Leaf functional trait variation in New World tree communities across a latitudinal and diversity gradient: Implications for coexistence

Classical competition theory asserts that greater numbers of trophically similar species coexist in species-rich tropical communities by virtue of greater niche specialization and narrower niche breadth in the tropics.  A contrasting hypothesis is that the more diffuse and unpredictable pairwise interactions in species-rich tropical communities select for ecological equivalence and broadly overlapping niches, representing a shared generalist strategy. We tested these hypotheses across three tree communities than vary nearly an order of magnitude in species richness, and spanning a latitudinal gradient from Georgia to Panama. We compared within- and among-species variation across the tree community in a suite of leaf functional traits as proxies of each species niche.  We chose key leaf functional traits that are known to be critically important to light capture, including specific leaf area (SLA), Leaf Nitrogen Content (LNC), carbon:nitrogen ratios (C:N), as well as a number of morphological indices of leaf size and shape.  Among canopy trees in all the communities, we found a high degree of intraspecific variability and broad trait overlap among many coexisting species.  However, understory shrubs were often exceptions to this rule, with relatively narrower niche breadth, at least for the traits we examined.  We conclude that there is only weak support for the hypothesis of a latitudinal gradient in specialization or niche differentiation for the key leaf functional traits that we studied, especially for canopy trees, which constitute over three quarters of the species in these forests.