Trees may be key to building Everglades’ tree islands

Rising just a few feet above the Everglades’ vast marshes, “tree islands” are dry roosting and resting spots for birds and terrestrial mammals in an otherwise flooded landscape. They’re also known to accumulate phosphorus, which scientists believe may help keep nutrient levels in the overall Everglades ecosystem low.

Tree island in Florida Everglades

Yet, while the ecological value of Everglades’ tree islands is well established, why they exist and how they form remains up for debate. A new study in the Nov.-Dec. 2014 issue of the Soil Science Society of America Journal now offers new support for one leading hypothesis: Tree islands are built and maintained by the trees themselves.

Tree islands are mounded patches of ground just high and dry enough to support trees, and they used to be widely distributed across the Everglades. When people began managing water at artificially high or low levels in different sections of the ecosystem, however, tree islands were lost in large numbers.

For example, in one Everglades water management unit, Water Conservation Area-3, more than 1,235 acres of tree islands existed in 1995. But by 2004, one-fifth of them had disappeared, says Len Scinto, a wetlands scientist at Florida International University (FIU), who led the current study with his former grad student Andres Rodriguez and postdoctoral associate, Alexandra Serna.

Likewise, in Water Conservation Area-2 an estimated 98% of tree islands smaller than eight acres in size are now gone—and when tree islands go, not only the trees are lost. Ecological services such as biodiversity support, wildlife habitat, and sequestration of nutrients are lost, as well. “Even though they only occupy a very small area of the Everglades, tree islands are really important,” Scinto says. That’s why it’s critical to understand how they’re created and sustained.

Soil core from tree island

Proposed mechanisms for their formation include the idea that some tree islands originated centuries ago as the garbage dumps of prehistoric peoples. Another is that animals build the islands by depositing guano, bones, and other wastes on them. What interested the FIU team, though, was whether a process exists that allows tree islands to grow above the surrounding marshes without “outside” help or influence.

In other words, says Scinto, “Are tree islands self-organizing?”

Tree island soil

To get at that question, the researchers examined rates of soil buildup and changes in soil chemistry in four constructed tree islands in the Loxahatchee Impoundment Landscape Assessment (LILA). Built jointly by the South Florida Water Management District, U.S. Fish and Wildlife Service, and the Army Corps of Engineers to mimic the Everglades on a small scale, LILA is where scientists test restoration and management techniques before applying them in the larger ecosystem.

As part of LILA, eight tree islands were constructed either of mounded peat or a limestone core topped with peat. They were then planted with native tree species in 2006 and 2007.

What the team found during three years of study is that the constructed islands accumulated soils at an average rate of 0.7 centimeters (0.28 inches) per year—a fast pace of growth rivaling soil development rates in Florida mangrove forests, Scinto says. In contrast, soil buildup in the surrounding marsh is six to nine times slower, according to other studies, averaging just 0.08 to 0.12 cm per year. Tree island soil accretion even appears to outpace nearby areas loaded with phosphorus, where others have estimated soils accrue at 0.20 to 0.56 cm per year.

Moreover, the tree islands’ top one-inch of soil held significantly higher amounts of phosphorus, nitrogen, and carbon than deeper and older soil layers—indicating the islands were becoming enriched in nutrients over time.

This all suggests a process is developing that “could eventually create a positive feedback loop, and therefore, a self-sustaining tree island,” Scinto says. Not only do the findings indicate how tree islands might be fundamentally functioning, he adds; they also imply that constructed tree islands might not need much ongoing intervention to keep developing.

Trees as pumps

Previous research by Scinto, his FIU colleagues Rene Price and Mike Ross, and others, offers additional evidence for the self-sustaining process, which works like this. When trees are young and small, they tap only the top soil layers on a tree island and whatever surface water and nutrients are available there. But as they grow, their roots extend into shallow groundwater, where higher concentrations of ions such as phosphate are found.

Len Scinto examining soil core

This richer source of nutrients boosts the trees’ growth and productivity, leading to an increase in leaf litter. The leaf litter, in turn, builds the soil and the island, and fuels the trees’ growth even more.

In short, “the trees are “bringing to these islands a source of water that is higher in chemical nutrients than the surrounding surface water,” Scinto says. “So they’re acting as a kind of ion pump that feed backs to the topography or height of the island.”

Something else the current research revealed is that soil buildup and nutrient enrichment happened fastest at the highest elevations on the islands, where water reached infrequently or not at all. This has implications for managing the Everglades’ remaining natural tree islands. If, for example, water is held at a level that’s unnaturally high but not high enough to drown out tree islands completely, it may nonetheless stunt the islands’ growth.

There’s another important implication, as well. Although Everglades land managers have no immediate plans to restore tree islands, there is talk of plugging some of the ecosystem’s miles and miles of canals with soil. The first goal would be to rehydrate areas of the Everglades that are today excessively drained by ditches and channels and therefore much too dry. But then people are asking, Scinto says, “If we are going to plug the canals, can we mound the plugs high enough so that they can support trees?”

If this does come to pass, the trees, it seems, can take it from there.

From "Soil Accretion Influenced by Elevation, Tree Density, and Substrate on Reconstructed Tree Islands." This article will be open access for 30 days after the publication of this news story.

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