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The species diversity of the floodplain forest communities in the North-East of the Russian Federation was studied on the basis of 177 geobotanical releves conducted in the north of the Koryak Okrug, the North-East of Magadan Oblast, and the southern Chukotka. This work aims at analysing the species and community diversity of the floodplain forests depending on the habitat conditions and their position in the successional series. Geobotanical characteristics of communities in chosenia (Chosenia arbutifolia), Mongolian poplar (Populus suaveolens), willow (Salix udensis, narrow-leaf willow S. schwerinii), and alder (Alnus hirsuta) forests were displayed by dominant-determinant classification. The community diversity of the floodplain forests is represented by 19 associations, grouped into 5 series. The floristic composition and structure of the floodplain forests' communities were characterized, the Shannon and Pielou indices, species richness, species percentage coverage, and layer density were estimated. Compared to the forests of the Kamchatka Peninsula, the floodplain forests of North-East Russia were characterized by low syntaxonomic diversity and represented by floristically poor communities. The structure and dynamics of the floodplain forests' communities are discussed. The indicators of species diversity and evenness were calculated for each of the studied communities. The lowest Shannon index appeared to the pioneer chosenia communities with sparse herb cover, found on recent pebble sediments; the highest index was found for herb-moss poplar forests that occupied the highest elevations of the floodplain. The rest of the associations had intermediate values of the Shannon index. The species diversity tended to increase over the sequence of alluvial succession. The obtained data suggested that the species diversity of communities might increase during succession and depend on habitat floodplain regime and alluvial index decrease. [ABSTRACT FROM AUTHOR]