Successful germination, establishment, and growth of seedlings in the understory of diverse tropical forests requires a vulnerable life stage to survive a number of biotic and abiotic mortality agents or stresses. The quick turnover of the seedling layer means biotic neighborhoods are changing more quickly than the adult neighborhoods, and the dynamic seedling bank provides the pool of potential species to take advantage of canopy openings. Spatial variation in the biotic seedling neighborhood results from patterns of seed supply as well as density-dependent responses of natural enemies or resource competition, each of which may vary year-to-year with interannual climate variation, even at an aseasonal site. Long-term monitoring is required to understand the generality of patterns observed in any one year. We examined the relative importance of interannual variation in seedling recruitment and survival as a result of variation in the composition of the seed rain and structure of the biotic neighborhood using a 12-year community-wide seedling dynamics dataset from Yasuní National Park in eastern Ecuador, a highly diverse, aseasonal Amazonian forest.
Results/Conclusions
There was great temporal aggregation of seed rain for all species, with variable densities of seeds hitting the same sites across years. Within any given year, seed rain was also highly aggregated across census sites. However, seedling distributions were significantly more evenly distributed through years and sites due to negative density-dependent mechanisms occurring across the seed-to-seedling transition. This led to increases in the diversity of the seedling assemblage, relative to the diversity of seed rain, but the magnitude of these changes demonstrated great inter-annual variation. Species with certain life history strategies or functional traits were more correlated in their interannual recruitment patterns than comparisons between species of differing strategies. Variation in seed dispersal and the subsequent negative density dependence in seedling recruitment and survival over early years act in concert to promote species coexistence in tropical forests.