Tropical forests harbor remarkable levels of biodiversity characterized by an exceptional number of rare species and few common species. Human and natural disturbances are increasingly threatening the condition of these forests, especially the persistence of species with low population sizes. Thus, disentangling the drivers of species relative abundance represents a central goal in ecology and critical step in order to maintain and predict future changes in biodiversity. One potential explanation for the variation in abundance relays on the idea that species exhibit differential abilities to exploit resources that are heterogeneously distributed in space. This ability is expected to be associated with different organisms’ attributes (functional traits) that affect the individual performance and ultimately their distribution and abundance. The trait-base approach serves as a general framework for this dissertation that examines the role of species’ traits in influencing the observed variation in species relative abundance across plant communities. In the first chapter, I examine changes in functional composition of tropical tree communities during a key life-history transition, the seed-to-seedling transition, in order to determine the main ecological forces driving the high mortality occurring during these early stages. In the second chapter, I evaluate the importance of intra-specific trait variation as related with the species abundance. I show that abundant species have lower magnitude of intraspecific trait variation than rare species. In the third study, I compile a set of functional traits across several plant communities including tropics and temperate forest to quantify the contribution of rare species to the functional diversity of the communities. In the fourth chapter, I assemble individual-level trait information together with performance to predict seedling growth rates as related with trait dissimilarity. Together, these findings expand our knowledge on the ecological forces underlying patterns of species relative abundance and will help to foster decisions devoted to preventing biodiversity loss in tropical forests.