Vascular plant diversity in Swedish road verges of high conservation value is threatened by the invasive alien herb <i>Lupinus polyphyllus</i> Lindley

Road verges can be important habitats for vascular plant communities and the organisms that, in turn, depend on them. However, the plant diversity in Swedish road verges is threatened by the invasive perennial plant Lupinus polyphyllus Lindl. Therefore, this study aimed to investigate the effects of L. polyphyllus on species richness, diversity, and biotic and abiotic characteristics of dissimilar plant communities in road verges. The study was performed in 24 road verges of high nature value (i.e. with high flowering plant diversity) in central Sweden. We selected road verges within two study areas, one dominated by forest (west) and the other by agricultural land (east). In each road verge we established a pair of 1 m 2 plots. One plot was dominated by L. polyphyllus while in the other plot L. polyphyllus either had very low occurrences or was completely absent. We investigated whether species richness, effective number of species, and the probability of occurrence of species belonging to three functional groups (based on their competitive ability and affiliation to nutrient rich soil) differed between plots with vs without lupine and if responses varied between the study areas. We found that L. polyphyllus reduced species richness and diversity of plant communities in road verges. However, we found that even though the direction of the effect was similar, the magnitude was strongly dependent on the pre‐existing communities. Community composition differed between the study areas. The study area in the west hosted a lower proportion of competitive species typical for nutrient poor soils compared to the east. We conclude that invasion by L. polyphyllus is a serious threat to vascular plant communities but that the response is context dependent. Communities with high abundance of vulnerable and poor competitive plant species should be prioritised for eradication and control of the invasive.