Buffelgrass, however, has at least one advantage over its fellow desert invaders: While cheatgrass and red brome are annuals, fed by winter rains, buffelgrass is perennial. It responds to rainfall at almost any time of year, and bulks up as it sprouts and dries, able to fuel fires not just during a particular season but throughout the calendar. It can even flame up when green and by some estimates can burn at more than 1,300 degrees, literally hot enough to peel rocks.

In the 1940s, Burquez found, fires on the outskirts of the Mexican capital of Hermosillo were all but unknown. In the 1960s, newspapers began to report sporadic fires, and today, hundreds of fires break out on the edge of the city every year. "You see the very same patterns everywhere in arid Sonora," he says. The national telephone company lost so many poles to buffelgrass fires that it now sheathes its Sonoran poles in sleeves of sheet metal.

Today, the best fire protection in Sonora comes on four legs. "If somehow cattle were to disappear, we'd have a huge problem," says Burquez. Though cattle inspired the planting of buffelgrass, their appetites also contain it and keep Sonoran fires generally small, low to the ground, and easily extinguishable. Without heavy pruning by cows, says Burquez, buffelgrass would grow even denser and thicker, able to stoke still hotter, higher, and more destructive fires. Just like in Tucson. But we'll get to that in a minute.

In the early 1990s, from about 2,000 feet in the air, Sandra Lanham spotted the difference in the desert. Lanham was piloting her 1956 single-engine Cessna between her home in Tucson and northern Mexico, where she helps researchers survey endangered pronghorn, blue whales, and hundreds of other species. As she floated over the arid valleys of northern Sonora, she sighted something less charismatic, and more disturbing.

"I started to notice that the desert, in many, many places, was being bulldozed and planted," she says. "I knew something huge was going on, but I couldn't really see what was happening."

The bush pilot borrowed a video camera, which she handed to her passengers. "I literally didn't know how to turn it on," she says. She eventually collected footage not only from the air but also on the ground, showing bulldozers toppling columnar cacti like so many giant Gumby dolls.

Through flight after flight, Lanham saw that vast reaches of pale desert soil had been cleared and covered with buffelgrass. Once the native plants were felled and replaced, Lanham learned, they had little chance of recovery.

Working with a $500 budget, Lanham and her colleagues pieced their footage of the expanding pastures into an unvarnished 10-minute video. (A few years later, Lanham's work as an environmental pilot would win her a MacArthur Fellowship, but these were leaner times.) They made 125 copies and distributed them to potential converts in southern Arizona.

To many north of the border, the video was a shocking glimpse of an altered future. "I don't think anyone here recognized the scale of it," says Lanham. "I don't think they could have, unless they were flying in an airplane."

Buffelgrass seeds have long strayed across the international border, carried in tires, trade goods, or the clothing of illegal border crossers. But as in Texas and Mexico, buffelgrass was also invited into Arizona. From the 1940s until the 1980s, the federal Soil Conservation Service and other agencies planted the grass in test plots, where its unimpressive performance prevented more widespread introduction. Stuart Bengson, the former head of reclamation for ASARCO Inc., says the mining company used buffelgrass for erosion control at its copper mine south of Tucson until at least the late 1980s.

Buffelgrass did edge out of these original plantings and into native desert, but for years, the species was only a whisper in the desert ecosystem. Many biologists assumed it had little strength in the colder climes of southern Arizona.

"We thought that maybe (southern Arizona) wouldn't be like Sonora," says ecologist Van Devender, an expert on Sonoran Desert flora. In 1992, when he was working on a plant guide for the Tucson Mountains, "we had buffelgrass in there, but I remember writing that it wasn't very common, that it was just along roadsides," he says. "Well, right after that, it took off."

Sue Rutman, a botanist at Organ Pipe Cactus National Monument southwest of Tucson, says much of the blame for the buffelgrass boom lies with the climate. "Buffelgrass used to behave more like an annual grass in Arizona, because it would freeze in the winter," she says. "But with the warming trend we've had since the 1980s, there are fewer freezes, so buffelgrass has become this big bunchgrass that reproduces a lot more." The bigger the plant, the less vulnerable it is to freezing, and the more powerful its cycle of increased growth and increased spread.