Seed dispersal, seedling establishment and gap partitioning among tropical pioneer trees

1. We examined the abundance and distribution patterns of pioneer seeds in the soil seed bank, and of pioneer seedlings in 53 recently formed gaps, in a 50-ha forest dynamics plot on Barro Colorado Island (BCI), Panama. The aim was to assess the importance of dispersal limitation (failure of seeds to arrive at all sites suitable for their germination) and establishment limitation (failure of seeds having reached a site to germinate successfully and establish as seedlings) in determining patterns of gap occupancy. 2. The abundance of seeds in the soil seed bank was strongly negatively correlated with seed size, but was not correlated with the abundance of reproductive-sized adult trees in the plot. In contrast, the abundance of pioneer seedlings > 10 cm height in natural gaps was strongly correlated with adult abundance, but was not correlated with seed size. 3. Seedlings were non-randomly distributed among gaps, but seedling abundance was not directly related to gap size, and there was no evidence of partitioning of the light environment of gaps by small seedlings. Large differences in growth and mortality rates among species were observed after 1 year, and this may result in the gap size partitioning previously found in saplings of the same species. 4. Seedlings of most species, particularly those with large seeds, were relatively more abundant than expected in gaps close to their conspecific adults. Proximity to reproductives, and by inference dispersal limitation, therefore exerts some effect on seedling distribution. None the less, large differences between seed and seedling abundances for some species, and low seedling occupancy rates in some gaps close to adult conspecifics, suggest that seedling emergence probabilities and species-specific establishment requirements may also be important determinants of local abundance.