The functional underpinnings of tropical forest dynamics-Functional traits, groups, and unmeasured diversity

The structure and dynamics of forest ecosystems are the outcome of differential performance playing out at the individual level. Interactions between the traits of an organism and its environment determine performance. Thus, our ability to understand and, ultimately, model forest dynamics critically relies on knowledge regarding the functional biology of the organisms. In tropical forests, this is a daunting challenge due to the diversity of the systems. This has driven ecologists to focus on identifying a handful of fundamentally important trade-offs and a few traits that may indicate where a species falls along that trade-off axis. In other cases, some ecologists have argued that species can be roughly binned into a handful of functional groups or guilds that capture most of the information needed to generate realistic models of forest dynamics. Here, we discuss the functional biology of tropical forest dynamics. We identify a series of key trade-offs that should underpin forest dynamics and the traits ecologists have attempted to link to these trade-offs. We then explore how far we can get by using functional groups or guilds to model tropical forest dynamics, the conceptual frameworks used for promoting such approaches, and what this modeling framework does not capture. We then use this to identify key gaps that should motivate the future of tropical tree functional ecology. In this review, we discuss fundamental life history trade-offs that have been elucidated via traits, how these may or may not be linked to tropical tree "guilds" or functional groups, and how this information can be used to understand the structure and dynamics of tropical tree communities. Next, we will highlight key areas where a great deal of functional information is still lacking. Specifically, we will highlight the general evidence for negative density dependence in tropical tree communities, but the relatively little functional information that underpins these key results and how advances may be made in tropical tree functional biology to address these fundamental issues in the future.image