(truenature)

Duck Soup

In a floodplain near the Mississippi River is one of the country’s last remaining bottomland forests. The primordial ooze in these soggy woods is home to millions of small fish and insects—enough to feed 350,000 wintering mallards, gadwalls, teals, and other ducks.

By Dan Ferber

Cypress gardens: Porous trunk tissue allows more oxygen into cypress trees when their roots are submerged under standing water.
Photograph by Richard E. Hines

Down in the bottoms, the hard rain has stopped, and Larry Mallard and I slosh through ankle-high water into the deep-green forest of the White River National Wildlife Refuge, near St. Charles, Arkansas. Sturdy oaks, elms, sugarberries, and ashes tower over us as cicadas and chorus frogs break the afternoon silence. Mallard, the manager of the refuge, points to a line of light-brown silt on the thick trunk of an oak, about 16 feet up from the forest floor. “That’s the high-water mark,” he says. I peer up, amazed. Such massive flooding would kill most trees as easily as an overwatered houseplant. But the ones here are thriving. Even Mallard, who for decades has seen winter floodwaters submerge this forest, seems momentarily impressed by the height of the year’s flood. “That would be extreme water,” he says.

Mallard and I are walking just a few miles from the Mississippi River, in the closest thing the Mississippi Valley and most of the nation have to the forest primeval—the vast bottomland forest fringing Arkansas’s White River. It’s a land where centuries-old tupelo gum trees still sit in watery backswamps alongside bald cypress trees that grew here before Columbus.

An hour ago we were standing at the edge of the White River, across from recently flooded trees sitting far above its muddy bank. Now we’re in the forest, and Mallard, a bearlike man with a salt-and-pepper beard and a baritone drawl, is explaining how native river fish make full use of such floods: “The water comes out here and sits, currents are moving, and the fish move out here looking for something to feed on.” It’s a bit jarring to imagine a two-foot gar chasing minnows at the level of my face right now. But such sights are common here when the water’s high.

As the White River snakes toward the Mississippi across the flat delta land of southeastern Arkansas, its floodwaters nourish a unique 155,000-acre forest that still shelters most of its original species, including bald eagles, river otters, black bears, and bobcats. Up to 350,000 ducks—mallards, gadwalls, teals, and wigeons—winter here each year,  fattening up on insect larvae and other invertebrates before flying north to breed. All told, the White River refuge harbors more than 250 species of birds, including rare songbirds, herons, egrets, storks, and kites.

In the spring of 2004, a few miles upstream in the 64,000-acre Cache River National Wildlife Refuge, searchers videotaped the long-lost ivory-billed woodpecker. The discovery made headlines worldwide, and some scientists believe a remnant population of the woodpecker remains in the White River refuge. Still, Mallard says, “the compelling story is the ecosystem.”

That ecosystem once covered river floodplains throughout the Southeast, especially in the Lower Mississippi valley, whose flat floodplain spreads up to 125 miles and stretches from southern Illinois to Louisiana. For millennia the Mississippi and tributaries like the White, the Yazoo, the Tennessee, and the Tensas would abandon their banks each winter, linger in the woods for months, then retreat reluctantly each summer. Today just 4.8 million acres—or 20 percent—of the original 24 million acres of bottomland hardwood forest remain, much of it cut off by levees from the rivers that once fed it. But not here. On this day Mallard and I are wandering one of the finest such forests left on earth.

 

The heart of the bottomland forest is the river, and in the White River refuge, it still beats strongly. Here the forest widens to 10 miles, hugging 90 miles of river as it meanders lazily toward the Mississippi. The extent of the flooding can be mind-bending: When the Mississippi River rises just a few miles downstream and heavy rains swamp the White in northern Arkansas, the bottomland turns into a bathtub. During a typical summer, rivers, bayous, sloughs, and lakes cover just 4 percent of the 160,000 acres. In spring up to 90 percent of the refuge is submerged, and the only way to get around is by boat.

When the river first spills its banks in early winter, it flows through shallow swales and sloughs, eventually covering the ground completely. Huge numbers of invertebrate eggs, dormant all summer, hatch, producing larval dragonflies plus isopods, crawfish, and snails. Minnows devour the invertebrates. Smallmouth buffalofish, longnose gar, and other river fish—some several feet long—roam the flooded woods, consuming the minnows. The larger fish are then eaten by black bears, river otters, and mink.

Plants here have developed special adaptations to deal with flooding. Overcup oaks actually take advantage of the high water by producing a unique floating acorn. Trees in the wettest areas must overcome soils that are waterlogged for months or years on end. “Normally trees take oxygen in through their roots. When water goes in, that’s shut down,” says Richard Hines, the White River refuge’s wildlife biologist. Tupelo gum and bald cypress trees escape suffocation by producing spongy, porous tissue in their trunks at or above the waterline that pulls in oxygen directly from the air. “There’s actually breathing going on,” Hines says.

Bald cypress, which live for more than half a millennium here, and tupelo gum also pack that spongy tissue into the ridges along the widened bases of their trunks. Scientists believe the ridged shape and increased trunk circumference create additional surface area, which allows even more oxygen to diffuse into the tree and help it grow. In addition, cypress trunks shoot out thin, porous roots that float near the water’s surface, where oxygen is plentiful.

Animals, too, have evolved special adaptations that enable them to thrive in bottomlands. Just-hatched gar larvae, for example, cling to grasses and other vegetation until they’re big enough to swim off. When the summer heat traps adult gar in shrinking puddles, they gulp air to take in oxygen, which helps them survive until the late-autumn floods enable them to them swim back to the river. Black bears, which den in brushy clearings in higher forests, make their homes here inside hollow old cypresses and oaks, high above the floodwaters. “They just kind of hang out there until the water goes down,” says Hines.

When the water returns to the river, as it does most summers, it exposes a complex network of watery arteries. Slow-flowing bayous snake through the forest, and oxbow lakes, formed long ago when the river cut loose a meander, punctuate the landscape like wayward commas. No one knows exactly how all the waters connect.

Just a few feet of elevation makes a huge difference in this wet, flat land. On low but non-swampy sites that flood half the year, water hickories, overcup oaks, and Nuttall oaks thrive. Less water-tolerant species, like willow oaks, sweetgums, and mockernut hickories, do well on terrain a few feet higher, which might flood several days a year or not at all.

Annual floodwaters also shape and reshape the floodplain. As the turbid river spills from its banks, it drops sand first, creating low natural levees. Farther out in the woods, the waters slow and pool, then silt and clay settles, stocking the soil with nutrients. Rapidly decaying plant and animal detritus further enrich the fertile soil. 

Enlarged trunks help tupelos absorb oxygen
Photograph by Richard E. Hines

In a section of forest with an open, parklike feel, Mallard points out a shallow, saucer-shaped depression on the ground, about 10 feet across and a foot deep. He kneels down and grabs a handful of moist, dark-brown leaf litter. It has been chewed to pieces by small animals like water beetles and caddis-fly larvae. Each spring retreating floodwaters leave behind countless shallow pools like this one that linger in the forest for weeks. “Just think of the thousands and millions of little creatures and critters out here, and you lay water out here, and you end up with a soup,” says Mallard. Dabbling ducks like mallards and gadwalls, which can’t dive for food, dine on that soup, nabbing isopods, larval dragonflies, damselflies, and other invertebrates. The ephemeral pools also serve as amphibian factories, producing legions of salamanders, tadpoles, and frogs.  

Even top predators thrive. Although the wolves and cougars that once roamed the bottoms are long gone, more than 400 native black bears—the population here is growing—fish in backwater bayous, and bald eagles nest in the refuge. Swallow-tailed kites lived here in 2002 for the first time in a century; last year they hatched their first two chicks.

 

The bounty of the bottomlands emerged relatively recently, in geological terms. During the past 2 million years torrents of meltwater from successive continental ice sheets flattened and widened the Lower Mississippi River valley. After the last Pleistocene glaciers retreated about 12,000 years ago, the local climate warmed and precipitation increased, which allowed bottomland hardwood forests to replace the sort of boreal forests now common in Canada. For centuries the Mississippi and its tributaries meandered tens of miles across the floodplain, shaping and reshaping natural levees, ridges, swales, and backswamps. 

Before European settlers got to them, those forests soaked up the equivalent of 60 days of Mississippi River flow, preventing flooding downstream. By the early 1970s, when the most recent research was done, they absorbed just 12 days’ worth of floodwater. And like other wetlands, floodplain forests filter excess nutrients and impurities from the water, keeping them from reaching the rivers.

In a grove of mature trees, Mallard sweeps his arm toward a large, sunny clearing covered with greenbrier and grasses; muscadine vine hangs from trees at the edge. A big tree—one that’s since decomposed—dropped here, either from a windstorm or from simple old age. Such clearings renew the forest, allowing oak seedlings to sprout and low-lying plants to provide browse and cover.

When the bottomland forests stretched up and down the Lower Mississippi valley, songbirds that prefer tree-fall clearings, such as Swainson’s warblers or Acadian flycatchers, might have flown over thousands of acres to find one. Today even the White River refuge is too small to rely on nature to create such clearings, so Mallard’s crew makes them instead by taking out trees at random sites.

Other human interventions in the bottomlands have been far less benign. Starting in the 1800s settlers cleared entire river bottoms along the Mississippi and Ohio rivers to fuel wood-powered steamships, logged millions of trees for lumber, and knocked down wide tracts of these forests to farm the rich floodplain soil. After the disastrous Mississippi River flood of 1927, local drainage districts and the U.S. Army Corps of Engineers deepened and straightened rivers and built most of the thousands of miles of levees that now line the region’s waterways, cutting them off from the surrounding forests. Between 1947 and 1977 fully half the remaining bottomland forest—5 million acres—were cleared, often just knocked down, bulldozed into piles, and burned, the victim of federal subsidies to create farmland and of temporarily sky-high soybean prices. In the late 1960s, recalls Leigh Fredrickson, a wetland ecologist with the University of Missouri, Columbia, “if you sat on an airplane from Memphis and flew to Baton Rouge, you’d see lines of fire everywhere.”

The clearing took its toll. Species that depended on large tracts of forest, including the Bachman’s warbler, disappeared and are presumed extinct. But today the government is working to reverse the damage with programs like the USDA’s Wetlands Reserve Program, which pays landowners to restore or enhance wetlands on their property (see “Green Acres,” November-December 2005). About half a million acres of forest in the Lower Mississippi valley have been replanted since 1990. Science is helping, too. Research foresters Emile Gardiner and Ted Leininger of the U.S. Forest Service’s Center for Bottomland Hardwoods Research in Stoneville, Mississippi, have sped the succession of former farm fields to bottomland hardwood forests by using fast-growing native cottonwood trees to shelter red oak seedlings. Nine years after the oaks were planted in one experimental tract, they’re 15 feet tall. “You’ve got more or less instant forest,” Leininger says. 

In the Cache River refuge, established in 1986 on a tributary upstream from the White River refuge, wildlife biologist Dennis Widner’s team has purchased 64,000 acres of the floodplain, much of it cleared for farmland, and has thus far turned approximately 15,000 acres back to woodlands.
 
The day after Mallard and I walked the White River bottoms, he, Widner, and I stand on a sunny, humid summer afternoon on a levee road in the Cache River refuge. On one side of the road, half a dozen centuries-old cypress trees stand knee-deep in Bayou De View, like sentries guarding the forest beyond. Just six miles up the bayou, a tributary to the Cache, searchers had finally made what many scientists believe was the first trustworthy recording of an ivory-billed woodpecker in seven decades.

Still, in the 1960s a local farmer had cleared old-growth cypress-tupelo swamp right up to the edge of the bayou here, built the levee, and begun planting soybeans. The farmer went bankrupt; Widner’s crew bought the land from his bank. Then in the early 1990s they busted through the levee to reconnect the bayou with the field, and planted hardwoods to restore the swamp forest to its former grandeur.

“The work that I’m doing on the Cache, you’ve got to visualize it 100 years from now,” says Widner. “That’s what we’re really managing for.” If all goes right, the land will nourish a watery forest filled with thriving cypress and tupelo trees. Maybe even ivory-bills will find a home here, pounding huge holes in old oaks, alongside bears and fish and eagles and bobcats. The vast bottomland-forest wilderness of old is gone forever in much of the Mississippi River valley, the victim of our desire to control nature and harness rivers big and small. But here in this small corner, the pulse of the river may well once again nourish a vibrant forest.

 

Dan Ferber is a freelance writer and a contributing correspondent for Science.

 


© 2006 National Audubon Society
 

Sound off! Send a letter to the editor
about this piece.

Enjoy Audubon on-line? Check out our print edition!

HOME

The Nature of Bottomland Forests

Bottomland forests are shaped and fed by floods. Over the centuries the river cuts into the soft, muddy bank on the concave side of each bend, gradually pushing it outward and making the river more sinuous. Major floods occasionally cut through the forest to a downstream bend, stranding a meander as an oxbow lake. A time-lapse video over millennia would show the river whipping across the floodplain like a loose water hose with the faucet full blast. During annual floods, the river overtops its banks into the bathtub-shaped bottomland, quickly dropping its sand to build a low natural levee. The floodwaters pool and sit for months in the forest, where nutrient-rich silts and clays slowly settle, enriching the soil. Together the shifting river and annual floods create a complex landscape of flats, swamps, gently undulating ridges, swales, bayous, lakes, and sloughs. Water-loving trees like bald cypress and tupelo gum grow in the forest’s swampiest areas, which stay submerged for years at a time; overcup oaks and water hickories grow in areas a few feet higher that flood for months each year; and willow oaks and Nuttall oaks grow in higher areas that might flood for a couple of weeks every few years.

—Dan Ferber