To clarify how the seedbank persistence changes through succession and why small seeds tend to develop persistent seedbanks, secondary seed dispersal and seed survival were experimentally investigated. We hypothesized that interactions between the litter thickness that increases through succession and the seed size determined secondary seed dispersal patterns and the resultant seedbank persistence.
The seed movements from the ground surface and seed survival at various burial depths were monitored for a year on four species that produce different-sized seeds, Drosera rotundifolia (seed mass of 0.01 mg), Lobelia sessilifolia (0.25 mg), Rhynchospora alba (0.87 mg) and Moliniopsis japonica (1.82 mg). The experiments were conducted in three successional stages where litter thickness differed at 0, 4, 9 cm on a post-mined peatland. Litter retained large seeds longer than small seeds. The largest seeds of M. japonica stayed within litter throughout the experimental period, and were not buried into peat regardless of litter thickness. The small seeds were swept away when litter was absent or sparse, but were retained when thick litter was present and moved downward until the next early spring. The seed survival rates for all species were nearly zero on the bare peat surface. The survival on and beneath the pear surface increased with an increase in covering litter thickness.
The effects of litter on seedbank development differed considerably among seed sizes. Although litter promoted the downward secondary dispersal of small seeds, that of large seeds was not advanced. When seeds reached beneath the litter, seed survival and seedbank persistence extended. The differences in seedbank persistence between small and large seeds were derived from the patterns of secondary seed dispersal, and were manipulated by litter accumulation.