VARIATION IN SELF-FERTILITY AND THE REPRODUCTIVE ADVANTAGE OF SELF-FERTILITY FOR AN INVADING PLANT (SPARTINA-ALTERNIFLORA)

The factors responsible for the reproductive success or failure of ind ividuals in small, founding populations have received little attention . Previous work on a small population of smooth cordgrass (Spartina al terniflora) invading San Francisco Bay, California found that most clo nes flower prolifically but set little or no seed, while a few clones have high rates of viable seed sell producing most of the seeds in the population. This study first identifies recruitment from seeds as the main source of new smooth cordgrass plants during invasion and then t ests the influence of growing conditions and pollination treatment on viable seed set among clones established in San Francisco Bay. Field t ransplants indicated that a clone's seed set rate was not strongly dep endent on its site of establishment. Low and high nutrient greenhouse treatments also had little effect on viable seed set rates within most clones. In contrast, pollination treatment (self-pollination or outcr ossing) had a major effect on viable seed set rates. Most clones had h igh seed set rates after outcross-pollination, but clones varied widel y in their selfing capacity. Zero or low viable self-seed set rates we re most common; however, a few clones had high viable self-seed set ra tes, comparable to outcross seed set rates. A clone's selfing capacity was significantly correlated across years (r = 0.89, P < 0.001), and capacity to set viable self-pollinated seeds in the greenhouse was sig nificantly correlated with the clone's rate of viable seed set in the field (r(s) = 0.74, P < 0.005). In this growing population where cross -pollination is limited, only the clones with high selfing ability had high viable seed set rates in the field. Among primarily outcrossing plant invaders, variation in self-fertility among individuals may be a common phenomenon, with important implications for genetic differenti ation, effective population size and patterns of spatial spread during an invasion.