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 Ba y, California, and is rapidly invading open mud flats, growing in circ ular patches that we found to be individual genetic clones. We collect ed spikelet samples from more than 200 clones and observed germination rates ranging from O% to 59%, indicating substantial variation in rep roductive output among clones. Several experiments were performed to e xplore 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 i ncreased the proportion of spikelets containing seed, but failed to in crease germinations per spikelet. Spikelets from Willapa Bay, Washingt on, grown free of insect herbivores, had germination rates similar to San Francisco Bay. These results suggest herbivory is not limiting rep roductive output of S. alterniflora. Spikelet viability was not relate d to clone size; however, clones located lower in the intertidal or fa r up a drainage slough averaged fewer germinations per spikelet, sugge sting clones in areas with lower genet density may have lower spikelet viabilities. Spikelet samples from different sections of clones growi ng 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 o f San Francisco Bay are expected to cause gene frequency changes in th is rapidly expanding population.