How to Make a Desert

You don't need to destroy all the plant life you see--just rearrange it a little. Then let nature do the rest.

By Carl Zimmer|Wednesday, February 01, 1995
RELATED TAGS: ECOSYSTEMS
At times the desert can make people seem small and inconsequential and even foolish. Bill Schlesinger feels that way today. With spikes and tape measure, he and co-worker Jane Raikes have staked out some 100 square yards of desert land in the Jornada Basin, 15 miles north of Las Cruces, New Mexico. Their claim includes some low-slung olive-drab creosote bushes, a clump of wispy tan snakeweed, and a lot of bare soil. Some ants roam the ground. A palm-size Texas horned lizard tries to stay cool in the shade of a creosote. It is a patch of desert that looks pretty much like countless other patches of desert in North America.

Why anyone--even someone like Schlesinger, a biogeochemist from Duke University in North Carolina--should bother to mark and study this particular patch of desert is hard for an outsider to divine. Schlesinger is aware of this. So, as he calls out pairs of numbers to Raikes, he is not surprised by the puzzled looks he gets from passing weekend horseback riders. Raikes goes to the corresponding coordinates in the plot, armed with a rock hammer and a metal pipe. She drives the pipe two inches into the ground, the blows ringing out in the silence. Then she pries the pipe out and taps the clogged soil into a Ziploc bag. I can see it in the press now, says Schlesinger. ‘They banged a pipe in the dirt many times and found it exciting.’

Despite his habitual self-mockery, Schlesinger knows that the tablespoons of soil he and Raikes are collecting may help reveal a profound secret of the desert. If Raikes and Schlesinger had come to this spot 150 years ago, they would have been surrounded by almost uninterrupted grasslands stretching across the basin. Somehow the Jornada has since changed, and Schlesinger, Raikes, and the other researchers who work here think they know why. In many cases, they believe, a desert is like a living organism. Like a cactus or a sidewinder, it needs parents to give it birth, but once kicked into the world it can grow and thrive on its own. Deserts aren’t necessarily the product of outside forces like decreasing rainfall, they say. Rather, it’s the internal ecology of the desert itself--its web of plants, animals, and soil--that drives its growth to maturity and stability. Nor does the transformation of a grassland to a desert necessarily mean the creation of a place where life is more scarce--only one where life is rearranged.

Grasslands are changing into full-blown deserts not only in the Jornada Basin but around the world, on every continent but Antarctica--on every continent, that is, where humans have settled. In North America alone an estimated 1.1 billion acres have been desertified, and researchers suggest that global warming may generate more desert acreage in the coming century. Desertification is sufficiently serious a threat that representatives of 87 countries have drafted a treaty to combat it; only two other environmental crises--ozone destruction and global warming--have earned such attention. But scientists still argue about exactly how desertification happens, how much of it is a matter of natural fluctuations, and how much is man-made. The political impact of the debate is huge--witness the conflict between ranchers and Interior Secretary Bruce Babbitt over grazing rights on public land. If the model Schlesinger and his colleagues at the Jornada have built proves true, it will inject some desperately needed science into this debate.

There’s a touch of ecological hubris in trying to generalize to the entire globe from the Jornada’s 310 square miles, but then, the Jornada is an exceptional place. It is arguably the best-studied desert in the world. As Kris Havstad, the director of the U.S. Department of Agriculture’s Jornada Experimental Range, drives around on a tour of the basin with Schlesinger and Raikes, he recounts the Jornada’s long history: As early as 1600, Spanish wagons were traveling from Mexico City to Santa Fe with livestock. They tried to avoid the Rio Grande because the terrain was broken and heavy and they’d get bogged down. So they crossed over the mountains, up into the plains. The problem is, there’s almost no water here. You read the journals people kept of their trip, and when they get here, they get really quiet.

The 90-mile trail was soon given the name Jornada del Muerto-- Journey of the Dead. Dry as it was, though, the basin was not a desert at the time.

Nothing could exceed the beauty of the country we traveled over this morning, wrote May Humphrey Stacy after crossing the basin in 1857. The whole extent, as far as vision reached ahead, was a level plain, covered thickly with the most luxurious grass.

The Jornada Basin is at the northern edge of the Chihuahuan Desert, which runs south 1,000 miles from New Mexico, through the western wedge of Texas and down the backbone of Mexico. Creosote bushes and small, spiny mesquite trees dominate the landscape. Yet for thousands of years the Jornada had been a stable grassland. The closely packed tufts of golden- stalked black grama grass attracted Mexican cattle raisers, and after them, Americans.

But because those early settlers could water their livestock only at springs in the bordering mountains, they raised rather few cattle. It wasn’t until we brought the technology to drill wells in places like this, where the water is 400 feet down, that this land opened up to heavy grazing, says Havstad. After water was brought to the surface in the 1880s, there were 20,000 head of cattle out here. This place just got hammered.

By the turn of the century people had begun to notice that the Jornada was changing. The grass was thinning, and the mesquite and creosote were spreading. The ground became bare; in some places low dunes formed. Ranching was becoming less and less profitable. Similar changes were happening throughout the American West, and in 1912 USDA officials fenced off 192,000 acres of the basin and set up the Experimental Range, where they tried to understand what was happening and how they could stop it.

As Havstad tells the tale of the Jornada, he is looking out the truck window for some of that history. Half a mile down the road he finds a red ribbon tied to a barbed-wire fence; 50 yards from the road is another ribbon tied to one of the spear-shaped leaves that explode from the top of a soap-tree yucca. Nearby, four rusted steel posts sprout from the ground, forming the corners of a square. When the USDA took over the Jornada 83 years ago, it sent surveyors to stake out 104 such sites, and for decades thereafter USDA researchers faithfully sketched the vegetation in each square. Havstad has some of the early drawings of this plot in hand. They show scattered patches of grass. Now the four posts enclose only mesquite.

These early researchers did not consider themselves ecologists. They were range scientists, dedicated to figuring out how to make the Jornada grow food for cattle. They did everything they could to stop the basin’s transformation. They cut the herd down to a few hundred head. They tore up mesquite, poisoned the shrubs, seeded grasses, and dug giant pits to help water penetrate the ground. They failed. The Jornada researchers estimate that in 1858 about 5 percent of the range was dominated by mesquite or creosote, 37 percent had a few shrubs in it, and the remaining 58 percent was shrub free. Just over 100 years later, in 1963, 64 percent of the range was dominated by mesquite and creosote and none of the remaining 36 percent could unqualifiedly be called grassland any longer. Now, Havstad estimates, about 80 percent is classic desert shrub land.

Heartbreaking as the desertification was to range scientists, it made the Jornada a fascinating place for desert ecologists. You can find virtually every kind of vegetation unit there that you would find anywhere in the Chihuahuan Desert, says Walt Whitford. Whitford began studying the Jornada in 1964 when he came to New Mexico State University in Las Cruces. The short drive from campus and long history of research made it an attractive place for doing ecology. And most important, says Whitford, he could really do ecology there. You can go to national parks, but you can’t do experiments in them. The Jornada is a huge piece of land dedicated to research. We can move livestock, impose droughts, burn strips--we can do almost anything you can imagine.

In 1981 Whitford launched the Jornada Long-Term Ecological Research Project. His own expertise was in desert animals, and he enticed other researchers to join him to study the Jornada’s plants, its soil chemistry, its patterns of wind and water. But by the mid-eighties, as the data crammed their filing cabinets and computers, Whitford’s team began to feel more like bookkeepers than scientists. The big problem was to relate what we were doing at a single site in New Mexico to global desertification, explains Duke ecologist James Reynolds. Precisely how had the Jornada changed, they asked, and was the same process responsible for changes elsewhere?

The first glimmers of an answer came in 1988, on an afternoon when the senior Jornada researchers closeted themselves in a hotel room in Columbus, Ohio, during an ecology meeting. That day, in the course of their long conversations, they first realized that they had all noticed a simple but important pattern in the Jornada: the desert is patchy. Its vegetation is obviously patchy even to the untrained eye, looking like an archipelago in a drained ocean. But the researchers had discovered that the desert is also patchy in unseen ways, such as in the distribution of its water, nutrients, and microbes. Grasslands, on the other hand, are relatively uniform carpets of plants and resources. Find what drives an ecosystem from smooth to patchy, the Jornada researchers decided that afternoon, and you’ve explained desertification.

Over the next several years they constructed something they call the Jornada model, based on observations, experiments, and intuition. Like all models, it is a story, and it begins in the 1800s, when the basin was a grassland. Though its climate was precariously dry, the ecosystem had remained stable for millennia, thanks in part to its ability to create its own weather. Its spongy soil soaked up rain, and when the water evaporated back into the air, it formed clouds that then recycled the rain back to the basin.

The grassland was also able to shut out competing plants. While a few creosote bushes and mesquite trees grew in the basin, they had a tenuous existence. They’re a native element that was always there, just waiting, says Havstad. Shrub seedlings, though, were particularly vulnerable to the fires that swept the grassland, and the ones that survived faced a crippling water shortage: the shallow, dense roots of the grass absorbed the rain before it could percolate down to the shrubs’ deeper roots.

In the late 1800s the vast unmanaged herds of cattle that were made possible by the advanced drilling technology helped the shrubs penetrate the grassland’s defenses. Simply by eating the grass, the cattle impaired its ability to photosynthesize and grow. Less obviously--but just as important--they may well have made the landscape patchy. The ecologists speculate that as the cattle trampled the ground to their favorite feeding spots, the ground they habitually walked on became less able to absorb water. Rain flowed over this soil rather than into it, forming channels. No longer lingering in the upper soil, where grass roots grew, the water instead either escaped downstream or percolated through the channel bottoms, down to where only the deep-rooted shrubs could get to it. Hoofprints gave rise to pools of water that infiltrated the soil, creating spots for a seedling to take root and thrive. Water was no longer evenly spread over the basin, but now concentrated in scattered places.

By the time the giant herds left the Jornada, they had pushed it over a critical threshold. Now the ecology of the desert itself took up where outside influences left off. Thanks to patchiness, shrubs established a foothold, and they made the patchiness even greater.

The shrubs are deeply and widely rooted, explains Schlesinger, so they’re obtaining nitrogen from a big area of the desert. It’s at low concentrations out there, but they’re getting it and concentrating it in their tissues. Then as the shrubs drop their leaves, the leaves fall under the shrub. It’s like a pumping mechanism. They’re sucking nutrients in from far and wide and dumping them under their canopies. The leaves decompose under the shrub and the nitrogen gets circulated. The bulk of the nitrogen is what gets circulated from under the shrub, but every year there’s new nitrogen that’s also being added from these roots.

The shrubs rearranged the nitrogen in the basin from a smooth layer to concentrated and increasingly isolated islands of fertility. Wind and rain began to make these islands grow faster. Gusts scoured the bare patches, carrying away nutrient-rich dust, but when they hit the canopies of creosote and mesquite, they broke into whirling, weakened eddies, dropping their dust--as well as dead leaves and other organic matter--to the base of the shrubs. When a raindrop hit unprotected soil, it shoveled up the topsoil and carried it away in the overland flow of water, and when the water hit the downslope edge of the patch, it carved away even more soil. But the shrub roots protected their islands from the flow, and their leaves broke the fall of raindrops, which dribbled gently to the spongy ground below.

The mesquite and creosote also brought animals adapted to them, which in their own ways helped the islands grow. Termites and kangaroo rats collected food from a wide range and stored it in their nests, which they often made under shrubs. In this way they brought a lot of organic matter into the islands and took it away from surrounding areas. Meanwhile, nests they established away from the shrubs often became ideal birthplaces for new seedlings.

Even the shape of an insect’s mouth could help build the desert. In a grassland, most of the processing is done by chewers, says Whitford, and the droppings of these grassland insects--the scientific term is frass- -consist of tough, complex material that spreads over the ground before it finally breaks down. But the insects you find on shrubs, he explains, are predominantly guys with mouthparts like little hypodermic needles, and they’re sticking them directly into the vascular system of the plant and siphoning off its sap. The frass is basically a simple sugar solution. When that hits the soil, you’ve got a ready source of energy for the microbes to use and break down into the soil, where it’s available for growing plants.

According to the model, all these feedback cycles will eventually, over the course of millennia, reach an equilibrium in the Jornada. When a shrub dies, the island it leaves will be a nursery for a new one, which will be protected from fire by the distance from one island to the next. Neither hard, bare soil nor shrub-dominated islands will offer any hope to colonizing grass. And since the soil will hold less and less water, rain recycling will stop, making the desert even drier.

Ecology being such a slow science, the researchers knew that by proposing this model they were indenturing themselves for decades. Seven years after their first brainstorm, they sound hopeful. Just about all the evidence we’ve collected so far supports it, says Reynolds. New Mexico State University biologist Laura Huenneke, for example, has been measuring the mass of the Jornada vegetation, and she finds that the central tenet of the model holds true: arid grassland and shrub-dominated desert contain about the same weight of plant material. It’s just arranged differently.

The researchers even feel confident enough now to argue that the Jornada model explains desertification in other parts of the world. The grass may not be black grama and the shrubs may not be mesquite and creosote, but the basic process seems universal.

The model also ought to apply to the spread of deserts before humans began to change the landscape, with slow shifts of climate playing the role of grazing cattle. As a grassland experienced centuries of decreasing rainfall, this scenario goes, the upper level of the soil would dry out. Grasses, with their shallow roots, would suffer, but shrubs would still be able to tap the deep water that trickled into the ground from storm runoff, and they’d start building their islands. A simultaneous change in the composition of the atmosphere could speed the process. Grasses are much more efficient absorbers of carbon dioxide than are plants like mesquite and creosote, and so they can thrive on low levels of the gas. But when the atmospheric level of CO2 jumped from time to time, grasslands lost their competitive advantage and became vulnerable to shrubs and patchiness.

As they test the model further, the researchers crawl over the basin like a swarm of locusts. Schlesinger and Raikes, for example, bang their pipes into the dirt and find it fascinating because they want to see how the distribution of chemicals in desert soil changes over millennia. The model predicts that the chemicals important to life get concentrated under vegetation, while unnecessary elements like lithium and bromine remain smoothly scattered. Schlesinger is collecting soil samples from Jornada grassland and shrub land and comparing them with samples from the Mojave Desert in California, where a dry climate has allowed creosote to build islands of fertility for 10,000 years. If the model is right, he should see a progression in the distribution of chemicals from young desert to old.

However, it is not these last stages of the model that are most controversial, but the first ones--in which grazing supposedly gives islands of fertility their start. The human causes of desertification and their cures have attracted vast amounts of money and prompted political wrangling. Billions of dollars have already been spent in various schemes to fight desertification, even though researchers are only beginning to understand how it works. Critics maintain that developing countries use the fear of spreading deserts as a way to guarantee a flow of aid. But the argument isn’t limited to Third World nations. In the United States, environmental groups are urging Secretary Babbitt to increase fees for grazing on the 280 million acres of public rangeland. Cheap grazing rights, they say, degrade the land, drive species extinct, and lead to desertification. Ranchers claim that they’ve improved their grazing practices since the turn of the century, so that the only real effect of raising grazing fees will be their bankruptcy.

The Jornada researchers try their best to keep the hue and cry from affecting their work. The ranching industry doesn’t like us at all, says Schlesinger. They’d love to say that cows make no difference, that it’s all drought or climate change or kangaroo rats. But I don’t see these things as mutually exclusive. There’s no point in our singling out one thing at the expense of others.

The debate has a hell of a lot less to do with science than emotion, says Whitford. It’s about economics, about whose ox is getting gored. Hopefully we can provide some factual information. We don’t have unequivocal evidence that there are these links; that’s why we’re doing the experiments we’re doing.

So far those experiments have given them results that are suggestive but not conclusive. In 1982 the Jornada ecologists closed off some plots from the USDA’s cattle. Over the following ten years many of the plots that still had some grass in them dramatically improved, compared with unprotected grass nearby; meanwhile, shrub-dominated plots saw no change. Still, some of the plots untouched by cows also died out, suggesting that drought too must play a role in the long-term survival of grassland.

However, New Mexico State University ecologist William Conley has shown that droughts may not have been so important in the Jornada. Grass is indeed susceptible to drought, but only when it strikes during the summer growing season. The USDA’s 80-year record of rainfall in the basin shows that droughts this century have actually hit the Jornada more during the winter, the growing season of mesquite and creosote. If anything, they should have helped the grassland survive. Conley also made a statistical analysis of the rainfall record that shows that the droughts were not freakish; they had probably hit the Jornada every few decades for centuries. If they had the power to desertify, the Jornada should have become a desert long before Mexicans first passed through it.

That leaves grazing as the most likely culprit. Only now, though, are the Jornada ecologists performing the experiment that can document the steps by which cattle may initiate islands of fertility. Almost dead center in the basin, a wave of mesquite is rolling over some of the last remaining black grama. In 1993, on the border between the two plant communities, USDA researchers set up 18 fenced enclosures, each about 750 feet square. In 9 enclosures they hacked down the mesquite and painted the stumps with herbicide; the other 9 sites were left untouched. Whitford, who has continued to study the Jornada since he joined the Environmental Protection Agency in 1992, cataloged all the plants and animals. Schlesinger, who with Reynolds now heads the Jornada Long-Term Ecological Research Project, measured the distribution of dozens of chemicals in the soil. Before this winter is over, Havstad will bring two dozen head of cattle into 6 of the corrals and let them graze for 24 hours, in which time they should devour two-thirds of the grass and trample the ground. This summer he will let them loose in 6 others, while the 6 remaining will stay unmolested. In the next five years the researchers will measure how the soil, plants, and animals change in response. Two of the grazed plots will be burned and 2 others will be covered in rain-out shelters to see how fires and droughts enhance or reduce the effects of grazing.

If the story of the Jornada does turn out to be the story of other deserts, then ecologists should be able to foretell the story of deserts not yet born. And that’s a skill that may be in high demand in the coming decades. As we burn fossil fuels and add carbon dioxide to the atmosphere, we once again take away the competitive advantage grass has in a CO2-poor world. Simulations also suggest that global warming may make continental interiors drier. Grasslands that now get comfortable levels of rain may become vulnerable to grazing, as the Jornada was in the nineteenth century.

For over 20 years Reynolds has been turning data collected in the Jornada into mathematical equations. By calculating 200 variables, he can now accurately simulate the year-to-year evolution of a few square yards of the Jornada, whether it’s occupied by grass, an island of fertility, or bare soil. Now he’s stitching these patches together into a quilt that represents the entire basin. Using its actual topography and weather, he lets the patches interact. Shrubs deplete the surrounding soil, and bare patches help erode neighboring topsoil as water flows from one patch to the next. By measuring the growth of creosote and mesquite in an isolated, CO2- flooded area, Reynolds hopes to be able to predict how the basin will evolve in the near future.

Ultimately the simulation should be able to predict the fate of any grassland. An ecologist in Chile, for instance, could feed a computer data on the local topography, weather patterns, and patterns of vegetation and see how likely the land would be to shift over to desert. We’ll be able to say how sensitive places will be to grazing and climate change, says Reynolds. We can say, this area is not beyond the threshold and it would be worth trying to restore it, but this other area has changes that are irreversible.

Irreversible is a tough word, but the Jornada model makes clear why most efforts to restore self-sustaining grasslands are futile. Take away the shrubs, and the landscape, full of islands of fertility, is still perfectly suited for new creosote and mesquite to invade. We’ve talked about a homogenization experiment, says Reynolds, in which we’d go out to some of the big dunes where it’s really heterogeneous and just bulldoze those babies and see if when you distribute everything, the grasses would be successful again. This would just be an academic pursuit; it wouldn’t be a restoration tool. Deserts do turn to grasslands naturally, but only when thousands of years of steadily increasing rainfall counteracts the power of the islands of fertility.

Perhaps the name Jornada del Muerto should now be changed to Jornada del Desierto. Just as people have journeyed across the basin for centuries, the land itself is taking a journey that these researchers are now able to trace. According to Whitford, history may provide a glimpse of its destination.

Climatically, North Africa should be a grassland savanna, he says. You can go back to historical records and read about the trees on the mountains and lush grass, about how they were the breadbasket for Rome, providing grain and meat to the empire. Well, now it looks like parts of Nevada. You put these things together and say, ‘Well, the model probably worked there, and it looks like it followed the trajectory that we’re following in places like the Jornada.’ We’re well on our way.
Comment on this article
ADVERTISEMENT

Discover's Newsletter

Sign up to get the latest science news delivered weekly right to your inbox!

ADVERTISEMENT
ADVERTISEMENT
Collapse bottom bar
DSCSeptCover
+

Log in to your account

X
Email address:
Password:
Remember me
Forgot your password?
No problem. Click here to have it emailed to you.

Not registered yet?

Register now for FREE. It takes only a few seconds to complete. Register now »