ASSESSING LIMITATIONS ON THE GROWTH OF ENDANGERED PLANT-POPULATIONS .2. SEED PRODUCTION AND SEED BANK DYNAMICS OF ERYSIMUM-CAPITATUM SSP ANGUSTATUM AND OENOTHERA-DELTOIDES SSP HOWELLII

Herein we extend a related study of Erysimum capitatum ssp. angustatum (= ECA) and Oenothera deltoides ssp. howellii (= ODH), two highly endangered plants, by examining seed production and seed bank dynamics at Antioch Dunes National Wildlife Refuge in central California (USA). ECA and ODH were found to produce large numbers of seeds at the refuge, comparing favorably with closely related but non-endangered taxa. Extrinsic limits on seed production, such as the availability of effective pollinators, were found to be important rather than intrinsic (i.e. genetic) limits. More than half of the ovules initiated by ODH during the two-year study period failed to become seeds. These ovules were contained in ovaries that failed to develop because effective pollinators (e.g. hawkmoths) were not available to carry pollen between flowers of different individuals in this outcrossing taxon. Management of the ODH population, therefore, will require special efforts to protect local populations of effective pollinators in order to minimize the pollination limitation in the future. Pollination could also limit seed production in ECA, but only when the plant population was small and competition for pollinators from other species intense. Pre-dispersal insect predation was also more important when the ECA population was small. Management of the ECA population to maximize seed production, therefore, requires increasing population size by maximizing germination, seedling establishment, and growth in years of poor rainfall. Controlling competition from introduced annual grasses particularly for moisture, is most important in this regard The seed banks of ECA and ODH were large compared to the number of established plants, and exhibited typical seasonal patterns of depletion and replenishment. A minimum residual bank of viable ECA seeds remained during the entire year, suggesting there was overlap of generations and a buffer against catastrophic events that affect established plants. ODH, however, had a smaller residual bank and was, therefore, less well-buffered against the erosion of genetic variation and catastrophic events that affect the adult population. The long-term status and dynamics of the ECA and ODH seed banks will depend on management that reduces the seed density of introduced annual grasses in the refuge seed bank.