tamarisk – Conservation Sense and Nonsense

The Grand Delusion: Controlling Nature

“This is a book about people trying to solve problems created by people trying to solve problems.”

Elizabeth Kolbert’s earlier book, The Sixth Extinction was ground-breaking, not because it described the consequences of climate change in the 21^st Century, but because it put modern climate change into the context of similar events in the past 500 million years of life on Earth. Although the current episode of climate change is man-made, five previous mass extinctions were natural events. What past extinction events have in common with the sixth extinction is the inevitable consequence of such changes in climate: when the climate changes, all life on Earth changes with it. Plants and animals will adapt, change, or they will go extinct as they have for 500 million years. (1)

Kolbert’s new book, Under a White Sky, turns the page on this cataclysmic event in the Earth’s history to focus on the efforts being made to control nature to address environmental problems, including climate change. To say that Kolbert is skeptical of those efforts is to understate her critical evaluation of them.

Controlling Nature

In 1990, I was introduced to the human delusion that we can control nature by John McPhee’s The Control of Nature. His book had a profound influence on my thinking about nature. It was the basis for my belief that attempts to turn back the botanical clock to 500 years ago to a pre-settlement landscape, mistakenly believed to be pristine, are futile, misguided, and often damaging. Kolbert’s latest book is written from the same perspective as McPhee’s seminal work and she gives him credit for his pioneering work.

McPhee’s book predicted the catastrophic flooding of New Orleans by hurricane Katrina in 2005. Human engineering of the Mississippi River for over 100 years set the stage for that disaster. New Orleans sits at the Gulf end of the Mississippi River. Historically, the river flowed from Minnesota to the Gulf, accumulating sediment along its way and depositing it as it entered the Gulf, fanning out into streams and swamps that created the Mississippi Delta. The labyrinth of land and marsh created by the sediment deposited by the river created a barrier that protected New Orleans from storms.

However, the uncontrolled and episodic flow of the river caused periodic flooding that was not convenient for the human inhabitants of New Orleans and the Delta community. So, the flow of the river was controlled by levees and pumps were used to return water from the land to the river. Sediment from the river could no longer replenish the land because it was confined to the constrained river, which put the human engineers onto a never-ending treadmill of building higher levees and bigger pumps. It was inevitable that the river would eventually overwhelm the defenses built by the engineers and so it did during Katrina in 2005.

Kolbert updates this untenable situation in the Mississippi Delta in her new book. The underlying cause, as told by McPhee is recapped by Kolbert. Then new manmade environmental issues are added to the catastrophic circumstances that will inevitably doom the human inhabitants. Rising sea levels caused by climate change are one factor. The incursion of salt water into fresh water swamps killed vegetation that acts as a buffer during storms. Oil and gas exploration and extraction in the Delta has caused the land to drop further.

Many Delta communities and some neighborhoods in New Orleans have been abandoned because they are essentially underwater. Since Katrina, no serious effort has been made to change the approach to the issues. Bigger, more powerful pumps have been built and levees have been made higher and stronger. No one is seriously considering the need to relocate New Orleans or surrounding communities to higher ground. The delusion that humans can outsmart the river continues.

A comedy of errors

Kolbert introduces the many projects that are trying to solve problems that were created by bad decisions made earlier by other humans with a quote from Albert Einstein: “We cannot solve our problems with the same thinking we used when we created them.” These wise words from a wise man are clearly not being heeded by the masterminds of the projects Kolbert describes in her book:

High on the list of projects in which society is heavily investing is the attempt to prevent carp from entering the Great Lakes from the Mississippi River. After many different approaches were tried and failed, the current strategy is an electrified fence separating the Chicago River (connecting to the Mississippi River) from Lake Michigan that kills untold thousands of fish every day. This deadly project is the end stage of previous bad decisions. A link between Lake Michigan and the Mississippi River was created by a massive engineering project that reversed the flow of the Chicago River in 1887. Later, carp was introduced to the Mississippi River from China as biological control to address pollution issues. One species of carp was introduced to control aquatic weeds and another carp species was introduced to consume nutrients in sewage ponds. Kolbert says such biological controls became popular after Silent Spring was published because Rachel Carson considered pesticides a curse and biological control a panacea. (Which is not to say that pesticides aren’t a curse.) In other words, we traded one problem for another.
Island eradications of introduced mammals such as rats and mice are also popular projects (with some people). Genetic engineering is being aggressively pursued as a possible substitute for the rodenticides that are being used for these projects. These projects have the potential to drive an entire species into extinction or alter their physiology such that they could become killers or prevent them from being killers. Kolbert buys a genetic engineering kit for $209 from a young entrepreneur in Oakland that enables her to make E.coli cells resistant to an antibiotic. E.coli is a deadly bacteria that can be fatal if untreated by antibiotics. In other words, anyone with $209 can turn bacteria into killers with no special training or equipment. What could possibly go wrong, Kolbert asks rhetorically.

The promise and threat of geoengineering

Kolbert visits several different geoengineering projects that are trying to prevent the consequences of climate change without reducing greenhouse gas emissions, the underlying cause of climate change. One such project is turning CO₂ into stone. Apparently it CAN be done, but to do it on a scale that would actually prevent climate change would be to devote much of the surface of the Earth to that purpose.

Kolbert visits a project that believes injecting aerosols into the atmosphere to block the sun is the best bet to stop climate change. The proposal strikes Kolbert as both preposterous and dangerous. The researcher detects her skepticism and retorts, “People think of all the bad examples of environmental modification. They forget all the ones that are more or less working. There’s a weed, tamarisk, originally from Egypt. It’s spread all around the desert Southwest and has been destructive. After a bunch of trials, they imported some bug that eats the tamarisk, and apparently it’s kind of working.”

Tamarisk defoliation along Colorado River, near Needles, California

In fact, the introduced tamarisk beetle is working too well. It has spread far beyond the regions where it was introduced and produced wastelands of dead trees in Arizona and Southern California. Since one of the rarest desert birds depends upon tamarisk there isn’t much to celebrate about this over-achiever beetle.

Compounding the problem

Instead of addressing the source of environmental issues, we compound them by creating new problems with our theoretical “fixes.” The native plant movement, in their zeal to save native plants, sprays herbicides that kill as many native plants as non-native plants and poison the soil while doing so, stunting all new growth, both native and non-native.

I share Kolbert’s skepticism about the projects she describes for the same reasons she gives. Every “fix” has the potential to create new problems that could be more disastrous than the problems they are meant to resolve. And the resources used to develop new techniques such as massive geoengineering projects could be used instead to address the underlying cause of the problem, which is the greenhouse gas emissions causing climate change. We don’t want to give up our fossil-fuel driven economy, so instead we conjure up even more damaging ways to ameliorate the inconveniences of climate change. It’s a fantasy that prolongs and exacerbates the consequences of climate change.

Finally, let’s give Kolbert the last word: “This has been a book about people trying to solve problems created by people trying to solve problems…Geoengineering may be ‘entirely crazy and quite disconcerting,’ but if it could slow the melting of the Greenland ice sheet, or take some of ‘the pain and suffering away,’ or help prevent no-longer-fully-natural ecosystems from collapsing, doesn’t it have to be considered?…But to imagine that ‘dimming the fucking sun’ could be less dangerous than not dimming it, you have to imagine not only that the technology will work according to plan but it will be deployed according to plan. And that’s a lot of imagining…But let’s just say the record here isn’t strong.” (2)

Thank you, Elizabeth Kolbert, for calling out the grand delusions of humans who mistakenly believe it is possible to control nature to avoid inconveniencing human society.

Elizabeth Kolbert, The Sixth Extinction, Henry Holt and Co., 2014
Elizabeth Kolbert, Under a White Sky, Crown New York, 2021

Tamarisk beetle: A case study in the dangers of biological controls to eradicate non-native species

Our readers were introduced to Matt Chew in his guest post about the economic interests of ecological “restorations.” Dr. Chew is a faculty member of Arizona State University’s Center for Biology and Society and an instructor in the ASU School of Life Sciences.

The most recent newsletter (see page 8) of the California Invasive Plant Council (Cal-IPC) informed us that the beetle that was introduced in Arizona to eradicate tamarisk has spread to California, where it was not introduced. When the beetle was originally introduced, its spread beyond where it was introduced was not predicted, based on climatic restrictions on its life cycle. As usual, evolution overturns the best laid plans. According to Cal-IPC, “Rapid evolution in this developmental trait, however, allowed beetles to stay active later in the season and thus facilitated their expansion southward…”

Tamarisk defoliated by tamarisk leaf beetle along Colorado River, near Needles, California

The rapid defoliation of tamarisk throughout the southwest, including California, is an immediate threat to the endangered Southwestern willow flycatcher, which long ago adapted to tamarisk in the absence of its native host, willow. The native willow requires a great deal more water than tamarisk. Therefore, willow died off when water throughout the southwest was diverted out of riparian corridors for human consumption and agricultural production.

Dr. Chew is an expert on tamarisk and the role it plays in the ecosystems of the southwest and so we asked him to write another guest post for us on this topic. He has generously obliged with this detailed history of biocontrols and their use to eradicate non-native species.

Biocontrols are also topical because a new biocontrol was recently approved by the USDA to eradicate cape ivy. This biocontrol was eagerly anticipated by native plant advocates and is likely to be widely used by land managers in California. Therefore, this is a timely opportunity to learn about the pros and cons of biocontrols. How long will it take the introduced insect to start feeding on the many other species of ivy that are not considered “invasive?”

Evolution and natural selection are wild cards in attempts to eradicate non-native plants and animals. Although there are many dangerous consequences of using pesticides, the role that evolution plays in rendering pesticides useless is less understood and taken into consideration. Much like the hungry beetle that now is running rampant in the southwest, the weeds that are continuously sprayed with herbicide are also adapting and evolving defenses against the chemicals being used to eradicate them. There are now millions of acres of agricultural crop land infested by weeds that are immune to the pesticides that were sprayed on them for decades. Our pesticides are now useless on these “superweeds.” Instead of getting off the pesticide treadmill, we are developing stronger—and therefore more toxic—herbicides.

There are many reasons why we object to the eradication of non-native plants and animals. The tamarisk beetle is an example that illustrates a few of our objections:

Many of the plants being eradicated are providing food and habitat for animals. The animals that depend upon them are being harmed by their elimination.
The methods used to eradicate non-native species often have unintended, negative consequences, such as breeding “superweeds” that cannot be eradicated.
The puny tools of humans are often powerless against the much stronger forces of nature, such as natural selection and evolution. These forces of nature should be treated with greater respect, particularly by people who call themselves “scientists.”

Million Trees

From California to Texas and occasionally beyond, tamarisks are among the most talked-about introduced plants in the US. Most of that discussion consists of familiar anti-alien dogma, augmented by the long-obsolete assertion that tamarisks are profligate water-guzzlers. Suffice for now to say that anti-tamarisk sentiment led to state and federal suppression policies beginning around 1940, and eventually to legislation at both levels. Little more than accumulated bad reputation of tamarisk and its presence in the region of interest led the US Fish and Wildlife Service to include tamarisks among the supposed threats to the persistence of the Southwestern Willow Flycatcher (Empidonax traillii extimus) when that subspecies was formally listed “endangered” in 1993. All of that meant both political will and appropriations were applied to the US Department of Agriculture’s search for biological control agents to deploy as “counter-pests” against tamarisks. By 1998 they had their critter, an Asian leaf-eating beetle that putatively specialized on tamarisks and would rather die than eat anything else.

By that time, though, circumspection had set in, because especially in southern Arizona the endangered birds had taken to nesting in tamarisk stands. USDA promised USFWS that their armored foreign legion would not jeopardize flycatcher populations. USDA argued that the beetles they were about to propagate and release by the multi-millions were genetically incapable of surviving below 38° North latitude. In addition to famously dividing North from South Korea, that frontier runs from near the tip of Point Reyes through Stockton and Mono Lake; just south of Tonopah, Nevada; south of Canyonlands Nation Park; through Moffat and Swink, Colorado; on through the Garden City Kansas and increasingly irrelevant points east. Southern Arizona would surely never see a tamarisk leaf beetle. “Because SCIENCE!” Hold that thought.

In 1952 the otherwise obscure and perhaps pseudonymous writer Rose Bonne copyrighted a succinct cautionary account of biological pest control. Perhaps it was read or sung or shown to you as a child: I know an Old Lady [who swallowed a fly]. Ms. Bonne denied knowing how or why the old lady swallowed the fly, but considered it portentous: “Perhaps she’ll die!” Subsequent actions had definite (if sometimes puzzling) rationales. The next four animals consumed represented a hopeful trophic cascade: the Old Lady swallowed a spider to catch the fly, then a bird to catch the spider, a cat to catch the bird and a dog to catch the cat. At that point, distended and incoherent, she panicked, swallowing a goat to catch the dog, a cow to catch the goat, then finally, fatally, a horse. (Revisionists inserted a pig between the goat and the cow. If you doubt me, Google it.)

The history of biocontrols

We can barely pause to consider the long and checkered history of biological control. Its inception required a few conditions, which may have arisen in different orders in different places. A sense of ownership, territorial claims or resource collection rights seems necessary, as does dissatisfaction with the dictates of fate. Why attempt to affect an outcome without expecting to benefit from the effort? A bit of empirical, practical natural history knowledge is also indispensable. Together they add up to the possibility of acting on the basis that “the enemy of my enemy is my friend,” to garner a greater share of whatever natural product seems desirable. Dogs to guard flocks and cats to discourage rodents are biological controls. The more organized and concentrated agriculture became, the greater the need for knowledge of “natural enemies” to enlist as economic allies. Even after revolutions in industrial chemistry offered alternatives, better living was still sometimes available through biology.

With private property rights come boundary disputes, complaints about trespass and spillover effects of management decisions. Public property, especially where subject to intensive multiple use mandates, adds complexity and diversity (if not novelty) to the mix. Rights collide with powers and authorities. Politically compromised jurisdictions—like U.S. state authority over wildlife except where superseded by federal laws and treaties or licensed to private parties—are endless fodder for litigation and finger pointing. All the while, science reconstructs what is known or considered knowable, changing expectations, affecting policies and destabilizing political balances.

Modern civilizations depend upon the plants they have introduced

Modern agricultural, horticultural and forestry practices are all legacies of the Renaissance, Reformation and Enlightenment motivations underpinning European colonialism. Empires were assembled and contested primarily for their economic advantages. During the past half-millennium they generated new wealth and new social classes that developed new governments. Among the array of actions those governments continue to undertake is facilitating the redistribution of valuable plants and animals. A visit to any retail food market reveals our near-total embrace of that redistribution. Almost every staple ingredient in every foodstuff is raised or grown far from its “wild” point of origin. Even insistent locavores prefer locally raised food, not locally evolved food. A negligible fraction of us recognize never-transported, never-domesticated edible organisms. Fewer still could survive on them as hunter-gatherers. Such are among the generally intended, hoped-for, positive outcomes of imperial colonialism. Famine is unnecessary, though it is a political tool, deployable as a weapon.

Fish, meat and leather, plant and animal fibers, timber, pulp and derived products can still be wild harvested, but are mostly and increasingly farmed. Anything worth gathering is worth cultivating, from redwood trees to bison to sugarcane to minks to soybeans to insects, yeasts, and bacteria. Even aspirational exceptions like native plant gardening are actually impossible to accomplish: seed intentionally transported from one location to another has been biogeographically rerouted; plants sold by native plant nurseries are raised in multi-source, formulated soils in plastic pots. Even simply deciding to leave a plant where it was found can render it an artifact, and there may no longer be any wilderness so remote that the configuration of its biota remains uninfluenced by human agency.

Benefits of introduced species often outweigh harm

We are told that some of the consequences of all this redistributed and reconfigured biota are marginally negligible. Others are cutting into the profits. Some organisms are moved around unintentionally and unknowingly (zebra mussels, various “blight” fungi) often because unaware transportation technology designers and operators never prevented their distribution. Many intentionally abducted and marooned populations are behaving in unexpected ways, thriving without always accomplishing their intended purposes (alligator apples and cane toads in Australia; house sparrows and wild carrots in North America) or even significantly over-achieving (“Asian” carps and kudzu in North America; rhododendrons and grey squirrels in Britain). Even where post-colonial inclinations to recover and reinstate pre-colonial values are tolerated, they hardly withstand translation into economic choices. We are adeptly, fundamentally invested in moving things around. We are likewise invested in competition, and building coalitions and alliances to help us win competitions. Especially competitions we thoughtlessly or accidentally set in motion.

The Old Lady who swallowed the fly would probably have been fine had she not overthought the problem. The fly was doubtless well on its way to being digested by the time she found a spider, which was likewise moribund before a bird came to hand. Maybe should could have swallowed a willow flycatcher (already protected by the Migratory Bird Treaty Act) and skipped the spider? Had the US Army Corps of Engineers, the USDA and others not overthought the problem in the late 19^th and early 20^th centuries, they might have come up with suitable alternatives to planting tamarisks to stabilize Texas barrier islands, deepening Four Corners arroyos and fly-away Dust Bowl topsoils. Yes, tamarisks, too, were brought to us to biologically control problems of our own making and conception. Then we needed a beetle…

As things turned out, USDA scientists were either mistaken or disingenuous regarding the latitudinal limits of their tamarisk leaf beetles. Likewise, even about the identity of the beetles, which is why I haven’t inflicted their Latin epithets on you yet. By 2010, sniping between USFWS and USDA, abetted by various conflicting conservation NGOs, led to a new “Biological Assessment” for the federally imposed tamarisk leaf beetle invasion. (I usually avoid using “invasion” in such circumstances, because invading exceeds many capacities of so-called “invasive species.” This was a real invasion, though, planned and carried out by people, not beetles. Beetles merely bred and spread.) One species of beetle became five, four which had been introduced: Diorhabda carinulata; D. elongata; D. sublineata; and D. carinata. Some were quite well-adapted to life in southern Arizona (31-32° N) and beyond. Furthermore, the endangered birds were also nesting in tamarisks in southern Utah, c. 37° N. USDA washed its hands of the federal program and revoked federal permits to release beetles; but that had no effect on the State of Colorado, which was heavily invested in producing them and continues to do so.

Distribution of tamarisk leaf beetle. Tamarisk Coalition

Fast-forward to 2017. Tamarisk leaf beetles have been spreading along Arizona waterways at rates up to ten times faster than their most ardent cheerleaders imagined they could, and from multiple directions. They will arrive in almost every known Southwestern Willow Flycatcher nesting area sometime this year. By next spring those riparian thickets will be defoliated just at the point when the nestlings most require thermal cover (i.e., shade). Thanks to Reclamation-Era water diversion projects, attempts to re-vegetate those areas with willows will require constant gardening. Reclamation replaced willow habitat with tamarisk habitat. Nevertheless, the birds persisted. Beetle releases suppressed the tamarisks, but will almost certainly fail to eliminate them entirely. Beetles are just another evolutionary pressure on a tamarisk population that is already unlike any other in the world due to unforeseen hybridizing among several species. New tamarisks and new beetles are evolving. Maybe the beetles will try a bite of something else. They’re in California now; could they find something there? Maybe the birds will evolve to eat the beetles, although that hasn’t happened yet. Perhaps the day will come when the Southwestern Willow Flycatcher gives way to the Tamarisk Beetlebird. It might not even take very long. But don’t bet on it. And don’t bet on biologists, bureaucrats or any other ambitious adults to re-learn the lesson of unintended consequences they laughed at as children, then (like so many other lessons) forgot.

Matt Chew

Invasion Biology: Confusion about cause and effect

We have said before on Million Trees that eradicating non-native plants will not result in the return of native plants because the underlying conditions that supported those native plants have changed and they are no longer competitive within their historic ranges. In those earlier posts we have focused on higher levels of CO₂ and the resulting climate change as the environmental variables to which non-natives are better adapted. Changes in water quality and flows have also resulted in changes in animal and plant populations and we will provide a few specific examples in this post.

Water levels in the Sacramento River delta have been hotly debated for decades and that debate has recently heated up as a commission gets close to making recommendations that will be legally binding.(1) On one side of the debate, the cities of Southern California and agriculture throughout the state want more water from the delta. They have been getting a lot of it for decades, but they want much more of it. On the other side of the debate, environmentalists object to exporting “our” water because they believe that the decline in the populations of native fish such as smelt and salmon is a direct result of the reduction in water flow from the delta to the ocean via the San Francisco bay. They object to further diversion of delta water and have legally challenged historic levels of water diversion using the Endangered Species Act.

Sacto Delta0001 — USFWS Recovery Plan for Native Fish in the Sacramento River Delta

The non-native bass in the delta are the proverbial red herring in this debate. Those who want yet more water diverted to agriculture claim that the bass are to blame for the declining salmon population. They demand that the bass be eradicated and they predict that the salmon population will recover once their non-native competitor is removed.(2)

The diversion of fresh water flow from the delta reduces the speed of the flow of the water, making it turbid and brackish as the ocean water overwhelms the fresh water from the Sacramento River. The warmer temperature of the water also promotes the growth of water weeds and algae. The bass benefit from these conditions, but the salmon do not. Eradicating the bass will not change these underlying conditions. Salmon populations are unlikely to rebound unless these underlying conditions are changed.

This is not an isolated example of the fallacy of invasion biology. There are as many examples of similar arguments as there are non-native animal and plant species now occupying spaces previously occupied by natives. Native plant advocates and their allies want non-native turtles eradicated because they believe they are responsible for declining populations of native turtles. They want to eradicate non-native bull frogs which they believe would benefit the native red-legged frogs. Etc., etc., etc., ad nauseum.

And there are as many examples of how such eradication strategies may not benefit natives as there are demands for eradication efforts. Here are just a couple.

The Tamarisk or saltcedar tree is one of hundreds of non-native trees that are considered invasive by native plant advocates. Here’s a description of an expedition on the Colorado River to eradicate Tamarisk that was published by the Sierra Club magazine.(3)

“’Kamikase!’ The most enthusiastic team members start to yell…and fall upon the larger plants with samarai fervor…’Kill tammys!’ someone yells. ‘Boy, that was satisfying.’ says a fellow tammy warrior…” And these are their tools of the trade: “…a veritable armory of tamarisk-killing tools, 32 gallons of herbicide, more than 40 cases of beer…and a Virgin Mary votive candle that…the camp cook has christened with a label reading, ‘Our Lady of Biodiversity.’”

Herbicide is being used in one of the country’s most important watersheds, yet there is no evidence that the Tamarisk is harming the environment:

One study found the “mean values for 22 of 30 soil, geomorphology, and vegetation structure traits did not differ significantly between saltcedar and Fremont cottonwood stands.”(4)
The same study found that saltcedar increased floristic biodiversity.
Another study stated, “As for the claim that salt cedar has little or no value to insects, birds, and mammals, that has been obliterated by available data.”(5)

OLYMPUS DIGITAL CAMERA — Tamarisk in natural habitat in Isreal, taken by Michael Baranovsky, Wikimedia Commons

But more importantly, eradicating the saltcedar is not likely to result in the return of the native cottonwoods because the natural flood cycle upon which the cottonwood depends has been altered by man. The saltcedar thrives in the reduced water flow. Unless the water flow is restored, the native trees will not return no matter how many saltcedar are destroyed. Not only are we wasting our time and effort trying to eradicate saltcedar, we are also poisoning our water in the process.

In our final example, cause and effect were not confused, and a restoration was successful. The Yuba Pass area in California is one of the most important migratory bird routes in the state. The breeding population of Willow flycatchers disappeared from one of the wet meadows east of the pass. The native willows upon which the flycatcher is dependent were disappearing from the meadow because channels caused by man along the edge of the meadow diverted water out of the meadow and dried it up. Ponderosa pines and sage, which prefer the drier conditions, were taking over the meadow. If native plant advocates had been in charge of remediating this situation their reaction may have been to eradicate the “invading” pines and sage. That would have been fruitless effort; conditions in the meadow were suitable for pines and sage, not for willows. But in this case biologists provided a more sophisticated solution. They eradicated no plants. They redirected the water from the channel back into its original slow flow through the meadow. The meadow is again wet, the willows are now thriving, and the Willow flycatcher has returned.

“Invasion biology” is an ideology, not a science. It frequently confuses cause with effect. A proper diagnosis of what may superficially appear to be an “invasion” requires an understanding of the complexity of nature. Most often the underlying reasons for an “invasion” are man-made conditions such as pollution and competition for scarce resources that are extremely difficult to fix. It may be convenient to scapegoat a plant or animal for what man has caused, but it is unlikely to reverse the conditions that create an opportunity for a non-native plant or animal that is better adapted to those new conditions.

(1) “Delta plan may do more harm than good,” Oakland Tribune, 11/5/10

“Effort Falters on San Francisco Bay Delta,” NY Times, 12/15/10

(2) http://www.sfgate.com/cgi-bin/article/article?f=/c/a/2010/12/11/EDG21GN1MJ.DTL

(3) http://www.sierraclub.org/sierra/200407/grand_canyon.asp

(4) Stromberg, JC 1998, “Dynamics of Fremont cottonwood and saltcedar populations along the San Pedro River,” Journal of Arid Environments, 40:133-155

(5) Anderson, BW 1998, “The Case for Salt cedar,” Restoration and Management Notes, 16: -130-134, 138