Dispersal patterns and seed bank dynamics of pioneer trees in moist tropical forest

Seed dispersal patterns and seed persistence in the soil should strongly influence the distribution of pioneer tree recruits in gaps. Nonetheless, seed distribution patterns for pioneers are poorly known, and processes controlling the fate of seeds in the soil have been little explored. We examined patterns of seed rain, seed abundance in the soil, and seed mortality of two common pioneer trees, Miconia argentea (Melastomataceae) and Cecropia insignis (Moraceae), on Barro Colorado Island, Panama. For each species, we selected four isolated, reproductive trees within a 50-ha forest dynamics plot. Seed rain and soil seed bank samples were collected, respectively, in mesh traps and from soil cores sampled along transects radiating away from the tree crowns. At below-crown sites, seed rain inputs far exceeded soil seed bank densities measured at the end of the fruiting season. For Miconia, the below-crown seed bank in the surface 3 cm of soil accounted for only 23% of seed rain, and for Cecropia, only 2%. However for Miconia, at distances >5 m from the crown seed bank densities exceeded the annual seed rain input. For both species log seed densities in the seed bank declined linearly with log distance from the crown and also decreased dramatically through the year. The annual loss rate of Miconia seeds was >90% below the crown and declined to 65% at 30 m from the crown. The annual loss rate for Cecropia was >90% at all distances. Seed losses in the seed bank could be largely attributed to mortality from pathogenic fungi. Fungicide treatment significantly increased seed survival in the soil for both species. For these two gap-dependent pioneer species, rapid seed-bank turnover rates and logarithmic declines in soil seed density with distance from adults suggested that both the spatial distribution and timing of gap formation may have influenced their chances of successful gap colonization. Recruitment from distant seed sources, or from seeds surviving in the soil after the death of the parent tree, may be relatively rare for these species.