Expansion rates and recruitment frequency of exotic smooth cordgrass, Spartina alterniflora (Loisel), colonizing unvegetated littoral flats in Willapa Bay, Washington

We estimated lateral growth rates of nonindigenous Spartina alterniflora patches in Willapa Bay, Washington, over three intervals between 1970 and 1990, to reconstruct recruitment frequency since S. alterniflora was initially introduced c. 1890. Black and white aerial photographs (1∶24,000 scale) of four representative sites where S. alterniflora has invaded were analyzed. Individual patches were digitized on a computer, and diameter and area were computed for each patch for all years and sites sampled using CAD software. Lateral growth rates of individual. S. alterniflora patches increased linearly at 79.3 (± 1.674 SE) cm yr⁻¹. Back calculations of origin dates for individual patches based on mean lateral growth rates indicate that recruitment of new patches has been episodic and increasing in frequency since initial introduction. Environmental factors such as sea surface temperature, sea level, and precipitation may account for some of this recruitment variability. These data may be useful in existing and future models of S. alterniflora expansion in Pacific Northwest estuaries.