DESERTS DO NOT burn. Parched, unwatered, swept by dehydrating winds, the deserts of the world are marked by a scarcity of plant life. Thinly spread across the landscape, desert plants do not grow densely enough to fuel the conflagrations that sweep periodically through forests and prairies. Those fires that do start out among the creosote bush or spinifex, the ghost-gums or baobabs, caused by lightning or the casually tossed cigarette, are small and self-contained. When an unfortunate desert plant does go up in flames, it kindly spares its neighbors--if only because its neighbors are far away.

Deserts do not burn. At least they are not supposed to. Thus it came as a surprise when, on July 4, 1994, after an unusually well-orchestrated show of fireworks honoring the nation's independence, Tucsonans were treated to the spectacle of an A Mountain coated, like an erupting volcano, in flames, a spectacle repeated, albeit more modestly, the following year.

The scientists who work at the nearby Desert Laboratory of the University of Arizona on neighboring Tumamoc Hill, longtime desert watchers, knew the big burn was coming. They'd been tracking changes in the desert around Tucson for years, noting the small-scale, plant-by-plant alterations that have slowly but perceptibly been reshaping the valley for the last couple of decades, and they knew where to put the blame for the A Mountain fires.

The fuel was an exotic species called buffelgrass (Pennisetum ciliare). A sturdy plant with a thick, woody stalk, buffelgrass dries in the absence of frequent rain or irrigation. When it meets flame, it burns fiercely, and it takes out its neighbors as it does--in the case of the A Mountain fires, paloverdes, creosote bush, and a few young saguaros, all plants that have not evolved the means to protect themselves from fire.

"We'd been expecting the burn for several years," says Tony Burgess, a scientist at the Desert Laboratory. "We began to notice some buffelgrass coming into undisturbed areas of the desert around Tumamoc in the early 1980s, and more when the El Niño weather system came later. We knew it was out there, and we knew it was going to go up sometime."

Fire is essential to the grass' well-being, essential to the ecology of grasslands. Fire burns off dead growth, and it provides the seed of new grasses with a bed of ash rich in nutrients. That bed yields an even more robust growth of grass that surrounds other native vegetation until the next burn, which, inevitably, will come. This natural logic enables buffelgrass to crowd out native vegetation and to establish itself as the dominant and often sole plant in a given area.

Pennisetum ciliare comes originally from Lake Turkana, in East Africa's Great Rift Valley--the place, anthropology textbooks tell you, where humankind was born. A rich source of protein for browsers in its native savanna, buffelgrass was brought to South Africa as cattle fodder early in the 20th century. It produced a good feed, and it provided a boost to a flagging cattle industry in a land long on drought and short on standing water.

American rangeland scientists took notice of South Africa's successes. Forty-five years ago, Texas A&M agronomists planted an experimental crop of buffelgrass at a research station near Dallas, and it produced a yield so dense they were soon billing it as a "wonder grass." The dry llanos of Texas proved an ideal place for buffelgrass, which grows on coarse and sandy soils at elevations below 2,900 feet. Soon much of Texas south of San Antonio was carpeted in buffelgrass, as was the neighboring Mexican state of Tamaulipas. (Buffelgrass fueled the huge grass fires that swept Texas and southern Oklahoma last winter, consuming nearly half a million acres of grassland.)

It also came to Sonora, and to southern Arizona. How it crossed the line into the Grand Canyon State is a matter of some debate; rangeland ecologists point out that buffelgrass, with its abundant supply of windborne seeds, seems to follow roads, and it may have hitched a ride on trucks laden with the riches of NAFTA making their way from Mexico northward along American highways.

David Yetman, a research social scientist at the University of Arizona, suggests another, more likely scenario. Not far from the combustible mountains west of Tucson lies a U.S. Soil and Conservation Service plant-materials center that has for the last two decades been producing crops of buffelgrass, along with other exotic African imports like Lehmann's lovegrass. From that Romero Road farm, Yetman conjectures, a few seeds escaped to begin colonizing the nearby desert.

In any event, in the last few years buffelgrass has spread rapidly across Tucson, establishing itself along arroyos and culverts, on land cleared for construction, in parking lots, snaking its way across the valley to A Mountain, the face of the Santa Catalina Mountains, Saguaro National Park, even to the pristine acreage of Tumamoc Hill's Desert Laboratory, held in trust since the early 1900s. Buffelgrass is turning up elsewhere in southern Arizona: Organ Pipe National Monument is full of it, and it has been reported in the Cabeza Prieta, one of the Sonoran Desert's remotest spots.

Once buffelgrass invades, plant scientists say, it cannot be eradicated. It's here in the Sonoran Desert, and it's here to stay.

BY COMPARISON WITH Sonora, southern Arizona has only a minor buffelgrass problem, at least for now.

Confronted with a profound economic and agricultural crisis that has stretched out and spiraled downward for the last two decades, Mexico has been having an increasingly hard time feeding itself lately. The price of corn flour, maseca, has quadrupled in the last year to four pesos a kilo; the prices of beans, wheat flour and other basic foodstuffs have also gone up dramatically.

Faced with the need to feed an exploding population nearing 100 million, Mexican ranchers have had to adapt by increasing their yields, especially in the face of competition from North America. One step that ranchers in the arid state of Sonora took was to convert their herds from the hardy, lean criollo cattle that had roamed the desert since the time of the Spanish conquest to Charolais, Guernsey, and zebu cattle that yielded substantially more meat on the hoof.

Where the criollo had long since adapted to drylands conditions and were famed for their ability to go for days without drinking or eating, the new cattle were, as cattle go, finicky eaters; they could not convert "shrub proteins," and instead depended on well-watered grass, scarce in Mexico, where rain-fed grains are uncommon and expensive. Still, they brought in more money than the comparatively scrawny criollos. With no financial incentive to stick to proven customs--and, unlike their counterparts in the United States, Mexican farmers and ranchers operate without federal subsidies--the Sonoran cattlemen took a gamble on a new kind of herd.

That gamble paid off in the short term for some ranchers. Others, facing the long-term crisis without adequate cash, gave up ranching, and their holdings were absorbed into bigger agribusiness operations. Still others, the Centro de Investigacin Pecuario del Estado de Sonora (CIPES) reports, have mixed a small amount of ranching with a twist on ecotourism; some 80 large spreads in Sonora now bill themselves as "game ranches," given over to providing hunters from the United States and Europe with a place to come and fire their weapons at native animals.

The ranchers who stuck to ranching found an ally in the wonder of buffelgrass. Introduced to Sonora in 1958 by plant specialist Donald C. Johnson, one of the architects of the so-called Green Revolution, buffelgrass seeds throughout the year, unlike native grasses that grow largely during the summer rainy season. It found an ideal growing range in the dry desert plains of Sonora, and, during the agricultural intensification of the 1960s, along with the building of new roads and bridges throughout northwestern Mexico, buffelgrass was planted in quantity in one of those programs of good intention with which the road to hell is paved.

Johnson knew planting buffelgrass had its risks. In a widely cited report published in Sonora, he admitted "the high productivity and aggressiveness of buffelgrass allows for its invading and establishing itself in unseeded areas." But, Johnson argued, the introduction of the exotic grass would be less damaging to the desert than overgrazing the native vegetation, which, in the absence of any meaningful environmental restrictions, was not likely to cease; the thought that stopping overgrazing instead of introducing non-native vegetation seems not to have occurred to the Sonoran government.

With that introduction, says Yetman, the face of Sonora changed. "Parts of the Sonoran Desert are going to be permanently altered," he notes. "Huge parts already have been, and we can see the damage all around us."

Vast tracts of the Sonoran Desert are now being bulldozed, scraped clean of their vegetative cover of trees, shrubs, and cacti for buffelgrass cultivation. Behind the bulldozers have come the carboneros, workers who chop up the uprooted trees and reduce them to the mesquite-and-ironwood charcoal that so satisfies North American palates. Many ranchers and feedlot operators have paid for the desmonte, the land clearance, by licensing this coveted one-time sale, which has on a larger scale made charcoal production a major Sonoran industry.

At this writing, about a million hectares (2.5 million acres) of land are under buffelgrass cultivation in Mexico. The government of Sonora has announced plans to increase that clearance to six to eight million hectares, meaning that nearly half of the arable land in the state will be given over to buffelgrass production.

Donald Johnson, its introducer, has written that ideal land for buffelgrass cultivation in Sonora does not exceed two million hectares, and that crop failure is the likely result of planting outside that productive zone. His prognosis seems not to have swayed the Sonoran government to call for a more modest program of planting. Neither has Alberto Búrquez of the Centro de Ecología de la Universidad Nacional de Mexico, who has been studying this accelerated growth for a decade, and who warns that "zacate buffel is the greatest threat to biological diversity in the Sonoran Desert."

Countering Búrquez, CIPES scientists point to a carefully maintained 100-hectare plot of buffelgrass on their 1,200-hectare demonstration ranch outside Hermosillo, dotted with sleek, fat cattle. By mixing buffelgrass with other kinds of vegetation, they argue, an ecological balance can be maintained. That balance relies on careful monitoring, on weeding out errant buffelgrass, and on keeping the cattle population low enough to allow all kinds of vegetation to regenerate. And because buffelgrass exhausts the soil of nutrients, it must be deep-plowed and replanted every few years, a cost only wealthy ranchers and agribusinesses can afford to absorb.

But Sonoran cattlemen, seeing the successes of their American counterparts in combating grazing-fee hikes and opening wild areas to grazing, are not much interested in environmentally responsible ranching. Much of the new ranching industry in Sonora, grasslands ecologist Peter Warshall observes, is a front for laundering drug money, and ranching profit is secondary. For the ranchers who rely on licit agriculture for a living, short-term gains have necessarily overridden the long-term health of the land, and few ranchers have the time or money to maintain their fields with the same care the scientists can.

And so the wonder grass continues to be planted, and, wherever it is planted, to spread. Just as canary grass overran New Zealand, forever changing its native ecology, so buffelgrass is rapidly displacing native hilaria and grama tallgrasses, burning out the desert thornscrub forests, taking over from horizon to horizon. Where delphiniums and hibiscus, poppies and geraniums, paperflowers and zinnia once carpeted the ground, a dense mat of yellow, reedy grass, whether deliberately planted or spread by the winds, now dominates the landscape.

Humans have always tended toward monoculture when the economic indications seemed propitious. Of the 10,000 grass species available worldwide, only 40 account for 99 percent of the yield of all sown grasslands. But monoculture has come to Sonora with a vengeance. Where hundreds of native species once flourished, buffelgrass favors only one: the cow.

Faced with this argument, Donald Johnson replies that rabbits, locusts, and ants have established themselves in areas under buffelgrass cultivation. Alberto Búrquez and David Yetman add to these sightings with reports of a single rattlesnake 10 feet inside buffelgrass, a single deer, and a brace of quail. Thus only seven species, plus a small subset of Homo sapiens--industrial ranchers, feedlot operators, and charcoal makers--benefit where hundreds thrived before.

With buffelgrass, fire has come to the deserts of Sonora. The pages of Hermosillo's daily paper El Imparcial are filled with stories of grasslands fires burning out of control, filling the sky with oily smoke. In rural communities without fire departments, grass fires have raged unchecked for days, burning thousands of hectares of desert land.

Proponents of buffelgrass agriculture argue that Mexico, with its runaway population growth, cannot afford to preserve native vegetation when a "wonder grass" is readily available to feed the masses. (Because beef is expensive in Mexico, the masses rarely see it on their tables.) They suggest that the only realistic solution is to create preserves for native vegetation throughout the Sonoran Desert, far away from the fields where Pennisetum ciliare is grown, treating cactus and acacia as captives in a postmodern museum exhibit. Donald Johnson likens these proposed relic stands of native vegetation to the handful of prairie preserves that dot the American plains.

Whether the introduction of buffelgrass will save the agricultural economy of Sonora, in the long or short term, is open to debate. Yetman, echoing the late agricultural economist P. Lamartine Yates, argues, "Mexico has reached the limits of its agricultural productivity." He observes Mexico is no longer self-sufficient even for the staples of its diet, that it now imports corn and pinto beans from the United States. (Those pintos are grown in Wisconsin, of all places.) Slowly, and for a number of reasons, the rural economy of Sonora is changing: More and more wealth is being concentrated in the hands of fewer and fewer people; more smallholders are being driven from their farms, forced to seek work in Mexico City or the United States.

And, in the absence of environmental regulation, buffelgrass cultivation in Sonora is likely to go unchecked. When asked whether anything can be done to curtail its explosive growth, Homéro Aridjis, president of the Mexico City-based Grupo de Cien, an environmental policy organization loosely modeled on the Sierra Club, sighs, "There are so many environmental problems pressing Mexico. Certainly zacate buffel is one. But it is only one. We have so many battles to fight."

THE FIGHT IS a global one, and it affects the southwestern United States and northern Mexico in equal measure. Tony Burgess sees nothing but apocalypse in its outcome. "What we are seeing here," he says, "is two huge trends of complete landscape alteration in the Americas: the Mediterranization of California and Chile, with the total replacement of native plants by imported annuals, and the Africanization of the neotropics, with native scrubthorn woodlands given over to savanna grasses.

"These trends are remaking a good portion of the world. Here at home, the rich, incredibly lush landscapes that ring the arid core of the Sonoran Desert are slated to go because they're not profitable. The only thing that will keep this from happening are subdivisions, the creation of cities full of people who would not want to see their homes go up in flames due to grassfires.

"So we have three choices for the future of the Sonoran Desert: suburbanization, the aristocratic hunting preserve where wild desert is set aside, or large-scale profitable ranching operations that rely on buffelgrass."

The future is with us already. To glimpse it, stop outside Espinazo Prieto on Highway 15, just north of Hermosillo, and take in the view of a desert that is being remade to resemble an African savanna. Or drive farther to the south and west, where the climate is drier, the soils sandier, and where a different future prevails: Fields of buffelgrass lie dead and abandoned, fallen to drought, dotted by the occasional surviving brittlebush.

The desert is being remade, following a pattern that the distinguished ecologist Eugene Odum has discerned throughout history. "Humans," Odum writes, "have had a persistent history of misuse of grassland resources by virtue of allowing overgrazing and overplowing. The result is that many grasslands are now human-made deserts."

And many deserts are now human-made grasslands. Sonora, famed in colonial times for its productive grainfields, is rapidly becoming one of them as its coastal plains, its thornscrub and tropical deciduous forests, are covered by a blanket of buffelgrass. Much of its native desert has simply disappeared. "Even if the bulldozing stops today," says Sandy Lanham of Tucson-based Environmental Flying Services, who has documented the plant's growth throughout Sonora over several years, "buffelgrass is here to stay forever."

And Sonora is not alone. More than 20 million acres of Australian dryland is now under buffelgrass cultivation. Large fields are being cleared for its plantation in Brazil and Argentina. Even the Hawaiian Islands, ever more given over to livestock production, are now being planted with Pennisetum ciliare.

With increased international trade has come the specter of increased biological invasions. The spread of the Africanized bee would not have been possible without long-distance trucks, or the fire ant without container ships. Harold Mooney, a Stanford University professor and editor of Biological Invasions: A Global Perspective, warns that no ecosystem anywhere in the world is safe from incursions by non-native species, incursions that will change the face of the planet--and, Burgess adds, that will turn out to be far more significant than global warming as an agent of ecological change.

Undaunted by such dire forecasts, Soil and Conservation Service scientists and their counterparts in Mexico have been experimenting with genetic engineering to produce a frost-resistant buffelgrass that, with red brome and Lehmann's lovegrass and other exotics, will complete the Africanization of the Sonoran Desert, and with it the conversion of what government reports call "useless scrub" into productive rangeland--a huge belt that runs on both sides of the international border from Lukeville, Arizona, to Big Spring, Texas.

Faced with the prospect of southern Arizona as a biological colony of a Sonora that has itself been colonized by South Africa by way of Texas, David Yetman struggles to find hope. "I don't want to be overly pessimistic," he says. "The Sonoran Desert has been around for 10 million years, and it's resilient. Predators on buffelgrass will evolve one day, and some kind of balance will occur. But in the short term, I expect that in those areas where buffelgrass is entrenched we'll lose the trees, the columnar cacti, the plants that make the desert what it is. They won't return in our time--or in our grandchildren's time."

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