Background/Question/Methods
The uplands or highest parts of tree islands in the Shark River Slough constitute the only areas in the central Everglades that are above water in the wet season, thus resulting in tree islands having two sharply distinct plant communities: (i) the upland community consisting of flood intolerant hammock species and (ii) the flooded margins with a hydroperiod of 3-7 months that have flood tolerant species.  The upland areas have built up a largely organic soil layer consisting of decomposing leaf litter that is thought to provide an unsaturated rooting zone for the flood-intolerant hammock species, with potentially greater nutrient availability in comparison with the flooded communities.  Two potential water sources exist: (i) entrapped rainwater in this organic soil (referred to as soilwater) that is nutrient-rich, yet very localized and small in volume and (ii) the regional surfacewater/groundwater that is nutrient-poor.  We investigated the differences in water source utilization in these two communities and then linked these differences with their foliar nutrient levels and photosynthetic performance as measured by δ¹³C abundance.  Three tree islands were sampled in a gradient from slough to ridge, 50 trees on each (20 upland, 30 lowland ) bimonthly over a year.
Results/Conclusions
Upland plants used soilwater in the wet season and shifted gradually to greater regional water uptake in the dry season, while lowland plants used regional water throughout the year.  Consistent with the nutrient concentration of the two water sources used in the two communities, uplands had a greater annual mean foliar nitrogen and phosphorus concentration at the community level over lowlands as well as a higher leaf area index, thereby supporting the idea of tree islands being nutrient hotspots in the oligotrophic Everglades.  High foliar N concentration in upland plants was associated with eventual stomatal limitation of photosynthesis.  Upland  species being intolerant of flooded soils are restricted to water uptake in the shallow unsaturated soil layer in the wet season, yet access the lowered groundwater table in the dry season because of drying up of surface soilwater. This dependence on a relatively narrow annual range of water table levels should be considered in South Florida water management and restoration of tree islands, which add ecological complexity to the predominantly flooded ecosystem and are critically important to fauna for being the only terrestrial habitat in the slough.