Soil Accretion Influenced by Elevation, Tree Density, and Substrate on Reconstructed Tree Islands

Tree islands provide a relatively dry habitat for flora and fauna and are biogeochemical hotspots within the oligotrophic Everglades' marsh. Tree islands occupy a small percentage of Everglades' surface area, yet they provide critical ecosystem functions. Hydrologic manipulations throughout the 20th century resulted in a significant loss of tree islands. This study was conducted to determine previously unknown characteristics of soil development important to creating self-sustaining tree islands. Physicochemical characteristics of surface soil (0-3 cm), considered newly accreted, were compared with deeper, older soil (3-10 cm). Soil at varying relative elevations (leading to differences in hydroperiods) and under different tree densities were evaluated in reconstructed tree islands at the Loxahatchee Impoundment Landscape Assessment (LILA). Accretion rates, using feldspar markers, averaged 0.70 cm yr(-1) and maximized at high elevations. Soil nutrients were positively correlated with organic matter. Surface soils exhibited greater total P (TP, 374 mu g g(-1) dry wt.), total N (TN, 14.4 mg g(-1) dry wt.), total C (TC, 190 mg g(-1) dry wt.), and organic matter (OM, 0.36 g g(-1) dry wt.) compared to 3- to 10-cm soils (TP, 216 mu g g(-1) dry wt.; TN, 10.2 mg g(-1) dry wt.; TC, 132 mg g(-1) dry wt.; OM, 0.25 g g(-1) dry wt.). Concentrations of TP and available P, determined by sequential fractionation, were greatest on surface soils in the densest planting. Findings indicate that tree islands gradually increase soil nutrient concentrations under the influence of plant activity, supporting previous work suggesting a mechanism by which groundwater nutrients are imported via the transpiration stream, and ultimately build soil.