The hardwood forests of south Florida, commonly referred to as hammocks, persist as well-drained patches of broadleaf forest, embedded in a matrix of brackish water swamp, freshwater marsh, or pineland. Little is known about the patterns of community assembly of these subtropical forest communities in the landscape mosaic of south Florida. We used a functional trait approach to understand the composition of these communities and their responses to environmental variation across four areas of south Florida: the pine rocklands and freshwater marshes in Everglades National Park, and the lower and upper Florida Keys, where the hammocks are surrounded by halophytic swamp communities. These sites represent an environmental gradient from less-productive, more xeric sites in the lower Florida Keys, to more-productive, wetter sites in the Everglades marshes. We examined the patterns of trait variation at three levels (individual, population, and community) to examine the underlying processes driving assembly in these hammock communities. To understand processes governing community composition in each site, we used methods that partition variance in six traits (maximum tree height, specific leaf area, wood specific gravity, leaf nitrogen, leaf phosphorus, and leaf stable carbon isotope ratios) into internal and external filtering components. Community-weighted mean trait values for three traits (specific leaf area, height, and leaf phosphorus) increased significantly from dry, less-productive coastal sites in the Florida Keys to the moist, more-productive areas on the mainland, while wood specific gravity and leaf δ13C showed the opposite pattern. For one or more traits per site, standardized effect sizes differed significantly from null expectation. Processes such as competition for resources (e.g., water, nutrients, light) and species sorting across microhabitats (i.e., within site) operate to increase local functional trait variation within communities and among species across sites. External filtering on individuals for height and leaf phosphorus differed significantly from null expectations across sites, while external filtering on species was only observed for specific leaf area, maximum tree height, and leaf phosphorus. These results are consistent with strong environmental filtering across the region, among local communities differing in freshwater accessibility or that occupy different positions along strong edaphic gradients. Our results confirm the importance of intraspecific variation among species and reflect a high degree of trait plasticity across the environmental gradient.
Subedi, Suresh C.; Hogan, J. Aaron; Ross, Michael S.; Sah, Jay P.; and Baraloto, Christopher, "Evidence for trait-based community assembly patterns in hardwood hammock forests" (2019). Faculty Publications - Biological Sciences. 46.