Alien species of Lepidium in the flora of Romania: invasion history and habitat preference.
The history of spread and habitat preferences during invasion of four alien species of Lepidium in Romania, namely L. densiflorum, L. oblongum, L. sativum, and L. virginicum are presented in this paper. The floristic records of these species, collected between 1816 and 2013, were used in the study. L. sativum was sporadically mentioned by earlier botanic references, as a casual alien plant in ruderal places. L. oblongum was reported from a single locality, but it may be considered a potential invader of ruderal habitats from southern regions. For L. densiflorum and L. virginicum, distribution maps were made in the Universal Transverse Mercator system, and exponential regression models were fitted to the cumulative number of records against time. Both L. densiflorum and L. virginicum are invasive in Romania, and their spread was initiated at the middle of last century. The slope of invasion curves, which was used as a measure of the invasion rate, did not differ significantly between the two invasive species. Although both L. densiflorum and L. virginicum were first reported in the North-West of the country, they followed different paths of migration in Romania. Ruderal habitats associated with railways played an important role in the invasion process of both species, but invaded habitats became more diversified over the time. Among the four species, only L. densiflorum tends to invade agricultural crops.