Finding Polygonum arenarium Waldst et Kit. in the West Siberia

When studying flora and vegetation of the northern forest-steppe of Armizon District, Tyumen Region, in the field season of 2017, herbarium material was collected. Determination of its species affiliation made it possible to identify it as Polygonum arenarium Waldst et Kit. - sand knotweed - of the subsection Arenaria Tzvel. of the typical section of Polygonum L. genus that was found in the south of Tyumen Region after more than a century break in a new habitat. Previously, this species had been collected here by Slovtsov I.Y. "in the vicinity of Perevalova Village..." in 1891 and by Gordyagin A.Y. "... the southern part of the Tobolsk Governorate..." in 1901 (labels of Tomsk State University). However, there are no Siberian specimens of this species in the Herbaria of ALTB, NS, NSK, TK. The research shows a new location of this very rare species in Western Siberia -“Tyumen Region, Armizon District, vicinity of Prokhorova Village, near Solonets Cemetry, 7 July, 2017", as well as a description of the halophilic plant community in which the species was found: 100 m2; cut by tracks of vehicles, the soil is wet; grass cover is uneven, the projective cover ranges from 60% to 80%; the grass stand height is 40-55 cm, the richness in species is 21: Artemisia nitrosa Weber ex Stechm. and Puccinellia tenuissima Litv. ex V. I. Krecz. (cop1) are dominant, Artemisia laciniata Willd., Chenopodium glaucum L., Limonium gmelinii Kuntze, Juncus gerardii Loisel., J. compressus Kunth, Veronica spicata L., Alopecurus arundinaceus Poir., Elymrous mutabilis (Drob.) Tzvel., Rumex pseudonatronatus (Borbas) Murb. Myosurus minimus L., Polygonum neglectum Besser., Lepidium ruderale L., Psammophil-iella muralis (L.) Ikonn grow in lower abundance (sp); Inula britannica L., Allium an-gulosum All., Agrostis tenuis Bastard ex Roem et Schult., Potentilla argentea L., Tri-pleurospermum perforatum (Merat) M. Lainz are also present (sol). Individual P. are-nariums are crowded in this plant community, in the form of one spot in abundance sp, they are in a budding phase. The knotweed population can be classified as generative and full-bodied. The nomenclative and morphological characteristics of the species are given, the species is compared to a closely related species of the same subsection P. pseudoarenarium Klokov, and to species of the close subsection Patula Tzvel. In the Siberian enclave, the species dwells in its characteristic alkaline community which convinces of the aboriginal nature of the species, however given the place of growth (near the cemetery) and the anthropogenic disturbance of the community (vehicle tracks) it cannot be ruled out that it was introduced here. It is not possible to make a final conclusion due to insufficient study of the issue.