Asclepias syriaca L. (Apocynaceae) and its invasiveness in the southern part of the Boreal region of Europe - evidence from Lithuania

Common milkweed, Asclepias syriaca L. (Apocynaceae), has become a problematic invasive species in southern regions of Europe and has been added to the list of invasive alien species of concern of the European Union. For a long time, this alien species had not been considered a potential invader in the northern regions of the continent. This study is an attempt to evaluate the potential invasiveness of A. syriaca in the southern part of the Boreal biogeographic region of Europe as by observations made in Lithuania. To this point, the plant has been recorded from 38 sites and the total area occupied by its stands is about 1.29 ha in the country. The size of A. syriaca stands at the different sites varies significantly, from a few individuals to stands covering an area of 0.74 ha. Most frequently, A. syriaca occupies anthropogenic herb stands, unmanaged xeric and mesic grasslands, and was recorded from arable field habitats. In eight of the 17 studied sites A. syriaca sets fruit and produces viable seeds. We suppose that stands setting fruit and producing viable seeds are represented by individuals of several genotypes, whereas some quite large stands are represented by vegetative descendants of one single individual and, therefore, fruit set is absent. Although most of the currently known stands are relics of former cultivation or escaped from areas of recent cultivation, at one site it is considered to be of seed origin. As by the quite large number of A. syriaca localities in Lithuania, its intense vegetative spread, and production of fruits and viable seeds, we predict further future spread and invasion in Lithuania. Thus, A. syriaca should also be treated as a potentially invasive species in other locations of the southern part of the Boreal biogeographical region of Europe