Brassicaceae (by 7%). Flora of the Ild river is represented by 42 species from 18 families and 29 genera. Most numerous families: Cyperaceae (21%), Ranunculaceae (14%), Poaceae (12%), Plantaginaceae (7%). We found two rare species: Carex bohemica – on the Ild river and Ranunculus reptans – on both rivers. The difference in the species composition is explained by the greater riparian area of the TFZ on the Korozhechna river, characterized by a wide low floodplain. Phytocenosis form multicomponent complexes and are represented by associations Scirpetum lacustris, Phalaroidetum arundinaceae, Caricetum gracilis, Phragmitetum communis – on both rivers, Hippuridetum vulgaris и Batrachietum circinati – in terrestrial form on the Korozhechna river, and Eleocharitetum palustris and Oenantho-Rorippetum on the Ild river. In the seed bank composition of the TFZ of the transition zone of the Korozhechna river receiver 25 species were found, Ild river – 30 species. The ecological structure of the flora of both rivers and the seed bank is dominated by species of waterlogged and humid habitats. Their share (hygrophyte and hygromeso-and mesophyte) accounts for about 50%. With prolonged drying, they can normally develop in the TFZ. In the biomorphological structure, the main part of the flora and seed bank species are perennial grasses, characterized by great environmental plasticity, adapted to extreme conditions of habitat, having wide ecological amplitude with respect to the ground and which can withstand considerable fluctuations in the water level. As a result, our work has confirmed that the seed bank is a hidden component of plant communities and reflects the floristic potential of ecologically unstable territories.
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