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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.

Cardamine occulta Hornem. (Brassicaceae), an East Asian weedy bittercress, have introduced globally in recent years. First record of this neophyte species in European Russia was made in 2003. According to current data C. occulta was found in nine regions: Leningrad Oblast, Moscow Oblast, Saratov Oblast, Vologda Oblast, Yaroslavl Oblast, the Republic of Karelia, the Udmurt Republic, the city of Moscow and the city of Saint Petersburg. All known populations of C. occulta occupied predominantly man-made habitats. Currently, the species is considered to be a casual or naturalized alien plant. Uncontrollable transfer of the weed along with ornamental plants might lead to its successful spread across the European Russia. Distribution history of the species, as well as a potential for further spread in European Russia are briefly discussed. The main diagnostic morphological features are presented in comparison with those for allied C. hirsuta L. and C. flexuosa With.

Questions: The human-related spread of alien plants has serious environmental and socioeconomic impacts. Therefore, it is important to know which habitats are most threatened by invasion and why. We studied a wide range of European grasslands to assess: (a) which alien species are the most successful invaders in grasslands; (b) how invasion levels differ across European regions (countries or their parts) and biogeographical regions; and (c) which habitat types are the most invaded. Location: Europe. Methods: We selected 97,411 grassland vegetation plots from the European Vegetation Archive (EVA) and assigned a native or alien status to each of the 8,212 vascular plant species found in these plots. We considered only neophytes (alien species introduced after 1500 AD), which we further divided according to their origin. We compared the levels of invasion using relative neophyte richness in the species pool, relative neophyte richness and cover per plot, and percentages of invaded plots among regions and habitats. Results: Only 536 species, representing 6.5% of all grassland vascular plant species, were classified as neophytes. These were mostly therophytes or hemicryptophytes with low habitat specificity. Most of them were present in very few plots, while only three species were recorded in more than 1% of all plots (Onobrychis viciifolia, Erigeron annuus and Erigeron canadensis). Although invasion levels were generally low, we found more invaded plots in the Boreal and Continental regions. When considering only non-European neophytes, the Pannonian region was the most invaded. Among different grassland habitats, sandy grasslands were most invaded, and alpine and oromediterranean grasslands least invaded. Conclusions: In general, natural and semi-natural European grasslands have relatively low levels of neophyte invasions compared with human-made habitats or alluvial forests, as well as with grasslands on other continents. The most typical neophytes invading European grasslands are species with broad ecological niches., IA, VK, JDa, MV, IK and MC were supported by the Czech Science Foundation (EXPRO grant no. 19-28491X), PP and JP by the Czech Science Foundation (EXPRO grant no. 19-28807X), JDa, PP and JP by the Czech Academy of Sciences (long-term research development project RVO 67985939), IB by the Basque Government (IT936-16), RGG by REMEDINAL network (S2018/EMT-4338), AL by Komarov Botanical Institute, RAS (AAAA-A19-119031290052-1) and AK by the National Research Foundation of Ukraine (grant no. 2020.01/0140). JCS considers this work a contribution to his VILLUM Investigator project “Biodiversity Dynamics in a Changing World” funded by VILLUM FONDEN (grant 16549).

Background The "Flora of Russia" project on iNaturalist brought together professional scientists and amateur naturalists from all over the country. Over 10,000 people were involved in the data collection. New information Within 20 months, the participants accumulated 750,143 photo observations of 6,857 species of the Russian flora. This constitutes the largest dataset of open spatial data on the country’s biodiversity and a leading source of data on the current state of the national flora. About 87% of all project data, i.e. 652,285 observations, are available under free licences (CC0, CC-BY, CC-BY-NC) and can be freely used in scientific, educational and environmental activities.