North American history of two invasive plant species: phytogeographic distribution, dispersal vectors, and multiple introductions

The North American historic phytogeographic distribution of mugwort (Artemisia vulgaris) and Japanese knotweed (Polygonum cuspidatum), two invasive perennial species introduced from Eurasia and East Asia respectively, was recreated using herbarium records. The putative initial introduction of these two species differs by c.a. 400 years, but their patterns of geographic distribution, introduction pathways, and local dispersal pathways are similar. Both species showed the expected logistic growth relationship between range size and the time following introduction, with lag phases of nearly 400 and 50 years for mugwort and Japanese knotweed respectively. The intrinsic growth rate was greater in Japanese knotweed than mugwort for the US, Canada, and North America. Both species were frequently found along waterway, railroad, and road rights-of-way. Introduction pathways differed, with Japanese knotweed commonly labeled as an ornamental escape (151 collections), while mugwort was commonly cited as an inadvertent component of ship ballast (20 collections). These potential founding populations were located across the final distribution for both species, suggesting anthropogenic large-scale dispersal across North America with local secondary spread. Range expansion appears to be active for both species in the US while nearing the carrying capacity in Canada. Managers of mugwort and Japanese knotweed can make use of this information on their range expansion dynamics and dispersal pathways by reducing anthropogenic dispersal and focusing resources on satellite populations and invasion corridors.