Seed limitation and the coexistence of pioneer tree species

Seed limitation, defined as the failure of seeds to arrive at sites favorable for recruitment, may be a critical force structuring plant communities. When seed limitation is strong, interspecific competition is reduced, and competitive exclusion may be slowed to the extent that diversity can be maintained through speciation and migration. In mature tropical forests, seed limitation may be especially important in determining the recruitment patterns of pioneer tree species. These species depend on dispersal to infrequent and ephemeral tree fall gaps for successful seedling establishment. Despite this requirement, pioneers show wide variation in the life-history traits that affect dispersal ability. Here we use seed trap data for pioneers from the 50 ha forest dynamics plot on Barro Colorado Island (BCI), Panama to show that seed limitation has a significant effect on seedling recruitment patterns. We then assess whether the effects of limited dispersal in space can be offset by prolonged dispersal in time through the accumulation of a persistent soil seed bank. Using a simulation model we show that variation in dispersal in space may have surprisingly little effect on overall seedling recruitment rates. This is because there is a trade-off between the number of gaps colonized and recruit density per gap. While long-term seed persistence increases the fraction of gaps colonized, it cannot fully compensate for limited dispersal in space and carries a substantial fitness cost resulting from increased generation time.