ИСТИНА

Floodplain ecosystems of small rivers in the Upper Volga basin are impacted by the disturbance regime created by activity of recently increased populations of European beaver (Castor fiber L.). The long-term influence of beavers on vegetation is expressed in the formation of herbaceous communities instead of forests. Over a few recent decades, in small river floodplains of the Central Forest Nature Biosphere Reserve (Tver` province, NW Russia), as well as in many regions of the forest zone in Russia, dynamic processes related to the activity of beavers have regularly been observed. The spreading of beaver settlements into the territory of the reserve upstream small rivers began after catastrophic windfalls of 1987 and 1996 that caused destruction of tree stands along the originally forested floodplains. These processes led to the formation of herbaceous vegetation in these floodplains. The diversity of such communities remains unexplored because they only emerged not long ago. The aim of my investigation is to reveal how diverse these herbaceous communities are and analyze the major factors determining the structure of hygrophytic vegetation in the beaver-impacted floodplain of the Mezha (one of the main rivers in the Reserve territory) using syntaxonomical and ordination approaches. I analyzed 75 relevés made in August 2016. The herbaceous vegetation was classified into one class, Phragmito-Magnocaricetea, order Magnocaricetalia and alliance Magnocaricion elatae. I revealed 4 associations (Caricetum rostratae Rübel 1912, Caricetum gracilis Savich 1926, Caricetum vesicariae Chouard 1924, Phalaridetum arundinaceae Libbert 1931), 1 variant and 4 facies. All these associations are typical for floodplain habitats and widely distributed geographically. Nevertheless, Phalaridetum arundinaceae (the most widespread association in the investigated territory) has local peculiarities expressed in high abundance of Urtica dioica, Filipendula ulmaria and high constancy of Alnetea glutinosae species. Finally, using DCA-ordination and phytoindication assessment, the main driving ecological factors (soil moisture, soil particle size and nitrogen pool) were designated.