By Keith Kloor

"I hate it when I sink," says Debra Willard, a paleoecologist with the U.S. Geological Survey (USGS). Though water lilies are floating around her knees, she keeps moving, determined to find the remnant "tail" of a tree island called L-67. Nearly all of the mile-long island is gone, its soil and flood-tolerant trees washed away by the unnaturally high waters of the past several decades. Willard seizes on the rotting willow poking out of the marsh as evidence that she's in the right vicinity. Her other marker is a white blur in the distance, which attests to the island's "head." There, several dozen white ibis are roosting on the gnarled limbs of a few scraggly trees, the last vestiges of what had been a narrow, unbroken stand of green willow and pond apple.

We're about 15 miles from our launch point, just west of Miami. Bill Orem, a USGS colleague who is trailing Willard, turns around to check on all the splashing behind him. He watches me totter to the side, then grasp a clump of dead branches to steady myself. Apparently I have either stepped into a slight depression formed a minute ago by one of the scientists, or I've experienced one of the marsh's sudden drops in depth. Either way, I am no longer ankle deep; the muddy water is now riding up to my knees. Orem flashes me a wry grin and says, "You have to remember that in the Everglades, inches mean everything."

Here in the wetland's interior, where pumps and levees divert water for storage and flood control, nothing illustrates this more dramatically than the fate of tree islands. In 1940 there were 1,251 of the elongated, tear-shaped islands in the central Everglades, which is about the size of Rhode Island. Today 581 remain. The islands were long ignored because of their relative inaccessibility, but now scientists are racing against time to discover their ecological secrets.

The Nature of Tree Islands Greg Hargreaves About 4,000 years ago the deepwater sloughs covering much of the Everglades were interrupted by patches of sawgrass marsh growing on high points in the underlying limestone. After a drop in water levels about 1,200 years ago, shrubby plants such as wax myrtle and button bush begin colonizing the mounds. Their decaying remains built up the peat soil into islands, where willows and other water-tolerant trees eventually became established. The tree islands reached their mature state about 800 years ago. The "head" of an island features water-tolerant trees such as willows, pond apples, and gumbo-limbos; sawgrass and shrubby plants form a transition zone between the island and the adjacent marsh.

Though the Everglades is primarily flat and dominated by sawgrass, its topography is defined by a series of alternating ridges and sloughs, through which an imperceptible flow of water courses south from Lake Okeechobee. The ridges are only six inches higher than the sloughs, but the slight gradation makes a huge difference in this sodden world. In effect, sloughs are the valleys of the Everglades; tree islands are its mountains.

Because they are thought to form on elevated bedrock just a few inches higher than the surrounding marsh, the islands provide what Orem calls "havens of biodiversity." Tree islands can be a half-mile to two miles long, and ecologists estimate that some have two to three times more plant and animal species than the surrounding marsh. Wading birds use the islands' trees to nest; alligators and turtles lay their eggs on the dry land; and deer, snakes, lizards, and many other animals find refuge there from rising water, which has prompted ecologists to declare that tree islands also "act as a fleet of Noah's Arks."

One of those ecologists, Fred Sklar, works for the South Florida Water Management District. He has agreed to show me some of what's left of the "fleet"--from a helicopter, 500 feet above the marsh. At first I see nothing except a muddy carpet of sawgrass. Then we pass over a long pocket of solid green called Rescue Strand. This island, one of the last vibrant rookeries in the central Everglades, is covered with lustrous pond apple trees that support what Sklar calls "wading-bird condominiums." The pond apple tree grows in clumps, spreading out to form many levels of branches, which is ideal for nesting birds. Wood storks, great blue herons, and other large birds nest on the top "floors"; snowy egrets, tricolored herons, and other smaller birds occupy the canopy below.

Peering down at the island, I spot a large hole outside its head. Sklar tells me it's an alligator hole; the reptiles like to dig holes in or around an island, he explains, so they can lay their eggs on dry ground.

When we land a few minutes later at the head of another lush, unnamed island, I'm relieved to see a long wooden catwalk leading to the island's interior. Looking around, I can see the walkway is necessary for other reasons as well: On the shores of a tree island, an unforgiving mix of scrubby marsh plants and dense sawgrass makes for a difficult entry.

The catwalk leads past an alligator hole and into a partial clearing of willow and pond apple trees. Suddenly, we've entered the island's jungle interior. A few steps in any direction leads into a crunchy thicket of shrubs, ferns, and broken twigs, all wrapped in curling vines. The South Florida Water Management District launched numerous field studies here in 1998; now Sklar stops to inspect an open wooden box set on the ground and enclosed in wire mesh. The "litter trap" is designed to capture falling leaves from the trees. "Litter fall is critical to maintaining elevation of land," Sklar says. The study, he explains, will give some indication of how much litter is amassed over a given period and how much is required to form peat--the decayed plants that compose the main soil of the Everglades.

A few feet away, near an enormous strangler fig tree, sits a night camera attached to a motion sensor; the camera photographs animals that cross the sensor's beam. "We need to know what's out here," says Sklar. "We need to document whether there are endangered or threatened animals using these islands." To him, the tree islands represent uncharted worlds. "You walk onto one of these islands, and you're not sure what you're going to find," he says. "You recognize 90 percent, maybe 95 percent of the flora and fauna, but the other 5 percent you say, what is this?"

Until recently, not much was known about the ecology of tree islands, much less about the role they play in the larger Everglades ecosystem. Scientists are only now beginning to understand how the islands originated and developed. "I'm amazed at how little is known about them," says Debra Willard, whose investigation of the ancient pollen record from seven islands is just starting to fill in some blanks.

For example, before Willard and Orem began their joint study in 1998, some scientists had assumed that tree islands were a young feature of the Everglades, perhaps no more than 100 years old. According to this hypothesis, the islands emerged when the marshlands dried out in the early part of the 20th century, after the first major canals and dikes began channeling water out to sea. The extended dry period was thought to have encouraged woody vegetation to flourish on the raised areas of the marsh. Under this theory, the prolonged flooding that followed over the next 50 years--when these areas were used for water storage--did not threaten the islands.

But recent findings by the two USGS paleoecologists have proven otherwise. Their radiocarbon dating of sediment and pollen from the underlying peat reveals that the tree islands have, according to Willard, "evolved along with the marshes, which are about 5,000 years old." Today's mature tree islands, they discovered, took their final shape 1,000 to 2,000 years ago.
Willard speculates that tree islands began with sawgrass marshes growing on high points in the limestone, surrounded by water lily sloughs and deeper water. Over time, peat accumulated in the marshes. Then, during dry periods, conditions became favorable enough for ferns and shrubby plants like button bush and wax myrtle to move in, forming a "head." As the plants died and decayed, they added to the soil buildup; as the islands grew, more and more peat and sediment accumulated. This fostered the emergence of additional species, such as the water-tolerant pond apple and gumbo-limbo trees.

"If you go back and date the pollen and sediment over the past 2,000 years," says Willard, "you'll see that tree islands were relatively stable until the 20th century." There's a big change in the pollen record of drowned islands about 1960, which corresponds to the conversion of the central Everglades into a massive holding pen for water. "We picked up a huge increase in water lily pollen," says Willard. "This plant only gets abundant when the water level is high for very long periods of time." About the same time, she says, "tree island species dropped out almost completely."

Willard blames human alteration of natural water patterns. In the northern and southern ends of the Everglades, where not enough water reaches the marshes, tree islands are actually starved for water. The dry conditions often lead to muck fires, and whole islands can go up in flames, burning away both vegetation and soil.

In the central Everglades, where the marshes now stay perpetually flooded, the tree islands rarely have a chance to dry out. "They can take seasonal flooding, but the trees can't survive with their feet in the water all the time," says Willard. When the tree roots can't get oxygen efficiently, the trees suffocate and die. And even the hardiest of water-tolerant trees, like willow, need the islands to dry out enough for their seeds to germinate in the moist soil.

On the day I accompany Willard and Orem to investigate what's left of L-67, they are joined by biologist Tim Towles, who is tracking the island's bird population for the Florida Fish and Wildlife Conservation Commission. In 1992 Towles documented 1,400 nesting birds there, including 700 great egrets and 450 white ibis. Now, he says, there are not enough trees to support more than a few hundred birds. In addition to the white ibis roosting on the last scrawny patch of pond apple, he spots about 50 anhingas through his binoculars. "Historically, this has been a really important rookery island," he says ruefully, "and now, in the blink of an eye, it's gone."

This is Willard and Orem's second trip to L-67. (Ironically, the island, a victim of engineering, is named after the L-67 canal, two miles to the east.) Last year they collected several samples of peat from the head and determined the island was 3,000 years old. Today's goal is to flesh out the island's history and corroborate that date with "coring" samples from the tail, which, after considerable effort, Willard has approximately located.

They have been surprised to discover that the peat samples from all seven of their study islands show a heavy abundance of naturally occurring phosphorus--especially on notable colony islands like L-67. "We think it's coming from the birds," says Orem. Bird droppings break down into phosphorus, which leaves a record in the peat. The high concentrations show up even in the oldest islands in their study, which date to 4,000 years ago. This suggests that wading birds have flourished in the Everglades since the watery wilderness was born, five millennia ago--at least until the 20th century, when their populations dropped an estimated 90 percent. "There may be a link between the disappearance of tree islands and the decline in wading-bird populations," says Orem.

A good scientist won't state any conclusions without absolute proof, but it only takes a few minutes in a helicopter for an average observer to see what's wrong with the Everglades. The coffee-colored expanse of the northern Everglades appears dry and brittle; Florida is in the middle of its worst drought in recorded history, and this area--always starved for water--is feeling it the worst. Sklar and I pass over some large bushes sticking out of the parched landscape, what he calls "ghost islands." We ride in silence for a bit, and then he turns to me and says with a pained look, "We've got a little Galápagos system down there, and we're really messing it up."

Like all the other scientists at work in the Everglades, Sklar is pinning his hopes for the tree islands on the vaunted $7.8 billion Comprehensive Everglades Restoration Plan. If engineers are able to restore natural water flows, he says, then the remaining tree islands may yet hang on. In the meantime, the South Florida Water Management District is gearing up for its own tree-island-restoration project, which will include shoring up the peat on distressed islands, eliminating exotic plants, and replanting native vegetation.

In addition, the results of studies by other scientists, including Willard and Orem's groundbreaking findings, will be published later this year in a book on tree islands. The reference text--which Sklar is editing--will be the most comprehensive body of knowledge ever assembled on the ecology of Everglades tree islands. By all accounts, the information couldn't come soon enough. "The health of the tree islands reflects the overall health of the Everglades," says Sklar. "So all this new interest and research will go a long way toward helping us preserve these hot spots of biodiversity."

Keith Kloor is an editor at Audubon magazine.

© 2001  NASI

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