Metacommunity structure of dry tropical forests at the distal end of the Florida peninsula

Background/Question/Methods

Dry tropical forests in the continental United States reach their best development in South Florida, in locations where flooding and freezing temperatures are rare.  These include not only the Florida Keys and the southeastern mainland coast, but also climatically-buffered “tree islands” in the interior Everglades, surrounded by an extensive marsh or pine forest matrix. Along with close analogues in the “coppice” of the Bahamas, these iconic ecosystems represent intra-regional hot spots of floral and faunal diversity.  In this study, we examined spatial variation in tree composition within this network of forest fragments, which may hold clues to the potential for northward movement of tropical tree species along the Florida peninsula in response to climatic warming.  We performed an analysis of metacommunity structure among 145 south Florida forests, tested for associations with selected climatic, hydro-edaphic, and geographic variables, and characterized the northern range distributions, successional status, and dispersal mechanisms associated with sub-regional groupings.

Results/Conclusions

Forest patch size was the strongest single correlate with composition and species richness, but several inter-related variables, especially mean January temperature, distance to the coast, and a neighborhood index denoting degree of isolation from other patches contributed significantly to regression models. The species data exhibited strong spatial autocorrelation structure suggestive of highly integrated local networks.   Notably, the species-by-site matrix was highly nested, with tree species common to small upland fragments in the interior of the Everglades representing a small but distinct subset of the richer assemblages found in sites closer to the coast.  Interior forests were smaller, more isolated, and subject to colder minimum temperatures than more coastal forests.  While dry forests throughout the region were overwhelmingly dominated by species of tropical origin, trees of the Everglades interior had range distributions that extended much further north, and tended to be early-successional and fleshy fruited, suggesting long-distance dispersal. The spatial patterns evident in this network appear to implicate the role of strong environmental filters in shaping the region’s forests.