Fluctuations of floodplain meadows of the river Big Yugan

The regularities in the development of vegeta-tion cover in floodplain meadows directly depend on fluctuations in ecotopic factors, as shown by the results of 2015-2016 studies of meadow vegetation in the valley of the Big Yugan river (in the vicinity of Yugan village of the Surgut district of Khanty-Mansi Autonomous Area - Yugra). Different years varia-bility of floristic and cenotic diversity of herbaceous vegetation on 7 permanent test plots of studied ter-ritory depends on the conditions of the river of Big Yugan. Meadows, confined to lowland forms of re-lief differ most contrastly in composition of species composition due to the fluctuations in maximum water levels (3-4 m) over the years of research. Trial areas with dominant Carex acuta and Carex aquatilis have the difference in the values of the total projective coverage of the grass stand of 10-15 %, and the increase in relative participation of species in 2016 along with significant increase in productivity of communities. Hydrological conditions have weak effect on meadows that develop on the surfaces of floodplain terraces with predominance of cereals in grass stand (Phalaroides arundinacea, Elytrigia repens, Calamagrostis epigeios, Cala-magrostis purpurea, Poa pratensis, Descha-mpsia cespitosa), which is expressed in relative constancy of the species composition of the communities. In connection with the availability of more comfortable conditions for the development of most species of mesophilic herbage of terraced phytocenoses in 2016 the above-ground phytomass levels increase by 2-2.5 times. At the same time mixed-grass-cereal meadow due to the location on the mane near the channels of the river of Big Yugan, experiencing fluctuations similar to the phytocenoses of the low-lands: the predominance of Carex acuta along with other hygrophilous species in 2015 and the devel-opment of herbage and grasses (Potentilla an-serina, Poa pratensis, Naumburgia thyrsiflora, Co-marum palustre, etc.) in 2016 gave an increase in total productivity community.