Soil, Water, and Vegetation Interactions in Tree Island Development At Lila: A Physical Model of the Everglades.

The Everglades are composed of several key ecosystem components that contribute to its diversity and sustainability. The historic Everglades were characterized largely by a Ridge and Slough landscape punctuated by tree islands. Although tree islands comprise relatively little area, they are a numerous and vital element of the landscape. Tree islands provide the most terrestrial of habitats in the Everglades and are therefore important as “dry-land” refugia for wildlife and avian rookeries. Concentrations of soil total P have been shown to be as much 100 times greater than that in the surrounding marsh making some tree islands potential “biogeochemical hotspots” in an otherwise P-limited system. Despite their importance, hydrologic modifications to the system, since approximately the 1950’s, have greatly altered the number, size and distribution of tree islands in the Everglades. Rehabilitation or restoration of Everglades tree islands is now considered an important part of the continuing Comprehensive Everglades Restoration Program. The objective of this work was to determine the mechanisms by which vegetation, hydrology, and soil dynamics interact to establish viable self-organizing tree islands. The restoration of a fully ecologically-functioning tree island system may occur over longer time scales than the typical evaluation timeframe of restoration projects. Therefore this research was meant to evaluate response trajectories to determine if mechanisms thought to be responsible for tree island self-organization were operating. Changes in tree island elevation, soil accretion, litter fall, CO₂ efflux, tree growth and biomass and surface and groundwater dynamics were measured over time and along hydrologic gradients on constructed tree islands in the Loxahatchee Impoundment Landscape Assessment (LILA; ARM Loxahatchee National Wildlife Refuge, Boynton Beach, Florida, USA). Soil development (accretion) averaged 0.68 ± 0.05 (mean ± 1SE) cm y^-1 and was more rapid at higher elevations (drier) where trees are maximally productive and where evapotranspiration appears to concentrate nutrients from the groundwater. Soil decomposition was negatively correlated to water depth with efflux of CO₂ greatest at highest elevations. Regardless of soil accretion, surface elevation tables (SETs) showed a net elevation change of -0.28 ± 0.23 (mean ± 1SE) cm y^-1 which predicts subsidence of 0.96 cm y^-1. This study documented the mechanisms of several tree island processes that may be crucial to the successful restoration and management of the Everglades.