Inbreeding depression in smooth cordgrass (Spartina alterniflora, Poaceae)invading San Francisco Bay

The magnitude of inbreeding depression in invading plant populations is oft en presumed to be small and of little consequence. The purpose of this stud y was to assess the magnitude of inbreeding depression in a pollen-limited, partially self-incompatible, invading plant population. The magnitude and timing of inbreeding depression were compared among ten maternal plants sam pled from a population of smooth cordgrass (Spartina alterniflora) invading San Francisco Bay. Selfed and outcrossed progeny were compared for embryo abortion, survival of seedlings, and growth/survival at the end of the firs t growing season in three greenhouse environments. Estimates of inbreeding depression varied among environments, with competitive environment > high-n utrient environment > low-nutrient environment. Population-level estimates of inbreeding depression ranged from 0.61 to 0.81; however, maternal plants varied significantly in their magnitude of inbreeding depression, ranging from 0.1 to 0.97. The 95% confidence interval for inbreeding depression for some maternal plants included zero. There was a significant negative corre lation between the overall magnitude of inbreeding depression and self-fert ility rate among maternal plants. The few maternal plants with high self-fe rtility carried relatively little genetic load, and their selfed progeny ar e likely to survive on open mudflats. The noncompetitive, pollen-limited gr owing conditions associated with invasion may allow self-fertility to sprea d in this population.