Invasion level of alien plants in semi-natural agricultural habitats in boreal region

We quantified the level of invasion (i.e. number or proportion of aliens of all species) of archaeophytes (introduced before 17th century) and neophytes (introduced after 17th century) in five semi-natural agricultural habitats and identified the factors affecting the occurrence of alien species in boreal region. The differences in native and alien plant species richness were analysed with generalized linear mixed models. One-third of the recorded plant species were aliens. The highest levels of invasion were detected from frequently disturbed field and road margins, whereas the lowest levels were in grasslands and forest margins. All species groups had temperature-related decreasing trend northward, and increasing trend towards east. Archaeophytes responded like neophytes to geographical location and the amount of bare ground. Factors related to disturbance (the amount of bare ground, mowing) may increase the level of invasion. To prevent the establishment and spread of invasive alien species, management practices, which increase the disturbance, should be limited.