Southeast Environmental Research Center and Department of Environmental Studies, Florida International University, Miami, FL
Tree islands appear in many forms in the Everglades landscape, each with a characteristic regional distribution, physiography, shape, and plant species assemblage. Surface elevation at the center of most southern Everglades tree island types is higher than the surrounding marsh. Consequently, a gradient between well-lit, frequently flooded edge locations and shaded, better-drained interior positions can usually be identified, with individual species arranged in a predictable sequence along it. However, the responses of Everglades tree species to this environmental gradient have rarely been tested under controlled conditions. In this presentation we describe the response of twelve tree species to three realistic hydrologic regimes that were applied while holding other variables (e.g., light, nutrients) constant across treatments. Seven species (Annona glabra, Magnolia virginiana, Myrica cerifera, Persea borbonia, Ilex cassine, Chrysobalanus icaco, and Salix caroliniana) were characteristic of Shark Slough Bayhead and Bayhead Swamp forests, while five (Coccoloba diversifolia, Mastichodendron foetidissimum, Eugenia axillaris, Bursera simarouba, and Simarouba glauca) were common in Hardwood Hammocks in the region.
Recently emerged seedlings of all species were collected from Everglades tree islands in May-June 2001, and planted in standard potting soil in small peat pots. After several months in greenhouse conditions, healthy seedlings of each species were transferred to larger pots and an organic soil medium, where they were maintained in a shadehouse (50% full sunlight) erected on the FIU campus. All plant individuals were treated with a systemic insecticide and slow-release fertilizer three weeks before the experiment commenced in April 2002. The experimental design was randomized complete block, with 36 Species x Treatment combinations represented twice in each of four blocks. The blocks were plastic-lined tubs in which water levels were managed to mimic variation among weekly means from the years 1990-1999 at a single Shark Slough water level recorder. Treatments representing Hammock, Bayhead, and Bayhead Swamp hydrologic regimes were maintained in each tub by supporting the three sets of pots on stages (plastic pots and wastebins) established at the appropriate relative heights (Hammock at 57 cm, Bayhead at 27 cm, and Bayhead Swamp at 0 cm) indicated by topographic surveys in Shark Slough tree islands. Plant structural and physiological responses were monitored at 1-6 week intervals throughout the experiment, which was continued for 30 weeks.
Upland species showed signs of stress from inundation by Week 12, though most remained alive through the end of the experiment. Some species typically found in Bayhead and Bayhead Swamp sites appeared to be better adapted to rising water levels than others. Our data suggested that A. glabra, M. cerifera, M. virginiana, and S. caroliniana responded most positively to flooding, while P. borbonia,C. icaco, and I. cassine were less flood-tolerant. The arrangement of species according to their response to the flooding treatment were for the most part established within the first 12 weeks of the experiment, and these rankings roughly paralleled their spatial distribution in the marsh landscape. Tracking hydrologic responses directly in the shadehouse may allow us to isolate the mechanism and timing of species' morphological and physiological responses. Results from such experimental treatments may eventually allow early warning of flooding stress in tree islands, and modification of water distribution – information that may be useful Everglades water management.