Understanding how woody plant species coexist in forest communities remains a particularly important and elusive challenge. Forests play a large role in the global carbon balance. However, they are difficult to work with empirically due to the long time scales of the relevant biological processes. Furthermore, with so few limiting resources to partition it seems counter-intuitive to find such a range of species diversity, and particularly, the high species diversity observed in tropical forests. A plethora of hypotheses have been posited to attempt to explain species coexistence in these systems. One important hypothesis is that many species can coexist under competition for light through life history differences enabling them to specialize on different light environments. There is much evidence for life history variation that qualitatively seems likely to lead to partitioning of the light environment. There is also evidence for at least some differentiation in functional traits thought to be related to life history strategy and light usage. However, a rigorous understanding of how species partition the light environment in forests and what kind of functional trait diversity is required to do so is still lacking. A traits-based understanding is desirable because it can provide a more tangible, physical picture that may lead to insight into structural similarities across communities, and that is critical for scaling from community dynamics to ecosystem properties. What is needed to rectify this situation is the development and testing of mechanistic theory of competition for light. The theory should capture key details of how demographic performance is influenced by the presence of competitors, and tie this competition-sensitivity of demographic performance all the way to the functional traits that determine it. This OOS will bring together researchers making important progress on the ingredients needed to develop and test this synthetic theory. Specifically, it will bring together: 1) theorists developing “demographic” models of forest dynamics, which describe the competition process in terms of birth, death, and growth rates and their dependence on the light environment, 2) functional ecologists using physiological principles and correlational studies to establish the relationship between plant functional traits and demographic performance in different environments, and 3) community ecologists elucidating the key axes of trait variation and patterns of trait diversity in forest communities. The aim of the OOS is to take a step towards achieving a synthetic traits-based understanding of coexistence under competition for light and its role in diversity maintenance in forests.