Patterns of Weed Migration in Northwestern U.S.A.

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Abstract
Patterns of spread of 85 weed species alien to the northwestern U.S. were simulated using principal coordinate analysis on primarily herbaria-derived data. The primary pattern detected was a west-to-east migration of weeds. The point of introduction for most weeds following this pattern appeared to be near Portland, OR, an important shipping port in the past. An east-to-west migration represented a secondary pattern. The major cattle and wheat-cropping areas of east-central Montana were the apparent introduction points for weeds following this pattern. The grain and pulse region of eastern Washington represented the point of introduction of species following a third pattern of migration. These weeds subsequently spread southward to Utah and then laterally to the east and west. Historical weed migration patterns provide insight to probable points of origin and routes of currently spreading weeds as well as those that will spread in the future. Such insight may aid in the containment of future spreading weeds.