Tree islands are defined as patches of woody vegetation within a freshwater wetland matrix dominated by non-woody species. Ecosystems with tree islands as a prominent landscape feature are found throughout the world, suggesting that they arise from a common mechanism of formation. This chapter considers the ecological processes that foster the development and maintenance of tree islands and compares tree islands found in the Florida Everglades to other tree island ecosystems. From the wide diversity of tree island ecosystems two common characteristics emerged: 1) a general mechanism of island formation and 2) vegetation communities that are a subset of the surrounding lowland forests. All tree islands form through a combination of directional, moving waters and biological activity. Islands are initiated with a physical-chemical point of formation such as a bedrock topographic high or low or a minerotrophic groundwater outflow. Biotic factors, usually plants, respond to that point of formation by raising the surface elevation of the island above the surrounding water level through deposition of plant litter. Plants also bind soil substrata or increase island sedimentation by stabilizing the point of formation. Other biotic factors of tree island formation include termites and seed dispersal by animals, primarily birds. Review of the literature also found that the vegetation on tree islands is a subset of the surrounding regional forest community. No endemics or rare plant species are reported to grow on tree islands. Hydrology is the primary factor affecting tree island vegetation in all systems and controls community composition, species richness, and vegetation zonation. Hydrology also controls succession and ultimately is linked to island development. Secondary succession is related to the ecosystem disturbance regime. Fire, flooding, and drought are disturbances common to all tree island ecosystems. All tree island ecosystems form in extraordinarily flat landscapes. If overland waters have a low velocity and climatic conditions support peat formation the result is a peatland tree island ecosystem, of which the Florida Everglades is an example. Higher velocity water flows, such as in or along rivers, result in non-peatland tree island ecosystems. Non-peatland tree islands are subject to abiotic factors common to riverine systems: high water velocity, rapid changes in hydrology, and alluvial geologic forces. These abiotic factors may have a greater influence on island formation than biotic factors, especially in the early stages of island development. Peatland tree island systems are just as dynamic, but biotic factors may dominate the formation of these islands. Tree islands have long-distance ecological links that extend far beyond the apparent boundaries of the island, requiring an expansive wetland complex to support them. Humans have affected nearly all tree island ecosystems through physical restructuring, discharge of wastes, or the introduction of exotic species. Although tree islands appear to be resilient, their alteration and destruction in the Everglades clearly illustrates that they can be destroyed or greatly modified by human activities.

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