Functional traits that moderate tropical tree recruitment during post-windstorm secondary succession

Understanding how functional traits moderate species' demographic responses along environmental gradients is a core pursuit in ecology, often to predict how species abundances will respond to a rapidly changing environment. The latter necessitates species demography, or at least abundance, to be modelled directly as a response; yet, most studies till date use abundance indirectly to calculate and then correlate community-aggregated trait indices to environmental variables, rendering the prediction of species demographic responses along environmental gradients unfeasible. Using a hierarchical Bayesian approach that stems from joint species distribution modelling, we modelled the recruitments of 53 woody plant species along windstorm-generated environmental gradients from twenty-eight 100-m(2) tropical secondary forest plots censused over 5 years in Singapore, and then examined whether environmental responses across species can be explained and generalized by functional traits. We found that species recruitment along the light-availability gradient was moderated by wood density, in that low wood-density species responded more positively to elevated light conditions by being more responsive in height growth. Seed mass moderated species recruitment along the leaf litter depth gradient, such that large-seeded species with greater energy reserves recruited more in sites with thicker leaf litter depths than small-seeded species. Shorter- and taller-statured species recruited more in historically short- and tall-canopied forest sites, respectively, indicating an influence from surrounding parent trees and the importance of considering dispersal limitation in successional studies. Leaf economic traits (specific leaf area, leaf dry-matter content and lamina thickness) did not moderate recruitment responses across environments, suggesting a weak role of leaf traits during the earlier life stages or their high plasticity. Synthesis. Wood density, seed mass and adult stature are the main traits that moderate sapling recruitment in our wind-generated forest gaps. The roles of multiple traits in moderating the shared environmental responses across species may be crucial in the maintenance of coexistence and diversity through niche partitioning. Most importantly, we estimated the strength and direction of trait effects on species responses to environmental changes during succession. These are potentially generalizable results that can directly predict the demographic responses of any species-given its traits-to environmental changes.