Tree Island Vegetation and Water Management in the Central Everglades

Tree islands occupy a small but distinctive portion of the Everglades landscape. They support high plant species diversity, provide habitat and wet-season refuges for upland species, and are essential nesting habitat for a variety of wetland reptiles and birds. Tree islands in the central Everglades have been dramatically altered during the past century, with drought, wildfire and prolonged high water identified as principle causes of damage to island vegetation and soils. Likewise, future changes in water management will surely affect these tree islands. Presently there is a lack of baseline biological data and of quantitative measures that can be used to predict the ecological response of tree islands to different hydrologic scenarios. This chapter reports on a study of plant communities occupying elevated tree islands in central Everglades water conservation areas 3A and 3B (WCA-3A and -3B). The study’s aims were, first, to obtain a baseline description of tree island vegetation; and second, to identify hydrologic correlates of vegetation that can provide tentative predictions of the ecological response of tree islands to future changes in water management. During 1997-1999, we collected data on plant species composition and vegetation cover from transects on 27 elevated tree islands in WCA-3A and WCA-3B. We also calculated island areas from 1994 aerial photographs and estimated maximum island elevation relative to the surrounding marsh. Historic hydrological conditions were estimated using output from the South Florida Water Management Model (v 3.5) for the years 1979 to 1995. Tree islands in our sample average 1.0 m in maximum height relative to surrounding slough bottoms, ranging from 0.25-1.6 m. The spatial extent of tree and shrub vegetation averages 18 ha per island, but varies over two orders of magnitude (0.3-260 ha). However, the area of an island that exceeds -0.5 m in height is typically less than 0.1 ha. A total of 147 species were identified in vegetation transects, including 43 species of trees and shrubs. Islands in the driest northern zone of WCA-3A were the most distinctive subgroup, with smaller areas of woody and upland vegetation, little or no sawgrass-and-shrub “tail,” reduced tree and shrub species richness, and limited or no canopy cover. These differences are most likely a consequence of drought and peat-consuming wildfires that have destroyed both trees and soils on islands throughout northern WCA-3A. Stepwise regression analysis identified hydrologic predictors for several vegetation variables, including species richness, woody vegetation cover, and island area. The strongest association was between three hydrologic predictors — a measure of low groundwater depths, frequency of island flooding and marsh hydroperiod - that together accounted for 81% of the variance in tree and shrub species richness on WCA-3A tree islands. Our results suggest that a diverse community of woody species can be supported by a hydrologic regime that prevents prolonged flooding of upland portions of islands while providing sufficient depths and hydroperiods to maintain soil moisture and prevent peat fires. Quantitative analysis thus confirms interpretations by previous authors based on smaller data sets and field observations, and provides suggestions for some specific hydrologic variables that could be used to evaluate future water management scenarios.