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The Thirsty Tree: Confronting Invasive Salt Cedar in the American Southwest

by Melissa L. Lamberton
  

I think that I shall never see
A poem lovely as a tree.
A tree whose hungry mouth is prest
Against the sweet earth’s flowing breast…
    — Joyce Kilmer

Tamarisk was a convenient scapegoat for the complex problems encountered by government water managers, be they true believers in the monster or otherwise.
   — Matthew Chew, “The Monstering of Tamarisk”

  

Thirst for New Water
  

Like all stories about the American Southwest, this one begins with water. The open, rugged landscape had everything else—mountains laced with copper ore, vast stands of timber, fertile soil, and warm weather. But rivers, running rampant through red-rimmed canyons and rocky gullies, held only a fraction of the water demanded by the influx of 19th-century settlers. Scorching months could pass without an inch of rain, and then a single storm would tear the topsoil to pieces. Water never came in the right amounts, or at the right time.

Salt cedar in fall colors
The gold leaves of salt cedar in the
autumn along a western river.

Photo courtesy Texas Tech University.

By the early 20th century, desert cities like Las Vegas, Phoenix, and Los Angeles had become sprawling webs of lights, surrounded by acres of agricultural fields. Development drove the Southwest region. It required ever-increasing amounts of water in a landscape that had little to spare.

Dams offered one solution, halting rivers in their tracks and preventing valuable water resources from disappearing into the sea. The grand sweep of Hoover Dam, completed in 1935, held enough space behind its bulk to store two years’ worth of the Colorado River’s flow. Aquifers provided another source of water. Deep wells drew up rainwater stored between soil pores sometime in the Pleistocene era, when mammoths still roamed the savannah-like plains of the Southwest.

But dams held only what river and rainfall provided, and depleted aquifers soon began cutting ominous fissures into the ground. The old adage says that when wells run dry, we know the worth of water. History suggests we merely look elsewhere. Midway through the 20th century, the fight to find more water settled on an unlikely candidate—a tree.

In the two centuries since salt cedar took root in America, it has been welcomed, vilified, battled, and exhaustively researched. Targeted as a needless waste of water, this tenacious, non-native species is a somber reflection of our drive to supply the thirst of western states. Attempts to control salt cedar call into question our understanding of what we label invasive, unwanted, and destructive, and what has come to belong to the transforming desert.
  

The Unbeneficial Tree
 

Salt cedar trees grow rampantly in the Middle East, Asia, and parts of Africa, accustomed to harsh landscapes with little rain. They first appeared on the East Coast of America in the early 1800s, carried over the sea and sold to plant nurseries as ornamental shrubs. The brushy trees flourished in saline soils, their grey-green leaves rough-textured with salt. Scientists call the species Tamarix, but most people know it by the nickname “salt cedar” or tamarisk.

At first, Americans welcomed the new addition to their gardens. The exotic tree, with its slender branches and miniscule rows of leaves, was beautiful. When it bloomed—an event that could take place from spring to autumn, sometimes even in winter—the tree burst forth into masses of feathery pink blossoms. Flowers gave way to hard, brown seedpods that broke open to reveal a cottony fluff, packed with seeds. 

Merely ornamental in the gardens of the lush East, salt cedar had a different role to play in the settlement of the Southwest. As farmers and homesteaders wrestled wild rivers into submission with dams and drilled deep wells to reach intractable aquifers, native trees began to die out. The long gallery forests of cottonwoods and willows that once graced desert streams disappeared. Silt crumbled into the water, filling reservoirs and eroding the fragile banks of streams. Cutting winds blew the topsoil away from cultivated fields.

By the early 1900s, farmers in Arizona, southern California, and other western states had begun planting salt cedar to act as windbreaks, stabilize the soil, and provide much-needed shade over the evaporating surfaces of streams. The hardy desert shrub took a liking to its new ecosystem. Downy seeds scattered in the dry desert breeze. Soon, slender salt cedar shrubs sprouted on their own accord along river bottoms.

Pink spring flowers on salt cedar
Spring flowers on a salt cedar.
Photo courtesy U.S. Army Corps of Engineers.

But trees require water—and nothing that claims water in the Southwest remains unchallenged for long. In Arizona, a new hostility toward salt cedar arose in concert with urgent water demands. The Phelps Dodge Corporation, for instance, made plans in the 1930s to expand a copper mine in Morenci, a tiny company town in the southeastern part of the state where layers of lavender and pink ringed the ore-rich hills. Phelps Dodge had legal rights to Eagle Creek, a thin tributary of the Upper Gila River. But the creek didn’t have enough water for the company’s grand plans.

Finding water in Arizona—let alone claiming it in court—proved to be a formidable task. Western water law required Phelps Dodge to meet two requirements. First, it had to put the water to a “beneficial use.” As the mining operation expanded beneath the ominous shadow of World War II, the company found it easy to fulfill this requirement. After all, the war effort required copper, and Phelps Dodge could supply it.

The second rule of western water law, called “prior appropriation,” states that the earliest claim on a water source has the highest priority. The rivers and aquifers of Safford Valley, downstream of the copper mine, had long since been divvied up among developments, farmers and corporations. War effort notwithstanding, Phelps Dodge would have to wait last in a long line to get a portion of the existing water. They needed “new” water, untouched by other claims.

Around the same time, the United States Geological Survey (USGS) began to inventory the water available in the Upper Gila River Basin. By definition, water consumed by plants didn’t count as a “beneficial use.” Surveyors eyed the long strips of riparian forests, lining the banks of the rivers, with transits and calculators in hand. Completed in 1941, the study estimated that killing the riparian vegetation in Safford Valley would free up 70,000 acre-feet of water annually—roughly a quarter of all the legally claimed irrigation rights in Safford Valley at the time.

As the concept of killing trees for “water salvage” gained momentum, a botanical hierarchy emerged in the minds of scientists and resource managers. The USGS invented a new term—“phreatophytes”—literally, well plants, or a species that regularly feeds on groundwater rather than depending on rainfall and rivers. The phrase was oddly mechanical for the teeming, vibrant ecosystems along river bottoms, but it suited resource managers. Trees became classified as machines, sucking up water with their roots the way a farmer laboriously pumps the handle of a well.

The Phelps Dodge Company wrote a list of 19 phreatophyte species in Safford Valley, including native trees like cottonwoods, and asked the USGS to continue its investigation. In 1950, the USGS published the agency’s conclusions. For the first time, salt cedar appeared as the lead villain, set apart from other phreatophytes as an invasive plant that didn’t belong on western streams. The USGS reported that salt cedar had high rates of evapotranspiration—the trees were literally breathing precious water resources into vapor through their leaves. It seemed that westerners had created their own worst enemy. Logically, removing the trees would free up water for truly beneficial uses—like copper mines and exploding suburban growth.

For Arizona, salt cedar became the home front of a new kind of war. Researchers attacked the dense jumble of trees along the Upper Gila River with flamethrowers. Clouds of ash and smoke choked the river valley. But salt cedar fought back. Green shoots arose in the rich black cinders, spouting anew from the deep-reaching root system. When fire failed to kill the trees, the researchers returned with bulldozers. One of the authors of the USGS report, hydrologist Thomas Robinson, later wrote that “new growth appeared from each severed root, so that two plants grew where one had grown before.” Like the mythical hydra, salt cedar always sprang new heads.  
  

Map of "suitable tamarisk habitat"
Map using Terra and Aqua satellite vegetation data
from the Moderate Resolution Imaging
Spectroradiometer (MODIS) sensor combined with
ground observations indicates areas that are
suitable habitat for salt cedar.

Graphic by Jeffrey T. Morisette, courtesy NASA.

A Monster in the Making
  

“This is an account of scientists creating a monster,” Matthew K. Chew writes in “The Monstering of Tamarisk,” appearing in Journal of the History of Biology. A biologist at Arizona State University, Chew studies the way people create categories to recast plants into malevolent roles. His work describes how, two centuries after Mary Shelley published her Gothic horror novel, the story replayed itself in the botany of the American Southwest. Scientific studies proclaimed salt cedar trees to be voracious, hazardous, aggressive, useless, and alien. Above all, they labeled it thirsty, in a landscape that could not afford to satisfy another thirst.

The theory of water salvage pivoted on one solid, irreproachable fact: Salt cedar did not belong in the Southwest. In 1959, the University of Arizona published a seminal report, optimistically titled “Recovering Rainfall: More Water for Irrigation.” The author, George W. Barr, proposed replacing high-water use plants like trees with low-water use grasses along western rivers. Invasive salt cedar seemed the perfect target.

On the heels of that study, federal agencies like the U.S. Bureau of Reclamation, the U.S. Forest Service, and the Agricultural Research Service embarked on what river scientist William Graf calls “an evangelistic crusade against botanical water pirates.” Only crude methods existed to measure phreatophyte water use, and millions of dollars poured from the Senate floor into research that resulted in little trustworthy data.

Nevertheless, by the 1960s few people questioned the perceived worthlessness of the salt cedar tree. Its strong taproot greedily siphoned up groundwater, claiming the region’s most important natural resource. Edward Abbey, champion of the desert wilderness, wrote in The Journey Home, “Tamarisk is not native to the American West. It comes from North Africa, and as is often the way with exotics, has spread like a plague in its new environment, clogging the desert watercourses and driving out the willow, the cottonwood, the hackberry, the box elder.” Plague, monster—epithets like these became commonplace in biological studies examining the salt cedar tree. 
  

A Surge of Fresh Water
  

In 1999, a research ecologist from the University of Arizona named Edward Glenn took a plane flight over the Ciénega de Santa Clara in northern Mexico. From overhead, he could see the blazing sunlight striking the surface of the wetland. Locals called this place simply la laguna—the lake—a rare oasis of water in the desert. Upstream, Arizona farmers along the banks of the Lower Colorado River had been discharging salty wastewater into a drainage ditch. Around this whisper of water, a lush wetland of cattails and mirror-still pools sprang to life.

The wetland wouldn’t last forever. The U.S. government planned to reopen the Yuma Desalting Plant, reclaiming the wastewater for human use. In the meantime, researchers like Glenn studied the ecosystem as an example of a remarkable resurgence of life in a trammeled desert region. At the end of the aerial survey, Glenn asked the pilot to fly back to Yuma by going up the channel of the Lower Colorado River, rather than taking the direct route across scorching acres of agricultural fields. He wanted to take a closer look at the river.

Actual extent of salt cedar invasion in the U.S.
Actual extent of salt cedar invasion in the U.S.
Graphic courtesy Allen Press Publishing Series / Southwest
Climate Change Network.

In his book A River No More, Richard Fradkin described the Lower Colorado, where it flows over the border into Mexico and loses itself in the Gulf of California, as “utterly devoid of any vitality . . . a tepid ditch for urban crowds.” Glenn couldn’t shake the troubling image. Many scientists lamented that the once-mighty river had been choked into silence by invasive salt cedar trees. As the pilot winged westward, toward the silver glint of the river, Glenn gazed anxiously out the narrow window.

 Below, a winding corridor of green wove through the brown desert scrub, cradling the blue shimmer of the river. Migratory birds flickered from tree to tree, their wings catching glints of sunlight. Poking up through the green-grey canopy of salt cedar, Glenn recognized flourishing stands of cottonwoods and willows. The lushness of the riverbanks caught him by surprise. Everyone knew native trees couldn’t compete with salt cedar’s deep roots and tolerance for salt.

Later, Glenn learned that the mystery had a simple answer. Recent rainstorms, more than usual, had forced managers of the Lower Colorado River to release water stored behind dams. The surge of water swept downstream, clearing away leaves, sediments and salts from the rank, choking riverbank. Native seeds, lying in wait for that ancient signal—a pulse of fresh water—germinated and came up right through the salt cedar.

 The incident set Glenn to wondering. What if salt cedar didn’t deserve its reputation after all? What if western rivers clogged with salt cedar trees simply lacked the natural ebb and flow that allowed native species to survive? Below him, the vibrant riparian ecosystem of the Lower Colorado winked in the sun. Glenn did what any good scientist would do. He applied for a research grant.
  

How Much Water?
  

After his epiphany over the banks of the Lower Colorado, Glenn discovered a growing community of scientists who questioned the common wisdom about salt cedar trees. As early as 1980, Benjamin Everitt from the Utah Division of Water Resources had issued a plea for balanced research. “Profound changes in riparian vegetation involving the spread of salt cedar have coincided with equally profound changes in channel geometry and streamflow,” he warned. “The cause and effect relationships involved are sometimes not at all obvious.”

Beginning in the late 1990s, freshwater ecologist Juliet Stromberg pioneered new research about the complex interactions between salt cedar and western rivers. With a love of botany inherited from long hours playing in her mother’s garden as a child, she determinedly forged a fresh look at salt cedar trees. In a 2009 article in Restoration Ecology, she describes a classic case of chicken-or-the-egg. Which came first: eager stands of salt cedar crowding out native species, or the decline of native species followed by the colonization of salt cedar?

Most policy documents labeled salt cedar as the instigator, a marauding plant whose long taproot and salt tolerance allowed it to outcompete natives. The National Park Service listed salt cedar on its “Least Wanted” website, warning that salt cedar replaces native plants, monopolizes water supplies, and increases the danger of fire and floods. But some scientists began to realize the story was more complex. Conditions on rivers had changed over the last two centuries. Groundwater pumping, overgrazing, and the construction of dams altered the natural pulse of a river’s seasonal cycle. Was it possible that salt cedar, well-equipped to deal with salt and drought, simply moved into niches where native trees could no longer take root?

This new look at salt cedar threw into doubt long-standing claims about the tree’s voracious water use. Scientific methods had greatly improved since the first surge of phreatophyte research, when the best tool available to measure water use involved large cement pots. Yet Glenn worried that an instinctive prejudice against invasive species had blinded scientists to unbiased inquiries. Armed with a NASA grant, Glenn recruited the help of Pamela Nagler, then a graduate student at the University of Arizona, to examine the questionable claims of salt cedar’s thirsty appetite.

Along the Lower Colorado River—the most altered river in the Southwest, its length gauntleted by dams and reservoirs—salt cedar accounted for 90 percent of the riparian vegetation. Flux towers, spindly metal poles poking out above the canopy of trees, measured the gases exhaled into the atmosphere. Originally intended for climate change research, Glenn and Nagler appropriated the technology to track the amount of moisture that the trees breathed back into the air.

Salt cedar in summer along the Colorado River.
Salt cedar in summer along the Colorado River.
Photo by William M. Ciesla, courtesy Forest Health
Management International.

The Middle Rio Grande in New Mexico, where salt cedar evenly matched native species in numbers, made another ideal case study. The San Pedro River offered a third example. Flowing northward over the Arizona-Sonora border, the San Pedro retained the sweet charm of a wild river—stretches of still blue waters, chattering gravel runs, and tunnels of native trees cradling migrating songbirds in their branches. Some call the San Pedro the last undammed river in the West. Like other unaltered rivers in Mexico, the San Pedro nurtures only a few scattered stands of salt cedar trees. Cottonwoods and willows line the banks in a winding ribbon of green, broken by patches of dense mesquite bosques, abuzz with insects.

In each river system, the researchers twisted thin strands of wire around tree trunks. The wire could detect a measurable loss of heat whenever water moved through the plant and evaporated from the leaves, adding to the data collected by flux towers. Images from satellites, trailing invisibly across the blue desert sky, provided further corroboration. The result: all three riparian ecosystems transpired an average of just one meter a year of water into the atmosphere, regardless of whether the trees were salt cedar, cottonwood, or willow.

The thirst of the salt cedar tree, on which many eradication efforts had been based, was a myth. Glenn and Nagler’s research lasted through a decade, several funding organizations and Nagler’s Ph.D. dissertation. Other investigators conducted their own studies, bolstering the conclusion that removing riparian vegetation from riverbeds would salvage little water for humans. Instead, bare soil eroded away without roots to hold it back, or new invasive species simply moved in.

“A mythology has been created about tamarisk,” Stromberg announced in the 2009 Restoration Ecology article, coauthored with Chew, Nagler and Glenn. The invasive tree had been blamed for thirst, competitiveness, and the destruction of rare river ecosystems. A closer look revealed, as the authors wrote, that scientists had participated in “a rationalized scapegoating of Tamarix as an agent of change because of its ability to thrive in anthropogenic habitats.” Ecologists began to understand that salt cedar had established itself so well in the Southwest because dams and diversions created an environment uniquely suited for them—in other words, because of our own thirst for water.
  

The Power of a Seed
  

Slowly, this new perspective began to appear in scientific journals, policy reviews, and newspapers. Yet eradication efforts continued. Some organizations, committed to hacking out salt cedar, refused to admit that little water could be salvaged from their efforts. Others eagerly reviewed the latest scientific studies and accepted salt cedar’s modest water use but maintained that good reasons remained for removing the invasive trees.

First and foremost, salt cedar didn’t belong. Along vast stretches of western rivers, collared by dams, salt cedar had become the dominant tree. Many environmentalists envisioned returning these streams to their former untrammeled glory, replanting the cottonwoods and calling back the native songbirds. Dense stands of salt cedar provide poor habitat for wildlife and tend to blaze up in devastating fires. Additionally, the tree’s extensive root system can channelize streams, increasing the danger of floods. Ultimately, environmentalists argued, a tree dislocated from Asia had no place in the fragile Southwest.

A passionate non-profit organization called The Tamarisk Coalition led the way in Colorado. Tim Carlson, an environmental engineer and one of the Coalition’s founders, dedicated his career to restoring native ecosystems along Colorado’s streams. The work began with organizing teams of volunteers to chainsaw, bulldoze, or chemically spray salt cedar trees. Soon the Coalition was hosting conferences, writing policy documents, and inventorying the health of watersheds across the Southwest. Federal agencies looked to the Coalition for technical assistance, recognizing Carlson’s commitment to understanding and interpreting the latest scientific data.

Native cottonwood and grasses along the Bosque del Apache
Native trees such as cottonwood will outcompete
invasive salt cedar, it was discovered at the Bosque
del Apache National Wildlife Refuge (shown here),
when regular spring river flows of fresh water,
resembling historic snowmelt patterns, are allowed
to occur.

Photo by Simmons B. Buntin.

“Our goal is riparian health, not killing tamarisk,” Carlson said. To that end, he insisted that every project have a carefully designed revegetation plan. Some eradication efforts, lacking foresight or funding, left tracts of bare soil exposed in their wake, increasing erosion and leaving room for new invasive species like Russian olive or buffelgrass to take root. Carlson’s volunteers coaxed native trees to flourish when they removed salt cedar, planting bendy willow saplings and broadcasting grass seed over freshly mulched soil.

In addition to improving habitat, environmentalists cited soil salinity as a good reason for eliminating salt cedar trees. True to its name, salt cedar draws up salt from the soil through its complex web of roots, storing it in leaves. When the leaves flutter downward, they carpet the ground in a saline layer of debris. Some naturalists took this as further evidence that salt cedar is a destructive, alien plant crowding out sensitive native species. The Nature Conservancy’s website stated that salt cedar “oozes salt from its leaves,” making it difficult for other plants to grow nearby. The authors of a 1997 book, Assessment and Management of Plant Invasions, claimed that “salinization of flood-plain habitats may be the most important single way that the invasion of salt cedar fundamentally alters ecosystems.”

Other scientists wondered if salt cedar merely colonizes streams that are already too salty for native species to tolerate. Along the Colorado River, for example, water that once ran swiftly downstream, thick with mud, now languishes behind dams. The increased evaporation from the surfaces of reservoirs concentrates the amount of salts in the water. Summertime floods no longer rinse the riverbanks, so salts accumulate in a pale crust. In contrast, natural floods still exist on the San Pedro, where saline soils—and salt cedar trees—are rare.

If humans created the niche that salt cedar loves so well, then could careful management undo the damage? In 1996, a dense stand of salt cedar trees caught fire on a stretch of the Rio Grande in southern New Mexico. The inferno spread to several thousand acres of native cottonwood trees. When Phil Norton, manager of the Bosque del Apache National Wildlife Refuge, walked through the ashes that spring, he noticed that the new seeds germinating in the scorched earth sprouted salt cedar trees. Something had to be done to rescue the native ecosystem.

High Country News reports that Norton had been battling salt cedar on his 13-mile stretch of the river for a decade. Every winter, he watched flurries of snow geese and sandhill cranes alight in the trees, and it was his idea to celebrate their arrival with a local festival. Salt cedar threatened the refuge that he had promised to protect. Norton sprayed herbicides and hired bulldozers, to little effect. His attempts to beat back the spreading salt cedar trees were becoming expensive. Seeds couldn’t be controlled. They scattered on the wind, new batches arriving with every gust.

The power of a seed—that’s where he found his inspiration.

Revegetation after salt cedar removal along a creek near Moab, Utah.
Revegetation after salt cedar removal along a creek near
Moab, Utah.

Photo courtesy The Tamarisk Coalition.

In May, just before the cottonwood trees dressed themselves in white, Norton flooded a stretch of the river to create a rich, fertile mudflat. He had a unique tool on hand—a senior water right to the river—that allowed him to open the floodgates on an upstream dam. The resulting surge of water mimicked the pulse of cold snowmelt that occurs on undammed rivers in the springtime. Over the next few weeks, Norton watched the mudflat dry out in the sun, dreading to see it sprout with salt cedar trees.

But when the first tentative green shoots appeared, they belonged to cottonwood trees. The experiment was a success: Norton had timed the flood to coincide precisely with the first flight of the cottonwood seeds, and they had taken root before salt cedar could colonize the area. The invasive tree was no monster after all, only a symptom of river systems that had been robbed of their seasonal rhythms.
  

Beetles Munch into Battle
  

In the late 1990s, the story of the war against salt cedar took an odd turn. In its native habitat, naturalists reasoned, salt cedar is controlled by several species of leaf-eating beetles that feed on nothing else. Why not import boxes of beetles to the beleaguered streams of the Southwest, an exotic predator to feed on an exotic pest?

Culver “Jack” DeLoach, an entomologist with the U.S. Department of Agriculture, spearheaded the biocontrol effort. DeLoach spent a decade traveling Europe and Asia looking for a worthy adversary for salt cedar trees. Returning to his lab in Texas with a handful of candidates, he conducted rigorous tests to ensure the beetles ate nothing but salt cedar leaves. At last, he settled on a species called Diorahadba elongata, a plain brown beetle about the size of a pencil eraser.

In 1995, DeLoach received permission to release his beetles in several Southwestern states. Then a fax appeared in his lab: The U.S. Fish and Wildlife Service had just listed the southwestern willow flycatcher, a tiny brown songbird, as endangered. All biocontrol efforts ground to a halt. DeLoach would have to spend another two years tediously documenting that eliminating salt cedar trees with beetles wouldn’t threaten the survival of the songbird.

Ironically, the U.S. Fish and Wildlife Service listed salt cedar invasion as one reason for the decline in willow flycatcher populations. Dense stands of salt cedar usually provide poor habitat for native creatures. But lacking actual willows to nest in, flycatchers along the Colorado River settled on salt cedar branches as the only available real estate. Now protected by the toothy Endangered Species Act, nothing that threatened the bird’s fragile existence would be allowed.

Entomologists didn’t give up on their beetles. Over the next several years, research labs across the Southwest examined the miniscule insects, searching for a way to satisfy the concerns of the Endangered Species Act. Dan Bean, director of the Colorado Department of Agriculture Palisade Insectory, discovered that a certain strain of Diorahadba from northwestern China required at least 14½ hours of daylight to reproduce. That confined them to the region north of the 38th parallel—southern Colorado and Utah—skirting the willow flycatcher’s habitat in Arizona entirely.

Diorahadba elongata, also called the Mediterranean tamarisk beetle or salt cedar leaf beetle
Diorahadba elongata, also called the Mediterranean
tamarisk beetle or salt cedar leaf beetle, has been
imported to help fight the salt cedar invasion.

Photo by Robert D. Richard, courtesy U.S. Department of
Agriculture.

The argument satisfied the Endangered Species Act committee. In 2001, after a three-year experimental phase with beetles in cages, the U.S. Department of Agriculture released the beetle in California, Nevada, Utah, Colorado, and Wyoming. Two years later, they extended the experiment to Montana, Oregon, and New Mexico. As an added precaution, the USDA promised they would not release beetles within 200 miles of willow flycatcher habitat, an area encircling the Colorado River Basin in Arizona, southern Utah, and southern Colorado.

 The beetles worked slowly. For a year or two after their release, little changed. Then tracts of trees along riverbanks began to turn brown. In the winter, when the beetles snuggled down beneath leaf litter, some salt cedar recovered. But most trees couldn’t withstand several years of repeated attacks. Beetle larvae robbed them of their photosynthetic power by removing their leaves. The experiment was working.

Jack DeLoach, Dan Bean, and their entomologist colleagues understood that the Diorahadba beetle was no silver bullet. Hoards of the tiny creatures could limit the spread of salt cedar, but not eliminate it entirely. And killing salt cedar alone wouldn’t return health to a river. Humans would have to follow in the beetles’ wake, removing stands of dead trees and replanting cottonwood saplings and willow sprigs. Nevertheless, the results seemed promising. Ecologist Tom Dudley, quoted in the Christian Science Monitor, greeted the beetles as “little green liberators,” waging war against the botanical pariah of the Southwest. 

Then local agencies and private individuals began to collect beetles for release on non-federal lands.  In 2004, a county weed supervisor introduced a slightly different strain of beetle—one from Kazakhstan, not China—into southwestern Utah, near Moab. With a rapidity that startled Utah residents, the beetles swarmed up the Colorado River corridor. In the hot summer months, they ate through 25 miles of the Dolores River and crossed the state line into Colorado.

No one had tested the photoperiodic requirement of the Kazakhstan beetles. But in his lab in Colorado, Dan Bean discovered that the China beetles could now reproduce with just 14 hours and 10 minutes of daylight, in what he called “probably the fastest case known” of evolution. The invisible barrier of the 38th parallel no longer existed. In 2006, the City of St. George, Utah, released beetles along the Virgin River, well below the promised boundary line. The insects thrived. Two years later, they entered Arizona and began munching on trees that cradled willow flycatcher nests.

In 2009, the Center for Biological Diversity and the Maricopa Audubon Society filed a lawsuit against the U.S. Department of Agriculture for violating their promise to keep beetles away from flycatcher habitat. The USDA quickly settled the lawsuit by agreeing to rethink the biocontrol program in cooperation with the U.S. Fish and Wildlife Service. In 2010, the USDA formally banned the release or interstate transport of the Diorahadba beetle in 13 western states, under the threat of stiff penalties imposed by the Endangered Species Act.

In the meantime, the beetles already released continued their southward march. “If they move down the Colorado, into central Arizona, then we’re going to have the extinction of the flycatcher,” said Dr. Robin Silver of the Center for Biological Diversity. Willow flycatchers, he warns, are incredibly faithful to their birthplace. If the trees disappear, the songbirds will go with them. Hanging in this delicate balance, the remaining willow flycatchers continue building their nests in whatever trees they can find.
  

Southwestern willow flycatcher
The southwestern willow flycatcher has
become rare across its natural range as
riparian habitats are lost.

Photo by Rick Fridell, courtesy Utah Division
of Wildlife Resources.

Old Ideas and New Research
  

Dreams of water salvage never quite disappeared from salt cedar control efforts. In 2006, Congress passed the Salt Cedar and Russian Olive Control Demonstration Act, which authorized $15 million a year to remove invasive species from western rivers. Among other things, the act required the Secretary of the Interior to “monitor and document any water savings from the control of salt cedar and Russian olive trees, including impacts to both groundwater and surface water.”

Water savings—that goal meant everything to the politicians and resource managers of the Southwest. To “save” water meant not to conserve it, but to rescue it from rivers and aquifers and make it available to humans. Demand for water continued to increase, while climate change studies troubled predictions for the future. Higher temperatures would mean more evaporation from reservoirs and canals. On the Colorado River, a series of agreements ensured that water shortages would be dealt with quickly and ruthlessly.

The Central Arizona Project operates a 336-mile long canal that supplies Colorado River water to fast-growing cities like Phoenix and Tucson. The project holds a junior water right on the river, meaning its supply is cut off first in a time of shortage. Recognizing this risk, CAP managers set out in search of “new” water. Desalination, cloud seeding, a new dam, a canal to Mexico—among all the newfangled options discussed in the 2009 annual report, “vegetation management” stands out as an echo of old ideas about invasive species, embedded deep in Southwestern culture.

“Non-native plants such as the tamarisk (salt-cedar) and Russian olive are pervasive along the lower Colorado River,” the CAP report states. “These thirsty plants grow quickly and cover a larger area than native vegetation such as cottonwood and willow. It is estimated that as much as two million acre-feet of water could be saved by removing the non-native invaders and replacing them with native plants.”

Two million acre-feet—enough water to supply the entire state of Arizona for three months. But this is “paper water,” disconnected from the realities of ecosystems. The old idea of “plant as machine,” a monster of our own making, imagines salt cedar roots drinking from a deep bucket of water, easily reallocated to human use. The reality is much more complex. Stark numbers ignore the intricate workings of tree, rain, soil, river—not a machine whose parts can be scrapped for other uses, but a delicate dance of interrelationships.

In response to the 2006 congressional act, the United States Geological Survey spent four years reviewing scientific literature about salt cedar and compiling a comprehensive document with the agency’s conclusions. Coauthored by dozens of scientists from the USGS, the U.S. Bureau of Reclamation, the U.S. Forest Service, and universities, including researchers Edward Glenn and Pamela Nagler, the Science Assessment firmly overturns the notion of water salvage. “To date, research and demonstration projects have not shown that is it feasible to salvage (or save) significant amounts of water for consumptive use by removing salt cedar,” the report states.

The authors list one possible exception. Native trees in river corridors, like cottonwoods and willows, can grow in stands just as dense, and drink just as much water, as the “thirsty” salt cedar trees. But higher up in the floodplain, smaller plants like desert broom, whitethorn acacia, and creosote subsist almost entirely on rainfall. Salt cedar can march into this drier zone by tapping into the water table with its long roots. Theoretically, clearing away salt cedar from these higher terraces could “save” groundwater. The report’s authors caution that this approach has never been effectively demonstrated. Recent studies by scientists at the University of Utah suggest that salt cedar trees on the upper terraces of the Colorado River consume only small amounts of water.

Even if a salvageable amount could be calculated, claiming the water in court remains a serious obstacle. A 2004 Colorado law review, dramatically entitled “Death Penalty to Water Thieves,” describes how a farmer named Harvey Phelps attempted to claim 181 acre-feet of water by clearing away salt cedar from a section of the Arkansas River. The Colorado Supreme Court reluctantly ruled against him, stating that the water was not “new” but simply reallocated from the existing supplies, and thus subject to the priority right system. Fearing the pandemonium that would ensue if they allowed farmers to salvage water from trees, the judges wrote that “thirsty men cannot step into the shoes of a water thief.” 

While the notion of water salvage is difficult to relinquish, scientists and resource managers are beginning to recognize the significance of hydrological and legal barriers. The Science Assessment signifies how far science has come since the USGS first torched salt cedar trees in Safford Valley for the sake of a copper mine. The document acknowledges the complexities of living ecosystems, and gently suggests changes to policies that simplify salt cedar as the scapegoat. It cites recent work by Juliet Stromberg and others showing that if underlying ecological processes support salt cedar habitat, even the most effective removal and restoration plan is likely to fail.

Neatly summing up two centuries of mistakes, the report cautions: “In many cases, tremendous effort, resources, and time have been applied to achieve an intended goal (for example, water salvage) with little yield on the investment, poor ability to quantify yields, or lack thereof, and little new knowledge to inform future efforts.”
  

Salt cedar, Russan olive, and cottonwoods at Canyon de Chelly National Monument.
Invasives salt cedar and Russian olive mix with
cottonwoods on the floodplain of Chinle Wash in
Canyon de Chelly National Monument, Arizona.

Photo by Lindsey Reynolds, courtesy U.S. Geological
Survey and Colorado State University.

A Question of Values
  

Today, salt cedar might cover as much as 900,000 acres of the western United States, although no detailed surveys have been conducted. As the climate warms, the trees are likely to march northward and upward, invading cooler regions and higher elevations. Drought, dams, and depleted groundwater create ready-made niches no longer suitable for native plants, further encouraging salt cedar spread. The imported beetles, meanwhile, are adjusting their life cycles to swarm southward. In time, the two species—eater and eaten—will adapt to a new equilibrium.

The image we’ve created for salt cedar trees—thirsty, voracious, and overcrowding—is a mirror we can hold up to our own interaction with the Southwestern landscape. We might learn something from invasive trees and imported beetles about coexistence. Ecosystems don’t collapse when a new, stronger species arrives on the scene. Landscapes have proven flexible and resilient, adopting new species with surprisingly little impact on their overall health. In most cases, the addition of a new species doesn’t mean the subtraction of an old one. Unlike clear-cutting or the destruction of a river, invasions alone don’t devastate a region.

This holistic perspective on ecosystem health stands at odds with traditional resource management in the Southwest. Sweeping policy decisions are often made based on a single target resource—water, timber, copper or an endangered species. Scientists are calling for a different kind of management, one that recognizes the needs of entire ecosystems, with all their complicated and marvelous interconnections. That means changing our notions of value, stripping away the stereotypes and labels we’ve crafted, and seeing what’s really there.

There’s no returning the Southwest to the idyllic environment that existed before salt cedar, or Anglo-American settlers, arrived. But there may be a way to restore some portion of that wildness, by recognizing wildness as a valuable asset in human history. Water slipping through a riverbank or evaporating from the broad surfaces of leaves isn’t a wasted resource. It’s exactly what ecosystems need to continue their quiet, everyday services, purifying the air, strengthening the soil, and providing the water we drink. A healthy ecosystem, given the chance, will heal itself from damaging invasions, or accept new species as belonging within its web of interactions.

To preserve that health, we need to recognize our own voracity, and relinquish some water from the demands of human consumption for the sake of the environment that sustains us. This is a new vision for the Southwest—a springtime pulse of cold snowmelt, branches cradling the twiggy nests of songbirds, and trees casting their seeds windward to settle in the rich black silt of the riverbanks.  

    
 

Melissa L. Lamberton has a degree in environmental science from the University of Arizona and is currently pursuing an MFA in creative writing and environment at Iowa State University. Her articles have appeared in The Tucson Citizen, The Arizona Daily Star, UANews, Arizona Water Resource, Vermilion Flycatcher, and Restoring Connections.
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Resources
 
 

Exotic Tamarisk Management in the Grand Canyon, National Park Service

Invaders of the Sonoran Desert Region: A Project of the Arizona-Sonora Desert Museum ~ Tamarisk

Invasive.org Salt Cedar Resource Page

National Invasive Species Information Center: Salt Cedar

Plant Conservation Alliance's Least Wanted: Salt Cedar

The Tamarisk Coalition

 
     

References

Chew, Matthew K. “The Monstering of Tamarisk: How Scientists made a Plant into a Problem.” Journal of the History of Biology. (2009) 231-266.

Larmer, Paul. “Tackling Tamarisk.” High Country News. 25 May 1998.

Nijhuis, Michelle. “Beetle Warfare.” High Country News. 26 Nov 2007.

Shafroth, P.B., C.A. Brown, and D.M. Merritt (eds.). Saltcedar and Russian Olive Control Demonstration Act Science Assessment: U.S. Geological Survey Scientific Investigations Report 2009-5247. (2010) 143 p.

Stromberg, Juliet C., Matthew K. Chew, Pamela L. Nagler, and Edward P. Glenn. “Changing Perceptions of Change: The Role of Scientists in Tamarix and River Management.” Restoration Ecology 17.2. (March 2009) 177-186.

  

    
  
 
   
    
  
 
   

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