Remote sensing is becoming a vital tool for understanding the changing vegetation patterns that are associated with broad-scale plant invasions. The establishment of North American east coast native Spartina alterniflora (smooth cordgrass) in Willapa Bay, Washington is a specific example of broad-scale invasion following local introduction. In this study, we examined a 120-y historical record of aerial photographs, oral histories, museum records, and publications to reconstruct the spatial, temporal, and historical elements of this invasion. We conclude that the most likely means for S. alterniflora to have reached Willapa Bay was the transport and translocation of oysters from New York harbour. Our data and analysis suggest that multiple areas were colonized between 1894 and 1920 coincident with sustained import of oysters from the Atlantic seaboard. We have evidence that S. alterniflora had been long established and growing in multiple widely spread locations by 1945, which is in contrast to a widely reported single introduction. Multiple foci would not only explain the colonization patterns we have observed, but could also increase the heterogeneity of the founding populations, helping to overcome barriers to reproduction that may have initially slowed the colonization in isolated populations during the first 50–70 y.
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