Pervasive distance effects (and not conspecific density per se) control early-stage tree recruitment in a lowland Amazonian rainforest

A widely-debated and critical aspect of the classic Janzen-Connell hypothesis for the maintenance of tree diversity in tropical rainforests is the relative importance of conspecific density versus distance from conspecific adults in determining early-stage progeny survival. Results from a three-year-and-running study in a lowland Amazonian rainforest involving >1300 individual seedlings representing 15 common species of canopy and sub-canopy trees provide strong support for the pervasive effects of distance from conspecific adults on progeny survival. More than half of the species displayed significantly higher survival at sites located far from versus close to conspecific adults, with the study design controlling for conspecific seedling density at sites. The role of host-specific invertebrate herbivores and microbial pathogens in causing seedling mortality near conspecific adults was confirmed by the use of mesh exclosures and a study design that combined “near” conspecific seedlings with “far” heterospecific seedlings that served as “controls”. Survival analysis based on multiple censuses revealed that the “distance effect” persisted and intensified over time, while the onset of differential mortality based on distance differed amongst species. Most of the species that did not show a significant “distance” effect on seedling mortality are known to experience high levels of seed predation by insects, and only two species demonstrated a positive correlation between survival and canopy openness. Overall, results from this study provide community-level support for the “distance-dependence” prediction of the Janzen-Connell model, and suggest that the earliest stages of tropical tree recruitment, at least in lowland rainforests of western Amazonia with intact floral and faunal assemblages, are strongly influenced by deterministic, biotic factors.