May 20, 2002 Picture a sunset in which a “forest” of that Sonoran Desert icon the saguaro cactus is silhouetted against the skyline. Now picture that sunset minus the saguaros and you will have an idea why researchers and resource managers across southern Arizona fear the take-over of the desert by invasive nonnative grasses.
Introduced from Africa into south Texas in the 1940’s and extensively in Sonora, Mexico since the 1960’s, buffelgrass has recently begun to spread vigorously in southern Arizona, invading even remote backcountry areas of national parks and wildlife refuges. Scientists at the U.S. Geological Survey are embarking on new research to provide resource managers the most cost-effective techniques for removing buffelgrass and restoring the desert’s native vegetation.
“Of the perennial exotic plants identified by researchers, buffelgrass appears to be spreading the most rapidly and is the most threatening to the Sonoran Desert ecosystem,” said Dr. Cecil Schwalbe, a USGS research ecologist with the Western Ecological Research Center in Tucson, Ariz.
Dr. John Hall, the Sonoran Desert Program Manager for The Nature Conservancy of Arizona, echoed Schwalbe’s concern. Buffelgrass, Hall said, is an example of an invasive species that can encroach into the Sonoran Desert and pose a significant threat to the region's biodiversity.
“Because buffelgrass resprouts vigorously after fire, it is capable of causing more frequent and larger wildfires, decreasing water infiltration to the soil and changing the way essential plant nutrients cycle in the desert,” said Todd Esque, a USGS ecologist in Las Vegas, Nev., and co-lead scientist in the study with Schwalbe. “Taken together, these changes could potentially convert Sonoran Desert shrublands to exotic fire-driven grasslands, completely altering the kinds of plants in the Sonoran Desert and even eliminating saguaros in some areas.”
Working collaboratively with colleagues in federal and state agencies, universities and private organizations in the United States and Mexico, the scientists will begin their study this spring at Saguaro National Park near Tucson, Ariz., to determine the most effective and efficient removal methods of buffelgrass and the response by native vegetation and wildlife, not only to the encroachment of the plant but also to its removal.
“In addition to its effects on native plants, buffelgrass may alter animal community structure,” said Schwalbe. “For instance, buffelgrass may decrease the open space required for some animals, like lizards, to escape predators. This, in turn, could lead to larger shifts in wildlife population abundance and diversity.”
At study plots with buffelgrass infestation, the researchers will assess chemical removal methods and compare those with manual removal of plants in another study. By studying the effectiveness of alternative methods for eradicating the largest, most continuous stands of buffelgrass in southern Arizona, the researchers also hope to identify methods for smaller-scale eradication efforts for roadsides and for smaller infestations that will reoccur periodically.
“Once results of this study are known, land managers throughout the Arizona-Sonora borderlands will be able to implement buffelgrass control programs,” said Sue Rutman, a resource specialist at Organ Pipe Cactus National Monument, who has led efforts to manually remove buffelgrass on park lands. “Pulling plants by hand is labor-intensive. For large areas, we need a more cost-effective means to address this problem.”
Buffelgrass, which is grown as livestock forage in parts of Texas and in Mexico, has escaped rangelands and invaded native desert ecosystems in Arizona, said Esque. At Saguaro National Park, for example, buffelgrass occurs up to 4,000 feet in elevation; it is also widespread at Organ Pipe Cactus National Monument.
In places, buffelgrass has also become established along roadsides, especially where runoff rainwater collects. Roadside infestations are common, for example, along Highway 8 between Gila Bend and the Colorado River, Highway 17 north of Phoenix and along Ajo Road at the south end of the Tucson Mountains west of Tucson.
In 1994, Schwalbe and Esque first became aware of the buffelgrass infestation in Saguaro National Park while studying the effects of an accidental fire that engulfed 1,150 acres in the park, including 340 acres of desert scrub habitat. Investigating the impacts of the fire and the role of red brome, an exotic annual grass, the researchers estimated that 11 percent of a desert tortoise population were killed by the fire and more than 20 percent of saguaros sampled died within five years following the fire.
“Losses such as these severely affect populations of long-lived species like saguaros and desert tortoises, Schwalbe said. “It can take decades for populations to recover from a severe fire. In fact, saguaros may disappear from areas having fires as frequently as every 20 years. Even low-intensity fires are causing long-lasting and adverse changes in desert plant communities.”
Mark Holden, a Saguaro National Park biologist, agreed. Holden said to maintain the natural structure and functioning of Sonoran Desert ecosystems, managers must know how best to control invasive grasses and ensure that invaded areas can recover after buffelgrass removal.
Like red brome, buffelgrass is capable of carrying fires across desert scrub during the arid months of June and July. Both grasses often colonize continuous expanses of desert, closing the open spaces that normally separate native desert plants and protect them from fire.
Rutman noted that buffelgrass can reach 4 feet tall in frost-free areas and that dead plant parts may persist as potential fire fuel for years.
What the researchers learn in this buffelgrass study may shed light on possible controls for other invasive species, such as fountaingrass, another perennial plant widely distributed in southern Arizona, said Schwalbe.
The U.S. Department of the Interior estimates that 4,600 acres of public lands nationwide per day are invaded by invasive species, and estimated losses to the American economy due to exotics are as high as $123 billion annually.
The USGS serves the nation by providing reliable scientific information to: describe and understand the Earth; minimize loss of life and property from natural disasters; manage water, biological, energy, and mineral resources; and enhance and protect our quality of life.
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