Factors that determine patterns of seedling recruitment in an Afrotropical forest

Tropical forests account for nearly 50% of all known species. Very little is understood about the processes that maintain or promote such diversity. Theoretical models suggest that processes limiting recruitment of new individuals into populations may be key to maintaining species diversity. By keeping population numbers of more competitive species in check, recruitment limitation should allow greater numbers of species to co-exist. Two opposing hypotheses have been proposed to explain how recruitment limitation might influence tropical tree diversity the seed limitation hypothesis and the establishment limitation hypothesis. These hypotheses are generally treated as mutually exclusive, and evidence of either is used to bolster competing theories of community composition that are tightly associated with each.In chapter 1, I develop a framework that views seed and establishment limitation as processes that occur at opposite ends of a continuum. I adopt this framework in a meta-analysis to assess the relative strength of seed and establishment limitation across a range of plant systems. I find that most species are seed limited, though the effects of seed addition are typically small. Establishment, on average, proves to be a stronger limiting force for most plants. I provide recommendations to improve experimental approaches used to examine the relative strength of these two processes. Chapter 2 applies these recommendations to a large scale experiment designed to tease apart the roles of seed and establishment limitation for five randomly-chosen tree species in an Afrotropical forest. I conclude that though seed limitation is relatively weak, it can balance the exclusion process of competition and niche partitioning at very high levels of seed arrival. Yet, niche-based, post dispersal processes more importantly limit seedling recruitment than seed arrival. Chapter 3 delves into the mechanisms responsible for post dispersal seed and seedling mortality. I evaluate the strength and relative importance light availability, soil fertility, competition, density- and distance-dependence, seed predation and herbivory at two stages of seedling recruitment. I conclude that seedling recruitment in the Congo Basin is most strongly dictated by generalist vertebrate seed predators and herbivores, with relatively weaker abiotic environmental filtering and density-dependence playing secondary roles.