Seed-rain-successional feedbacks in wet tropical forests.

Tropical forest regeneration after abandonment of former agricultural land depends critically on the input of tree seeds, yet seed dispersal is increasingly disrupted in contemporary human-modified landscapes. Here, we introduce the concept of seed-rain-successional feedbacks as a deterministic process in which seed rain is shaped by successional dynamics internal to a forest site and that acts to reinforce priority effects. We used a combination of time series and chronosequence approaches to investigate how the quantity and taxonomic and functional composition of seed rain change during succession and to evaluate the strength of seed-rain-successional feedbacks, relative to other deterministic and stochastic mechanisms, in secondary wet forests of Costa Rica. We found that both successional niches and seed-rain-successional feedbacks shaped successional trajectories in the seed rain. Determinism due to successional niche assembly was supported by the increasing convergence of community structure to that of a mature forest, in terms of both functional and taxonomic composition. With successional age, the proportions of large-seeded, shade-tolerant species in the seed rain increased, whereas the proportion of animal-dispersed species did not change significantly. Seed-rain-successional feedbacks increased in strength with successional age, as the proportion of immigrant seeds (species not locally represented in the site) decreased with successional age, and the composition of the seed rain became more similar to that of the adult trees at the forest site. The deterministic assembly generated by seed-rain-successional feedback likely contributed to the increasing divergence of secondary forest sites from each other during succession. To the extent that human modification of tropical forest landscapes reduces connectivity via factors such as forest cover loss, our results suggest that seed-rain-successional feedbacks are likely to increasingly shape regeneration trajectories in and amplify floristic heterogeneity among tropical secondary forests.

Gaps present a trade-off between dispersal and establishment that nourishes species diversity.

We took advantage of two natural experiments to investigate processes that regulate tree recruitment in gaps. In the first, we examined the recruitment of small and large saplings and trees into 31 gaps resulting from treefalls occurring between 1984 and 2015 in the 2.25-ha core area of a 4-ha tree plot at Cocha Cashu in Perú. In the second, we identified the tallest saplings recruiting into 69 gaps created during a violent wind storm in February 2000. In the established tree plot, we were able to compare the composition of saplings in the disturbance zones of gaps prior to, during, and subsequent to the period of gap formation. Recruitment in gaps was compared with that in "nofall" zones, areas within the plot that had not experienced a treefall at least since the early 1980s. Our results confirmed earlier findings that a consistently high proportion (~60%) of established saplings survived gap formation. Light demanding species, as proxied by mortality rates, recruited under all conditions, but preferentially during periods of gap formation, a pattern that was especially strong among gap pioneers. Similar results were noted, separately, for small and large saplings and trees recruiting at ≥10 cm dbh. One hundred percent of previously untagged trees recruiting into gaps in the first post-disturbance census were gap pioneers, suggesting rapid development. This conclusion was strongly supported in a follow-up survey taken of 69 gaps 19 months after they had been synchronously created in a wind storm. Ten species of gap pioneers, eight of which are not normally present in the advance regeneration, had attained heights of 6-10 m in 19 months. The 10 gap pioneers were dispersed, variously, by primates, bats, birds, and wind and reached maximum frequency in different-sized gaps (range <100 m2 to >1,000 m2 ). Both gap size and limited dispersal of zoochorous species into gaps serve as filters for establishment, creating a complex mosaic of conditions that enhances species diversity.

Gaps contribute tree diversity to a tropical floodplain forest.

Treefall gaps have long been a central feature of discussions about the maintenance of tree diversity in both temperate and tropical forests. Gaps expose parts of the forest floor to direct sunlight and create a distinctive microenvironment that can favor the recruitment into the community of so-called gap pioneers. This traditional view enjoys strong empirical support, yet has been cast into doubt by a much-cited article claiming that gaps are inherently "neutral" in their contribution to forest dynamics. We present concurrent data on seedfall and sapling recruitment into gaps vs. under a vertically structured canopy in an Amazonian floodplain forest in Peru. Our results strongly uphold the view of gaps as important generators of tree diversity. Our methods differed significantly from those employed by the neutralist group and can explain the contrasting outcomes. We found that seedfall into gaps differs both quantitatively and qualitatively from that falling under a multi-tiered canopy, being greatly enriched in wind-dispersed and autochorus species and sharply deficient in all types of zoochorous seeds. Despite a reduced input of zoochorous seeds, zoochorous species made up 79% of saplings recruiting into gaps, whereas wind-dispersed species made up only 1%. Cohorts of saplings recruiting into gaps are less diverse than those recruiting under a closed canopy (Fisher's alpha = 40 vs. 100) and compositionally distinct, containing many light-demanding species that rarely, if ever, recruit under shaded conditions. Saplings recruiting into gaps appear to represent a variable mix of shade-tolerant survivors of the initiating treefall and sun-demanding species that germinate subsequently.