For years, the scientists watched it spread. On Tumamoc Hill, a research preserve on the western edge of Tucson, botanists noted the first sprigs in 1968. The tousle-headed pasture grass, an import from Africa and Asia, gradually filled the open spaces between native plants. Below the spiky, succulent latticework of the Sonoran Desert - a landscape unaccustomed to fire - buffelgrass formed a thick carpet. A flammable carpet. The scientists watched, as they had been trained to do. But they also started to worry.
One day in 2005, U.S. Geological Survey researcher Julio Betancourt drove the winding, narrow road through the research preserve. Betancourt - an expert on the ancient climates of the Southwest - has made Tumamoc Hill his professional home for 26 years, and both the drive and the surrounding landscape were deeply familiar.
He paused at a hairpin turn and looked out at a steep, rocky hillside dotted with saguaro cacti, palo verde trees, and the rest of the botanical cast of the Sonoran Desert. Once again, he noticed the golden thickets of dry grass. But this time, Betancourt realized that buffelgrass was poised to win, no matter what.
"I realized I was looking at the very last saguaros on that landscape," he remembers, gesturing at the hillside. "We have saguaros embedded in an Africanized grassland. That's not normal." Whether or not a grass-fueled wildfire sweeps through the preserve, he points out, there's no longer enough space for the next generation of saguaros to begin their 150-year lifespans. "I realized that this was a microcosm of what was happening throughout the Sonoran Desert," he says. "This is one of the most impressive ecosystem conversions happening in North America."
Betancourt wasn't the first to notice the inexorable transformation of the Sonoran Desert, and the threat to the city within it. But it was his conviction - and his break with scientific convention - that would give Tucson a fighting chance. "Buffelgrass is the worst environmental crisis facing Tucson," he says flatly. "It's even worse than development."
Some weeds arrive in our lives by accident. Buffelgrass came by invitation, riding on the pioneer dream of turning desert into pasture. Beginning in the late 1800s, ambitious agronomists carried buffelgrass from South Asia, Kenya and South Africa (buffel is Afrikaans for buffalo) to the Southern United States. It seemed the perfect grass for the Southwest: resistant to both drought and heat, tenacious enough to control erosion, bulky and nutritious enough for cattle feed. By the 1950s, ranchers in south Texas were planting it with abandon.
"Some ranchers down there swear by it," says Byron Burson, a plant geneticist with the federal Agricultural Research Service in College Station, Texas. "When they've had severe droughts, its tolerance has kept some of them in business."
In the 1960s and early 1970s, during the Green Revolution, Mexico was eager to enrich its portion of the Sonoran Desert, and buffelgrass seemed the ideal tool. With the encouragement of Mexican scientists, some trained in the United States, the Mexican federal government subsidized a buffelgrass planting spree. Ranchers in northwestern Mexico, who found that the wonder grass made their property values blossom, were glad to help it along.
"The bulldozing just took off like crazy," says David Yetman, a researcher at the University of Arizona with long experience in the Mexican state of Sonora. "They'd put a chain between two bulldozers and knock over all the trees - a lot of mesquite and a lot of ironwood, some of the finest trees in the world - and they would bulldoze them all into long rows."
The rows, called chorizos after the spicy Mexican sausage, were themselves a source of income: Ranchers often sold the wood to carboneros, who sold it to Mexican and U.S. steakhouses as premium mesquite charcoal. The charcoal trade funded more bulldozing, more buffelgrass planting, and eventually more steak for the steakhouses.
"I enjoy my carne asada," says Sonoran ecologist Alberto Burquez, "but I call it reprocessed buffelgrass."
The Sonoran bulldozing, which peaked in the 1980s and 1990s, has slowed now, more from lack of available land than lack of enthusiasm. Burquez, a professor at the Sonoran campus of the National Autonomous University of Mexico, estimates that direct buffelgrass plantings now cover 2.5 million acres of Sonora, an area larger than Yellowstone. "Most of the desert has been invaded by buffelgrass, and it's increasing," he says. Yet state and federal subsidy programs continue, good intentions unbowed. "In the past, they were described as programs for planting buffelgrass, creating grasslands, conquering the desert," Burquez says. "Today, they're called restoration or range improvement. They have these contorted names ... but it's like calling impotence erectile dysfunction. They still have incentives for planting buffelgrass."
Buffelgrass, which has few enemies, soon moved from these plantings into less disturbed desert. Its dense growth crowded out native seedlings and began to beat even large shrubs to water. When Arizona-Sonora Desert Museum ecologists Tom Van Devender and Mark Dimmitt studied buffelgrass stands in Arizona and Sonora, they found that both native annuals and short-lived perennials had virtually disappeared.
Buffelgrass also carries fire, an element all but unknown in the Sonoran Desert. Unlike native plants, it burns easily and recovers quickly, creating yet more fuel for more and bigger fires - and burying slow-growing cacti and shrubs beneath a spreading buffelgrass savannah.
To many desert ecologists, this cycle is all too familiar. Red brome, an invasive Mediterranean grass, also feeds a quickening cycle of fire and invasion in many parts of the Sonoran and Mojave Deserts. In the Mojave - another desert once believed to be all but fireproof - U.S. Geological Survey ecologist Todd Esque and his colleagues found that in burned areas, growth of red brome and other exotics consistently, and often dramatically, outpaced that of native annuals. Cheatgrass, another Mediterranean native, is ubiquitous in the Great Basin, where its ability to dominate burned areas frustrates efforts to restore native sagebrush.