Elongate, tear-drop shaped tree islands of the Everglades began forming as early as 3,000 years ago, based on pollen and geochemical evidence from Gumbo Limbo and Nuthouse tree islands (WCA 3B). Before tree-island formation, sawgrass marshes with weedy annuals occupied heads and tails of modern tree-island sites. In early phases of tree-island development, abundance of ferns and shrubs increased, becoming dominant when tree islands matured. Vegetation on the tree island indicates shallower water than in the surrounding marsh throughout the history of the sites.

High phosphorous levels occurred on the head throughout the history of the site; on tails, they increased up to sixfold after tree-island formation. Initial baseline phosphorous levels in the marsh increased after tree-island maturation. Elevated phosphorous levels may reflect use of tree islands as rookeries by bird populations throughout their history.

Our results suggest that Gumbo Limbo island formed around 1200 BC, maturing by about 800 AD, whereas Nuthouse Island formed around 300 AD and matured by 1500 AD. These data suggest modern tree islands formed at various times over the past 3,000 years and that full development requires 1,000 to 2,000 years.

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