Alien flora in a boreal region of European Russia: an example of Kostroma oblast.
The manuscript presents the first comprehensive assessment of alien vascular plants in Kostroma Oblast (Upper Volga, central European Russia), a region with low human density and high proportions of natural and seminatural habitats (boreal forests). The data set combines literature published for 1866-2020 and direct field observations made in 2011-2020. Among the 1200 vascular taxa listed for this region, 330 are neophytes, including 125 casual and 172 naturalized species, with 21 species considered invasive; 33 casual species recorded for the region have vanished. Additionally, we identified 14 naturalized species as potentially invasive. Overall, naturalized alien plants made up ca. 14% of the vascular flora of Kostroma Oblast. Elodea canadensis, Erigeron canadensis, Heracleum pubescens, Lupinus polyphyllus, and Matricaria discoidea are the most widely distributed invasive species across the region. The majority of naturalized taxa originated from Eurasia (i.e., Europe, the Mediterranean, and Asia), while in invasive flora, there is a clear dominance of species from North America. Biennial/perennial herbs are the most common naturalized plants, followed by annuals, while woody species are poorly represented. Twenty-five naturalized species (ca. 8% of all neophyte taxa) were recorded in at least half of all districts in the region. The number of naturalized species per district was notably uneven, increasing in districts with higher human population densities. Compared to other European regions in Russia, Kostroma Oblast has poor alien and particularly naturalized flora; this condition is linked to the socioeconomic features and climatic conditions of the region.