When Lewis and Clark  traversed the West, cottonwood trees lined rivers, streams, and springs like threads of green stitched through the forbidding expanses of plains and desert. The tall trees with fat trunks and canopies of rustling leaves rising as high as a ten-story building formed groves whose green and billowing crowns stood out like beacons in the largely treeless landscapes, guiding travelers and advertising water, shade, and wood.

Lewis and Clark’s company floated up the Missouri in pirogues—canoes shaped from whole cottonwood trunks. Fort Mandan, which sheltered the explorers through their first Great Plains winter, was built from cottonwood logs; the wood, described by Lewis as “of a white color, soft spungey and light,” also supplied fuel for heat and cooking. The wheels and axles for their portage around the Great Falls on the Missouri were carved of cottonwood, and the explorers noted that Indians used the sweet inner bark of the tree as winter silage for horses.

“Native American cultures had a close connection with cottonwoods,” says Mike Scott, research ecologist with the U.S. Geological Survey (USGS) in Fort Collins, Colorado. Plains tribes used cottonwood trees for food, fuel, and to create children’s toys; cottonwood trunks also furnished the sacred pole for their ceremonies. One Plains tribe believed the spirits spoke through cottonwoods, because their angled stalks cause the leaves to rustle in the slightest breeze. In the Southwest, the Hopi still carve statues of sacred beings called kachinas from the roots of the cottonwood tree, and Pueblo people hollow out cottonwood logs for ceremonial drums.

Cottonwoods have a special resonance for most Westerners, perhaps because the trees, called alamo in Spanish and commemorated in hundreds of western place names, mark the presence of water and the surprising diversity of life it nourishes in these thirsty landscapes. Their rustling leaves even sound like water.

The trees that succored Lewis and Clark’s party were Plains cottonwoods, one of three types of broad-leafed cottonwoods that flourish along lower-elevation rivers and streams in the inland West. All are fast-growing with deeply furrowed bark and deltoid leaves, and they play similar ecological roles. The three are most easily distinguished by their geographic location. (See “A Field Guide to Plains and Desert Cottonwoods, page 29.”)

Scott calls these cottonwoods “keystone species”: The shady, open groves of riparian or bank-side forests formed by these trees provide critical habitat for many species of wildlife in arid regions. The trees’ open, leafy canopies attract unique insect species that feed birds from yellow warblers to yellow-billed cuckoos; their thick branches support the massive platform nests of bald eagles, hawks, and herons; cavities in the soft wood house wildlife from bears and raccoons to bats, owls, and bluebirds.

As in Lewis and Clark’s time, the cottonwood forests lining the West’s rivers still succor travelers, in this case, migratory wildlife. These tree-shaded corridors provide what Scott calls a “region-wide uniform habitat” from central Mexico to Canada that guides and feeds wildlife making the hazardous annual commute across deserts and plains: “Neotropical migratory birds [warblers, orioles, and tanagers] winter in cottonwood riparian forests in Mexico, use them as migration corridors, and nest in them in summer in the United States and Canada,” says Scott. “At each point on their journey, these cottonwood forests are home.”

Yet the West’s cottonwood forests are threatened. If Meriwether Lewis retraced his route on what is now the Lewis and Clark National Historic Trail, he would barely recognize the riparian forests. Today, their shady canopies are frayed or entirely gone; impenetrable tangles of brush choke formerly open groves.
What happened? Dams were built, streams dried up, and invasive species moved in.

The West’s broad-leafed cottonwoods are synched to the natural rhythms of the region’s rivers and streams, which flood after spring snowmelt or summer rains and ebb during fall and winter. Cottonwoods bear fruit during high flows, loosing prodigious quantities of seeds, which float downwind and downstream on the cottony “parrishoots” noted by Meriwether Lewis. The seeds stick in sodden drifts on puddles of receding water and germinate on damp, flood-scoured ground. The size of a pinhead, each seed sprouts a tap root that grows as much as an inch and a half every day, following the water deep into the soil. The tops can spurt up ten feet a year, maturing into trees with cushioned bark, sturdy trunks, and root systems that endure through the very floods that prepare the ground for new seedlings.

But the flooding necessary to cottonwoods is now rare on western rivers. Dams have evened out the flows, and withdrawals for human uses dry up rivers and streams, dropping groundwater levels below the reach of cottonwood roots. Global warming is likely to worsen water shortages, delivering smaller snowpacks and longer droughts. Without flooding, cottonwood seeds can’t sprout; because fast growth yields weak wood and thus a short life, the emblematic groves soon die out.

The human impact on stream and river flows also favors the spread of exotic invasive species, says Curt Deuser, supervisory restoration ecologist with the National Park Service’s Lake Mead Exotic Plant Management Team. The most widespread of these is tamarisk, a group of small trees native to Eurasia that sprout tiny, succulent leaves and masses of pink flowers. Planted as an ornamental species since the late 1800s and later distributed for erosion control, the various species of tamarisk now infest an estimated 3.3 million acres throughout the western United States, seriously affecting every national park and monument in the desert Southwest. At Big Bend National Park in Texas, for instance, botanist Joe Sirotnak estimates that half of the park’s 117 miles of Rio Grande riverbank are choked by tamarisk.

Tamarisk crowds river and stream banks, elbowing out native plant species and poisoning the soil with the rain of salty, needlelike leaves that inspired its other common name, salt cedar. Thickets of tamarisk turn diverse riparian communities into monocultures, fuel frequent fires, and narrow and deepen stream channels; some thickets are so dense they can literally dry up streams and springs. A single acre of tamarisk can transpire 2.8 million gallons of water a year, equal to about two minutes of Colorado River flow through the Grand Canyon in fall. Multiply that by millions of acres, and the impact on the region’s scarce water supplies becomes all too clear.

As tamarisk populations have exploded, myriad species that depend on cottonwood forests have declined. Populations of the southwest willow flycatcher, a songbird tied to riparian willow and cottonwood communities, plummeted; the diminutive bird was declared an endangered species in 1995. According to Park Service biologist David Bustos, tamarisk infestation has lowered stream flow and now threatens dozens of species, including the White Sands pupfish, a tiny desert-dwelling fish native to a few isolated springs and streams around White Sands National Monument in New Mexico.

But there is hope. Removing tamarisk allows streams and groundwater levels to recover, helping cottonwood forests to rebound. Deuser and his cohorts at Lake Mead have criss-crossed the West, controlling the invasive trees with chainsaws, prescribed burns, and herbicides at a cost that ranges from $400 to $2,000 per acre. Five years after tamarisk removal in one drainage, cottonwoods as tall as 20 feet stand next to dead tamarisk stumps.

Soon, a little-known but promising biocontrol method involving small green beetles could put Deuser and his crew out of business. The leaf-eating beetles, imported from tamarisk’s native range in Asia and the Mediterranean, were extensively tested before they were released at ten western sites in 2001. They have already defoliated 100,000 acres from Nevada to Texas, says C. Jack DeLoach, research entomologist at the U.S. Department of Agriculture’s (USDA) Agricultural Research Service in Temple, Texas.

Beetles released at Lovelock, Nevada, munched their way 100 miles upriver in just five years; in one year, beetles released near Canyonlands and Arches national parks in Utah defoliated every tamarisk along 11 miles of the Colorado River. “After four years of defoliation, the plants begin dying,” says DeLoach.

Converting tamarisk into native habitat is a benefit both biologically and aesthetically. At Big Bend, Sirotnak is working on an environmental impact statement for the release of the beetles as an alternative to costly and “heavy-handed” mechanical and chemical control methods.

At White Sands, nearby habitat suitable for the imperiled southwest willow flycatcher precludes the release of leaf-eating beetles, which may disturb the birds’ remaining nest sites. So Bustos is whacking away at an estimated 4,000 acres of tamarisk with the help of a National Park Service Exotic Pest Management team and as many volunteers as he can recruit. Cottonwoods flourish in unlikely places at White Sands. “Groves of big trees just pop out in the middle of the dunes,” able to tap groundwater under the snow-white sands, says Bustos.

Cottonwoods may be threatened, but these symbols of water—and life—in arid landscapes inspire broad support. During one of Scott’s presentations, the USGS ecologist encountered a rancher who groused about the trees’ water-wasting ways, only to confess in private that he actually likes them. USDA entomologist DeLoach admits he should have retired a long time ago, but he just had to stick around long enough to see the biocontrol project succeed. Lake Mead’s Deuser says simply, “Seeing the cottonwoods come back gives me hope.”

A Field Guide to Plains and Desert Cottonwoods

Plains cottonwood trees live east of the Continental Divide from Alberta south to the Texas Panhandle. The species offers shade in historic sites like Fort Laramie in Wyoming, Bent’s Old Fort National Historic Site in Colorado, and stretches of the Lewis and Clark National Historic Trail, and provides oases in the prairie at places like Badlands National Park in South Dakota.

Rio Grande cottonwood dominates the river it is named for in New Mexico and extends into the upper reaches of the Colorado River system. It’s the cottonwood whose leaves rustle in Canyon de Chelly, Petroglyph, White Sands, and Colorado national monuments and at Mesa Verde National Park.

Frémont cottonwood thrives in the hot deserts of the southern Southwest and northern Mexico. Look for it at national parks ranging from Channel Islands and Death Valley in California to Arches and Big Bend, Canyonlands, and Grand Canyon.