In low-elevation coastal communities, species zonation is driven by several factors, including those related to species’ flooding and salinity tolerances. In the lower Florida Keys, three freshwater-requiring communities (pine rockland, hardwood hammock, and supratidal scrub) have assemblages of glycophytic species and occur in close proximity to the coast. Species in these forests rely on a freshwater resource that fluctuates throughout the year with the seasonality of precipitation. This study investigated the relationship between precipitation, evapotranspiration, and changes in groundwater salinity using data from groundwater monitoring and electrical resistivity tomography (ERT) surveys conducted over one year (2011-2012). The study transects traversed a coastal gradient on a small and relatively larger island, Upper Sugarloaf (SLK) and Big Pine Key (BPK), respectively. Vegetation sampling and subsequent split-moving window analysis were used to delineate boundaries between adjacent communities along each transect. Using random forest classification and density estimation, we assessed the ability of ERT-derived groundwater salinity values from May and November, coastal proximity, and topographic location to predict the location of forest communities along the gradient.
Results/Conclusions
Decreases in groundwater salinity generally occurred after large rain events on SLK, whereas monitoring locations on BPK were less affected by those same events. Similarly, while vertical increases in the lens depth occurred between May and November on both islands, lateral increases in lens extent were restricted to SLK. Coastal proximity and groundwater salinity were identified as important variables in the prediction of coastal forest location. Pine rockland was restricted to salinities of 3 ppt or less and the supratidal scrub community was associated with groundwater salinities generally greater than 10 ppt. Distinct peaks in conditional probabilities for community types along the groundwater salinity gradient suggest a strong connection between the extent of the freshwater resource on these islands and coastal forest community zonation. Yet hardwood hammock and low elevation pine rockland occurred across a wider range of groundwater salinities and were compositionally more similar to one another than the other habitat types, suggesting a succession of pine to hammock resulting from increasing groundwater salinity.