Is Aquatic Fern Salvinia natans (Salviniaceae) Rare in Russia?

According to publication and literature, herbarium data, materials of Internet resources, the change in distribution of aquatic fern Salvinia natans (Salviniaceae) on Russian territory was traced since the beginning of XX century until 2021, and the current boundaries of its range were determined. Among the bioclimatic and temperature indicators that determine the distribution of S. natans, the minimum and maximum temperatures in October and January and the maximum temperatures in April and July showed the highest statistical significance, which is associated with important stages of the life cycle of S. natans. Comparison of species localities with climatic and temperature indicators for the time periods of 1960–1969, 2010–2018, 2011–2021 showed that up to 2021 in the European part, West Siberia, and the Far East there was significant spreading of S. natans to the north (more than 200 km). A notable increase in the number of records of the species has been found since the 2000s in different regions. For example, in the Ivanovo Region from 2018 to 2021 their number has increased more than three time. The expansion of the species occurs against the background of a noticeable increase in average annual temperatures and changes in other bioclimatic indicators. If present temperature trends continue, S. natans will expand its range even further north, east, and west in European Russia, north and west in West Siberia and the Far East. Due to the ongoing expansion of S. natans, the premises for the exclusion of the species from a number of regional Red Data Books were discussed.