Seed dispersal and density-dependent seed and seedling survival in Trichilia tuberculata and Miconia argentea

We investigated seed dispersal and density-dependence of seed, seedling and sapling survival in two BCI tree species of contrasting life history strategies, using data from seed traps, soil seed samples, seedling plots within the forest dynamics plot, and the plot census data themselves. Seed dispersal distances and seed production per unit basal area were significantly higher in Miconia argentea, a small-seeded pioneer, than in Trichilia tuberculata, a large-seeded shade-tolerant. Miconia seed survival and Trichilia seed-to-seedling survival were both strongly negatively related to local density of conspecific seeds and conspecific basal area. Seedling survival was weakly negatively related to local density in Trichilia, but not in Miconia; however, in these analyses, the power was greatly reduced by relatively small sample sizes. Survival of 1-cm diameter saplings was negatively related to local basal area density in both species, but the effect was smaller in magnitude than among seeds and seedlings. Our results suggest that Janzen-Connell effects are strongest on a per-year basis at earlier life history stages, and that more work is needed to assess their magnitude and impact at those stages. Characterization of seed dispersal is also important, as it determines what local densities of conspecifics seeds and seedlings will experience. Further studies of seed and seedling biology within forest dynamics plots have the potential to greatly increase our understanding of the ecology of tropical tree species, and elucidate the mechanisms behind spatial patterns in the larger size classes of the forest dynamics plot data. Muller-Landau et al.