Role of soil seed banks and newly dispersed seeds in population dynamics of the annual sunflower, Helianthus annuus

1 We explored the role of the seed bank in population dynamics of the summer annual Helianthus annuus. First, we determined seed survival under field conditions. Secondly, we conducted an experiment in which a dispersal treatment (plants allowed/not allowed to disperse seeds) was crossed with a soil disturbance treatment that was predicted to increase seed bank recruitment. Our goal was to evaluate the relative importance of the previous year's seed production vs. the remainder of the seed bank to numbers of plants. 2 Yearly seed survival was variable, ranging from 46 to 83% when seeds were buried in mesh packets and from 12 to 76% for seeds placed on the soil surface. Survival was higher for plots established in 1999 than in 2000. Survival was often higher in later years but was unaffected by the presence of litter. 3 By comparing seedling establishment between dispersal treatments, we inferred that approximately 10-23% of the seedlings were from the seed bank, independent of soil disturbance. 4 Although seed dispersal the previous year led to increased numbers of seedlings by at least 4.5 times, the number of adults only increased 2.5 times and head production only increased 1.5 times because of density-dependent processes. 5 Patches of seedlings emerging only from the seed bank (often at low densities) may have a disproportionate contribution to the next generation. Average head production/seedling was 3.6 for such seedlings vs. 0.8 for seedlings from both the previous year's seed set and the seed bank. Emergence of seeds from the seed bank in high-density seedling areas may, however, have little effect on patch reproduction as reproductive output was asymptotically related to the number of seedlings initially present. Studies of seed survival and seedling biology should therefore be done with consideration of the entire life cycle.