Lateral spread of invasive Spartina alterniflora in uncrowded environments

We present a stochastic model of clonal growth in uncrowded environments and use it to study data of 7,536 clones from the invasion of Willapa Bay, Washington by the Atlantic cordgrass Spartina alterniflora. The model incorporates effects on clone growth of covariates, spatial autocorrelation, and temporal trends. The deterministic component of the model assumes that growth rate of a clone’s area is proportional to its perimeter, resulting in constant radial growth of the clone. The full stochastic model is built assuming that the fluctuations of radial growth increments (differences of square root-transformed areas) are normally distributed with constant variance. Graphical fit analysis with the density probability plot technique indicates that the stochastic model provides an excellent description of the S. alterniflora invasion. Variation in Spartina growth was significantly but weakly (5%) related to intertidal elevation, substrate type, year of survey, and the two-way interactions between these variables, suggesting that factors intrinsic to Spartina, along with localized high frequency noise, dominate the effects of larger scale abiotic factors on clone growth. Our model of clonal growth is potentially applicable to other systems with approximately circular plants or lichens.