Variable reproductive output among clones of Spartina alterniflora (Poaceae) invading San Francisco Bay,California: the influence of herbivory,pollination, and establishment site

Spartina alterniflora has recently been introduced to San Francisco Bay, California, and is rapidly invading open mud flats, growing in circular patches that we found to be individual genetic clones. We collected spikelet samples from more than 200 clones and observed germination rates ranging from 0% to 59%, indicating substantial variation in reproductive output among clones. Several experiments were performed to explore the cause of variation. Pollination manipulations showed that S. alterniflora is outcrossing, but pollen supplements did not increase spikelet germination rates. Exclusion of the only insect herbivore (a phloem feeder, Prokelisia marginata) from developing inflorescences increased the proportion of spikelets containing seed, but failed to increase germinations per spikelet. Spikelets from Willapa Bay, Washington, grown free of insect herbivores, had germination rates similar to San Francisco Bay. These results suggest herbivory is not limiting reproductive output of S. alterniflora. Spikelet viability was not related to clone size; however, clones located lower in the intertidal or far up a drainage slough averaged fewer germinations per spikelet, suggesting clones in areas with lower genet density may have lower spikelet viabilities. Spikelet samples from different sections of clones growing across wide environmental ranges had similar rates of germination, suggesting some genetic influence on spikelet viability. Differential reproductive output among clones and the novel selective environment of San Francisco Bay are expected to cause gene frequency changes in this rapidly expanding population.