25 Mar 2011, 1:36pm
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Ancient trash heaps gave rise to Everglades tree islands

American Geophysical Union Release No. 11–12, 21 March 2011 [here]

SANTA FE, N.M.—Garbage mounds left by prehistoric humans might have driven the formation of many of the Florida Everglades’ tree islands, distinctive havens of exceptional ecological richness in the sprawling marsh that are today threatened by human development.

Tree islands are patches of relatively high and dry ground that dot the marshes of the Everglades. Typically a meter (3.3 feet) or so high, many of them are elevated enough to allow trees to grow. They provide a nesting site for alligators and a refuge for birds, panthers, and other wildlife.

Scientists have thought for many years that the so-called fixed tree islands (a larger type of tree island frequently found in the Everglades’ main channel, Shark River Slough) developed on protrusions from the rocky layer of a mineral called carbonate that sits beneath the marsh. Now, new research indicates that the real trigger for island development might have been middens, or trash piles left behind from human settlements that date to about 5,000 years ago.

These middens, a mixture of bones, food discards, charcoal, and human artifacts (such as clay pots and shell tools), would have provided an elevated area, drier than the surrounding marsh, allowing trees and other vegetation to grow. Bones also leaked phosphorus, a nutrient for plants that is otherwise scarce in the Everglades. … [more]


The Human Trigger for Development of Tree Islands in the Florida Everglades

by G. L. Chmura1, 2; M. Graf1, 2
1. Department of Geography, McGill University, Montreal, QC, Canada.
2. Global Environmental and Climate Change Centre, McGill University, Montreal, QC, Canada.

Mar 22, 2011 Presentation, American Geophysical Union’s Chapman Conference on Climates, Past Landscapes, and Civilizations, Sante Fe NM.


During the last glacial, the vegetation of South Florida was arid steppe distributed across a karst landscape. Over the Holocene rising sea levels caused the water table to rise, eventually creating a landscape with saturated soils and a “river of grass”, i.e., marsh vegetation dominated by sawgrass. The modern Everglades also contains elevated areas that serve as islands of terrestrial vegetation, many high enough to support trees. The slightly higher elevation (1-2 m) and vegetation diversity of the islands provides refuge for wildlife avoiding seasonal high waters and the islands are highly valued as hotspots of diversity within the Everglades system.

Sediment coring for paleoecological studies indicated that peats of the islands were younger and shallower than those of the surrounding marsh, supporting a widely held belief that the tree islands formed over localized high points in the underlying carbonate bedrock. More recent discovery of a hard mineral layer perched within the peat soils of many islands reveals that depth to bedrock, and age of islands has been misinterpreted, calling for a new theory of tree island development. The ~40-75 cm thick layer, a “pedogenic calcrete” is composed of reprecipitated calcium carbonates and calcium phosphates interspersed with bone, charcoal, and human artefacts. Below is a midden with cultural artefacts that date to the origin of the Everglades wetland system about 4,000 years ago.

It is likely that early human occupation on the landscape was responsible for development of the Everglades large tree islands as well as island stability and productivity. The elevated surfaces of middens provided refuge for terrestrial vegetation as surrounding soil became saturated. Bones in the middens provided additional bulk as well as phosphorous for plant growth in the otherwise phosphorous-depleted wetland. These better soil conditions allowed establishment of trees which were key to development of the perched mineral layer. Trees are known to play a major role in pedogenic calcrete formation, particularly in areas with pronounced wet and dry seasons, such as the Everglades. Transpiration by the trees draws carbonate-rich water through the soil and carbonates and phosphates excluded from the roots are deposited within the rooting zone. Our research indicates that this process probably began with establishment of trees on the island and is an ongoing process.

Fire is a regular phenomenon in the Everglades, resulting in combustion of peat soils. Because it does not burn, the calcrete layer protects the underlying soil, maintaining elevation and allowing regrowth of terrestrial vegetation. Ironically, it is human disturbance that now threatens these valuable ecosystems. Urban development has displaced much of the Everglades and water control systems threaten the remainder. The calcrete layer is subject to dissolution under extended flooding and if not maintained by tree growth. This is likely occurring in South Florida’s water control districts today, where high water levels are maintained and tree islands are disappearing.



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