The Mechanisms and Consequences of Seed Bank Formation in wild Sunflowers (Helianthus annuus)

1 Seed banks are important to many plant populations, yet we lack clear understanding of how environmental factors promote seed bank formation. We experimentally varied seed density, and soil disturbance size and timing, to examine the formation of seed banks and the resulting population dynamics in wild annual sunflowers (Helianthus annuus). Seed density treatments reflected the expected increase in seed production of individual wild sunflowers due to hybridization with Bt-transgenic crop sunflowers. Soil disturbances of two sizes (large and small) were applied either before or after seed dispersal. We followed the experimental plots for 2½ years measuring seedling emergence, seeds in the soil, and reproduction, while preventing additional seed input. We assessed seed bank formation by the numbers of emergent seedlings and seeds in the soil. 2 Soil disturbances, regardless of timing, were required for successful germination and reproduction of wild sunflowers. Large disturbances resulted in greater seedling emergence and reproduction than small disturbances. Reproduction on small disturbance plots was low, due to low seedling survival. Higher seed density resulted in more seedlings only in large-disturbance plots. 3 Post-dispersal disturbances led to significant seed bank formation. This effect was greater for large, post-dispersal disturbances. Higher seed density increased seed bank size only in the presence of large, post-dispersal disturbances. 4 Negative density-dependence in seedling mortality and reproduction meant that there was no effect of the high seed density treatment on reproduction. Thus the positive effects of increased seed bank size on population growth were tempered by density-dependence. To assess fully the impact of seed banks on population growth, studies should continue beyond the seedling stage. 5 Our results suggest increased seed production of individual sunflowers resulting from crop transgenes would be unlikely to increase population growth on the scale of small patches. Increased seed production could result in dispersal of seeds across larger areas, and thus could lead to larger seed banks in disturbed environments and an increased prevalence of sunflowers across the landscape. These results are relevant to other annual species, especially those with transgenic crop relatives.