Exotic plant records in the northwest United States 1950-1996: an ecological assessment.
A floristic database (INVADERS) was used to examine the status of incipient plant invasions in the northwest United States (Washington, Oregon, Idaho, Montana and Wyoming). INVADERS was queried for distribution records of plant species exotic to North America that were first recorded in the northwest states during 1950-1996. The query resulted in records for 288 species, of which 133 were judged to have become established (based on collector notes), or to have high potential to spread beyond artificial environments such as lawns and gardens. Inherent potential for invasion was based on examination of several invasive plant lists and the international literature on plant invasions. Thirty species have become moderately widespread (reported from >5 counties) in the five-state northwest region, and several are known to be aggressive invaders in other regions of North America or in other parts of the world. Five species discussed in the text are notable for rapid spread and/or indications of aggressiveness: Polygonum cuspidatum [Reynoutria japonica], Bryonia alba, Impatiens glandulifera, Hieracium pratense and Scorzonera laciniata. Compared with the early exotics (mid 1800s-early 1900s), which tended to be annual herbs, the post-1950 exotic flora shows a trend toward greater proportions of perennials and woody growth forms (shrubs and trees). Some applications of floristic databanks in regional-scale management of plant invasions are discussed.