Lowland tropical rainforests are one of the most productive ecosystems on the planet. However, their future as carbon sinks during times of global climate change is uncertain in part due to an incomplete understanding of tropical tree nutrient use. My dissertation project combines theoretical modeling with empirical field measurements and experimentation in the forest at La Selva Biological Station in Costa Rica. I use the perspective of individual trees and functional traits to examine nutrient limitation in mature lowland tropical rainforests, with a focus on competition for light in treefall gaps. This work has implications for species evolutionary success, forest composition, and ultimately the role of lowland tropical rainforests in the global carbon cycle.