Habitat Conditions and Neighborhood Interactions Drive the Phylogenetic Structure of Seedling Assemblages in a Tropical Forest Mosaic

Environmental filtering and ecological interactions can shape the phylogenetic structure of coexisting species. Given the high mortality rates early in plant development, studying the relative roles of these processes in structuring seedling communities across habitats highlights the importance of specialization in the regeneration niche. Additionally, because seedlings interact at fine spatial scales, focusing on smaller assemblages and source pools is essential. We analyzed the phylogenetic structure of seedling communities in a white-sand tropical forest mosaic on the southeastern coast of Brazil, influenced by seasonal flooding and varying soil conditions. We tested whether the occurrence- and abundance-based mean phylogenetic distance of seedling assemblages in three forest types differed from null expectation at both habitat and neighborhood scales and evaluated the influence of dispersal limitation by quantifying the proportion of seedling species also present as adults nearby. At the habitat scale, poorer sandy soils filtered closely related species, while the flooded habitat favored distantly related species, likely due to the effects of natural enemies. However, we cannot rule out the role of neutral processes and dispersal limitation, given the random phylogenetic patterns when species abundance was considered. At the neighborhood scale, assemblages with very closely related species were rarer than expected in all three forest types, emphasizing the role of ecological interactions at short distances. As seedling species occurrence within neighborhoods was not strongly associated with the presence of adults nearby, we argue that seedling phylogenetic structure results from processes acting on the regeneration niche rather than merely reflecting adult patterns.