Dissertation abstract: We share our planet with hundreds of thousands of other species, including more than 73,000 tree species alone. Making sense of this tremendous amount of biodiversity has remained an on-going challenge, and ecologists are often limited in our ability to study how species identities influence community dynamics and ecosystem processes at larger scales, particularly in diverse tree communities such as tropical forests. The use of functional traits as indicators of species’ life history strategies and other key aspects of their performance has the potential to help us find general trends in biological communities. However, the extent to which traits link to measures of species’ fitness for diverse species assemblages needs further development. Specifically, leaf traits play key roles in photosynthetic pathways, yet the functionality of leaf traits as indicators of tree species’ life history strategy remain ambiguous. This dissertation closely examines the role of leaves as indicators of tree species’ growth and mortality rates. I find that leaf traits play a greater role in characterizing how tree species’ growth responds to resource availability rather than their average or maximum growth rates, and have significant indirect effects on species’ life history strategies that are not evident when we consider only direct effects. I also found evidence for a trade-off between species’ allocations to individual leaves versus to their crown, which together uncovers a stronger role of leaves as indicators of species’ growth strategies, and that trait patterns in temperate forests sometimes are sometimes opposite to those found in tropical forests. Together, these results deepen our understanding of the role of leaves in tree species’ life history strategies and support our efforts to find general patterns in forests around the world.