Determinism and stochasticity in seed dispersal-successional feedbacks

Abstract

Regeneration of tropical secondary forests depends critically on seed input, and yet successional dynamics of seed dispersal remain poorly understood. We investigated the role of stochastic vs. deterministic processes in structuring seed rain in successional forests using four years of seed rain data collected at two time periods in four tropical secondary forest fragments representing a chronosequence and in mature forest. Determinism in successional trajectories is defined as predictable, directional, and orderly changes in community structure through time, resulting in convergence toward a climax community. We found that with increasing successional age, the community assembly of the seed rain in secondary forests became more deterministic, and community structure converged to that in the mature forest, both in terms of taxonomic and functional composition. Taxonomic similarity of the seed rain in successional forest to that of the mature forest increased with successional age, as did species co-occurrence and the percentage of shared species between the seed rain of successional and mature forests. The proportions of large, shade-tolerant species in the seed rain increased with successional age, although the proportion of animal-dispersed species increased only modestly. Analyses of the spatial variation in community structure in the seed rain among sites within each secondary forest showed evidence that assembly processes transitioned from being deterministic and convergent early on, to purely stochastic, and then to deterministic and divergent later in succession. Moreover, with increasing successional age, the composition of the seed rain became more similar to that of the mature woody stems in the forest, which could be an important deterministic driver of successional change, that, along with among site variation in landscape context and environment, could also generate idiosyncratic successional patterns among secondary forest fragments Our results suggest that the dominant processes influencing seed dispersal and assembly of the seed rain change during succession and point to successional feedbacks influencing the seed rain that are likely to shape regeneration trajectories.