VARIABLE REPRODUCTIVE OUTPUT AMONG CLONES OF SPARTINA-ALTERNIFLORA (POACEAE) INVADING SAN-FRANCISCO BAY, CALIFORNIA - THE INFLUENCE OF HERBIVORY, POLLINATION, AND ESTABLISHMENT SITE
Cc. Daehler et Dr. Strong, VARIABLE REPRODUCTIVE OUTPUT AMONG CLONES OF SPARTINA-ALTERNIFLORA (POACEAE) INVADING SAN-FRANCISCO BAY, CALIFORNIA - THE INFLUENCE OF HERBIVORY, POLLINATION, AND ESTABLISHMENT SITE, American journal of botany, 81(3), 1994, pp. 307-313
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.