Abstract
Quantifying the relative importance of the multiple processes that limit recruitment may hold the key to understanding tropical tree diversity. Here we couple theoretical models with a large-scale, multi-species seed-sowing experiment to assess the degree to which seed and establishment limitation shape patterns of tropical tree seedling recruitment in a central African forest. Of five randomly selected species (Pancovia laurentii, Staudtia kamerunensis, Manilkara mabokeensis, Myrianthus arboreas, and Entandophragma utile), seedling establishment and survival were low (means of 16% and 6% at 3 and 24 months, respectively), and seedling density increased with seed augmentation. Seedling recruitment was best explained by species identity and the interaction of site-by-species, suggesting recruitment probabilities vary among species and sites, and supporting the role of niche-based mechanisms. Although seed augmentation enhanced initial seedling density, environmental filtering and post-establishment mortality strongly limited seedling recruitment. The relative importance of seed and establishment limitation changed with seed and seedling density and through time. The arrival of seeds most strongly affected local recruitment when seeds were nearly absent from a site (∼ 1 seed m2), but was also important when seeds arrived in extremely high densities, overwhelming niche-based mortality factors. The strength of seed limitation and density-independent mortality decreased significantly over time, while density-dependent mortality showed the opposite trend. The varying strengths of seed and establishment limitation as a function of juvenile density and time emphasize the need to evaluate their roles through later stages of a tree’s life cycle.
Highlights
A fundamental challenge of community ecology is to determine the processes that govern patterns of species diversity and composition
Recruitment limitation can result from multiple processes occurring at various life history stages, including seed production, dispersal and viability, competition for space and resources, predation, and herbivory [4]: recruitment cannot occur without seed arrival, seed arrival is no guarantee of recruitment
The position of a species along the establishment - seed limitation continuum was most strongly explained by species identity and the site-by-species interaction term (Fig. 1b; or by site for species-specific models, Fig. S2), suggesting that differences in recruitment probabilities among species vary with site characteristics
Summary
A fundamental challenge of community ecology is to determine the processes that govern patterns of species diversity and composition. Individuals are both most abundant and vulnerable to death at the beginning stages of recruitment (i.e., the transition from seed to seedling), and processes operating early in the plant life cycle may disproportionately influence the structure, dynamics, and species composition of communities [6,7]. Two processes, (1) seed and (2) establishment limitation, are thought to largely explain how limitations to recruitment of seedlings influence overall community structure and diversity [5,8,9,10,11]. Seed limitation is defined as the failure of seeds to arrive in saturating densities at all potential recruitment sites because of low population-level seed production and/or a lack of dispersal to available sites. Establishment limitation is defined as the lack of suitable microsites for recruitment, given the arrival of a sufficient density of seeds. Establishment limitation can be partitioned into several processes or stages that occur between seed deposition and recruitment into the adult population [12]
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