Rockland pine forests of south Florida dominated by Pinus elliottii var. densa characteristically have poor soil development in relation to neighboring hardwood hammocks. This has led to the hypothesis that Everglades hammock trees are more reliant on soil moisture derived from local precipitation whereas pineland plants must depend more on groundwater linked to broader regional hydrologic patterns. Because soil moisture sources are likely to vary more than groundwater sources, we hypothesized that hammock plants would exhibit correspondingly higher levels of dry season water stress. This was examined by measuring predawn water potentials, and by analyzing water uptake in representative hammock and pineland woody species using stable isotopes of plant water and that of potential sources during wet and dry seasons.
Two species typical of each of the two communities were selected; a fifth species which was found in both communities, Lysiloma latisiliqua Benth., was also analyzed. Water content of soils in both communities decreased from wet to dry season. Consistent with our hypothesis, the change in predawn water potentials between the wet and dry season was less in pineland species than that of hammock species. Water potential changes in L. latisiliqua in both communities resembled that of hammock species more than pineland plants. Isotopic data showed that pineland species rely proportionately more on groundwater than hammock species. Nevertheless, unlike hammock species in the Florida Keys, mainland hammock species utilized a substantial amount of groundwater during the dry season.
