Seed rain is a critical step in forest regeneration because it establishes the initial spatial template for tree recruitment. While each tree was once a dispersed seed, not all seeds produced by a tree get dispersed. Dispersal limitation has been an important component of multiple studies intending to explain the maintenance of high diversity in primary tropical forest. Dispersal limitation is defined as the failure of a species to arrive to a possible recruitment site. Because of the focus on primary forests, we still lack an understanding of the potential variation of seed rain in secondary forests of different successional ages, as well as of the factors responsible for this variation, which may ultimately affect the structure and composition of secondary forests as they regenerate. We quantified spatial and temporal variation in seed rain in four successional forest plots with 12, 15, 20, and 25 years since abandonment after agricultural land use, and the identical areas after 20 years (32, 35, 41, and 45 years since abandonment), compared with primary forest. Two successional plots and the primary forest plot are embedded in continuous forest inside La Selva Biological Station, Costa Rica, and are protected from hunting, whereas the other two successional plots are nearby in fragmented, unprotected areas. We were thus able to use both a chronosequence approach and comparisons through time to evaluate succession, hunting, and fragmentation-related changes in seed rain.
Results/Conclusions
We found that there was significant variation in the seed rain species composition across secondary forests due to protection status, pointing to the importance of animal dispersers for community assembly. While species richness in the seed rain increased with time since abandonment, the increase was not monotonic and depended on seed dispersal mode. We found that animal dispersed species richness increased with time since abandonment, whereas richness of species with wind and explosive dispersal did not change significantly. Richness of the seed rain in primary forests was significantly higher than in secondary forests when grouping all animal dispersed species, but not for bird dispersed species. This result suggests that many birds may be less affected by hunting and fragmentation than other animals. While seed rain in these secondary forests appears to be on a trajectory towards convergence with primary forest, it is not a straight-forward one. Our results suggest that protection status of secondary forests is important in the recovery of seed dispersal processes during tropical forest succession.