Expansion Rates and Recruitment Frequency of Exotic Smooth Cordgrass, Spartina altemiflora (Loisel), Colonizing Unvegetated Littoral Flats in Willapa Bay, Washington

We es t imated lateral growth rates o f non ind igenous Spartina alterMflora pa tches in Willapa Bay, Washington, over th ree intervals between 1970 a n d 1990, to r econs t ruc t r ec ru i tmen t f r equency s ince & a/terMflorawas initially in troduced c. 1890. Black and white aerial pho tographs (1:24~000 scale) of fou r representat ive s i tes where S. alterMflora has invaded were analyzed. Individual pa tches were digit ized on a computer~ and d iameter and area w e r e c o m p u t e d for each patch for all years and sites s a m p l e d using CAD software. Lateral growth rates of individual S. alter~dflora patches increased l inear ly at 79.3 (--1.674 SE) CUl yr -L Back calculat ions of origin dates for individual patches based on mean lateral growth rates indicate that recru i tment of new patches has been episodic and increas ing in f r e q u e n c y s ince initial introduction. Environulenta l factors such as s ea sur face teulperature, s ea level , and precipitat ion may account for s o m e of this recru i tment variability. T h e s e data may be useful in exist ing and future m o d e l s of S. alterMflora expans ion in Pacific Northwest estuaries.