POSTSETTLEMENT PLANT MIGRATIONS IN NORTHEASTERN NORTH-AMERICA

European settlement of northeastern North America created opportunities for expansion of native species preadapted to the new human-created landscapes. I used Gleason's (1963) New Britton and Brown Illustrated Flora of the Northeastern United States and Adjacent Canada to test predictions about the broad geographic range expansions of native species since settlement. As expected, most of the migrants are herbaceous species from plant families that include many weeds. The migrants are more commonly from dry grasslands than from such well-known weed sources as riverbanks and sea beaches. Range expansion has been highly asymmetric: almost all migrants have moved east or north. Forest clearing and creation of continuous xeric migration routes along roads and railroads have been important causes of uneven range expansion. Such invasions of human-disturbed mesic environments by species of drier regions are probably a general phenomenon.