From the Arizona Star via Yahoo Daily News, by Tom Beal
The Sonoran Desert is burning.
It's not supposed to happen, and it could signal the transformation of our saguaro-dominated landscape into something that more closely resembles an African savannah as native plants that have developed no defense to fire die off.
You can blame an invasion of nonnative plants for many of this year's low-elevation blazes.
The 55,000-acre Goldwater Fire between Ajo and Gila Bend burned through a landscape that hadn't known fire in 10,000 years, said plant researcher Julio Betancourt. He blames waist-high fields of a Mediterranean weed known as Sahara mustard that moved in to dwarf the creosote bushes.
The Cave Creek Fire is burning now through higher elevation grasslands where fire is common. But the two fires that combined to give it a roaring start began lower, in the northernmost range of the Sonoran Desert, where the desert plants were linked to each other by fields of another Mediterranean invader called red brome.
The Sonoran Desert never saw fire below 3,000 feet until the 1970s, when the invading weeds began filling in between native plants that had kept their distance for centuries and competed for sparse rainfall, said Mark Dimmitt, director of natural history at the Arizona Sonora Desert Museum.
Over centuries, desert plants learned to weather heat and drought, but fire was not frequent or wide-ranging enough to force adaptation. Unlike plants that evolved with fire, they did not develop thick bark or cork layers to protect delicate tissue, and are easily killed.
The foothills palo verde, whose chlorophyll-filled bark is thinner than a succulent leaf, is particularly vulnerable. If fire girdles its thin bark, it's a goner. Its mortality rate in a good burn is 100 percent.
The saguaro, whose conductive tissue lies just beneath its waxy cuticle and whose dried needles easily flame up to scorch its tender green flesh, is also quite vulnerable, Dimmitt said.
Scientists have kept an inventory of native and nonnative species since the Desert Laboratory's establishment in 1903. The first count, in 1906, listed three nonnatives. In 2005, the number was 44.
The trick is identifying the ones that have the most potential for harm, said Betancourt, a senior scientist with the U.S. Geological Survey who works at the Desert Laboratory on Tumamoc Hill in Tucson.
In the Tucson area, the threat comes from buffelgrass, a North African species brought to the region as forage and now a principal fuel in roadside brush fires.
About 30 percent of Tumamoc Hill's 860 acres are thick with buffelgrass, said Travis Bean, senior research specialist at the Desert Laboratory. Nearby Sentinel Peak is infested to an equal or greater extent, he said.
And it is climbing the slopes of the Tucson Mountains and the Santa Catalinas.
Buffelgrass has 10 times the mass needed to carry fire in the desert, Bean said. He documented it as the fuel in five major brush fires in the Tucson area since May 5, including a May 30 fire that burned along two miles of Interstate 10 near the Houghton Road exit, he said.
Dimmitt bristles when fires are identified simply as burning through brush or grass because it leads people to believe they are a natural occurrence after a wet winter produces abundant wildflowers and other native vegetation.
Native plants, even when a wet year brings them out in profusion, lack the mass of the invaders and rarely burn, he said. "They disintegrate when they dry out."
Red brome, inadvertently introduced to the United States as a seed contaminant, was the most prolific of several invasive species that caused the Cave Creek Fire's rapid development, said Norm Ambos, forest soil scientist for the Tonto National Forest. It was also the main fuel in two other fires that burned near Roosevelt Lake this year. It has been carrying fires in the Tonto since the early '90s, he said.
Only 10 percent of the Cave Creek fire burned through the Sonoran Desert zone of giant saguaros and palo verdes, but much of that landscape may not recover, he said.
"I've seen areas where it's going to end up killing 90 percent of the saguaros," Ambos said.
The fire also burned through areas that had fire in them just last year, Ambos said. The red brome had come back thick enough to burn again.
Successive fires are the scary part of the scenario, said the Desert Museum's Dimmitt. Fire kills mature native plants and successive ones keep juveniles from returning. The nonnatives quickly fill in. "These fires in the desert really convert it to a monocultural wasteland," Dimmitt said.
Betancourt, who joined with other scientists recently to have buffelgrass listed by the state as a "noxious weed," said it's not too late to do something about the invasion. Programs at the Desert Laboratory and in nearby Saguaro National Park are studying effective eradication methods.
But Betancourt says the battle against buffelgrass and other nonnatives needs a full-fledged push - a "state Department of Invasive Species Control."
People need to be made aware of the threat posed by nonnative species and scientists need to identify the biggest threats sooner, said John Hall, Sonoran Desert program director for the Nature Conservancy, which has its own programs for invasive-species control.
This year could be an educational one as fires burn where they're not supposed to burn, he said. It's important to not dismiss them as an obvious consequence of a wet winter.
"The solution is not blaming the rain," Hall said.
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