Exotic plant records in the northwest United States 1950-1996: an ecological assessment

Nonnative plant species continue to be introduced into North America both a ccidentally and intentionally for horticultural and agricultural purposes. Some new species will spread extensively and some will become weeds of impo rtance. We used a floristic database (INVADERS) to examine the status of in cipient plant invasions in the northwest United States (Washington, Oregon, Idaho. Montana and Wyoming). We queried INVADERS for distribution records elf plant species exotic to North America that were first recorded in the n orthwest states during 1950-1996. The query resulted in records for 288 spe cies, of which 133 were judged to have become established (based on collect or notes), or to have high potential to spread beyond artificial environmen ts such as lawns and gardens. Inherent potential for invasion was based on examination of several invasive plant lists and the international literatur e on plant invasions. Thirty species have become moderately widespread (rep orted 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 ot her parts of the world. Five species discussed in the text are notable for rapid spread and/or indications of aggressiveness: Polygonum cuspidatum (sy n: Fallopia japonica, Reynoutria japonica), Bryonia alba, Impatiens glandul ifera, Hieracium pratense and Scorzonera laciniata. Compared with the early exotics (mid 1800s - early 1900s), which tended to be annual herbs, the po st-1950 exotic flora shows a trend reward 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.