Tree Islands

In South Florida, the term tree island is most often used to depict the island-like appearance of a patch of forest in an Everglades marsh. Tree islands provide nesting sites for turtles and alligators, protective cover for wildlife such as deer and otters, and roosting and nesting locations for many birds. Evidence also shows that Native Americans not only used, but likely initiated the formation of high ground on certain tree islands through their long history intertwined with the evolution of the Everglades.

Kinds of Tree Islands

Bayheads

By far the most common type of tree island. island. They are named for swamp bay,* but typically have several other wetland-tolerant trees, including sweet bay and dahoon.

Willows and Willow Heads

Coastal plain willow, a low tree that colonizes rapidly with wind-blown seeds; good marker of an active alligator hole.

Cypress, Cypress Domes, and Cypress Heads

Cypress forests of various kinds occur widely in the Big Cypress Swamp but are limited in the Everglades.

Tree Island Formation

Tree islands in the Everglades peatland have mainly formed by three processes:

Pop-up or Battery Tree Islands

Pop-up or battery tree islands occur in sloughs where water lilies are abundant. Lily roots and rhizomes commonly fill with gas; and additional gas bubbles — mostly methane — are produced in the peat by decomposition. As a result, slough-bottom peat can become buoyant. If an area of peat exceeds the peat’s cohesion, it tears free and rises to the surface as a pop-up or battery. Although pop-ups do not look substantial, their strength of interwoven roots can usually support the weight of a person.

Photo by Tom Lodge

Strand Tree Islands

Strand islands are huge compared to pop-up islands but relatively narrow and fixed in location. Investigation reveals they are made up of Gandy peat underlain by sawgrass peat. It is apparent that they evolved from sawgrass strands by shrub invasion.

US. Fish and Wildlife Service, A.R.M Loxahatchee National Wildlife Refuge

Fixed Tree Islands

Fixed tree islands are large and teardrop-shaped with an orientation aligned with the historic direction of water flow. The “fixed” designation comes from appearing to be “anchored” on an elevated area at their upstream end. Studies in the past decade have suggested that Native Americans played a role in strand tree island development.

Pond Apple (Custard Apple)

Photo by Tom Lodge

The most peculiar tree associated with tree islands is the pond apple, a species that thrives in a longer hydroperiod and deeper water than most. Though seldom taller than 20 feet, it develops a massive trunk and buttressed roots. Due to its common location in or next to open water, and its convenient configuration of branches, nesting birds such as the anhinga regularly use pond apples. Historically, there was a forest of pond apple at the head of the Everglades rimming the south shore of Lake Okeechobee.

Lodge, Thomas E.. The Everglades Handbook. CRC Press. Kindle Edition.