Invasive Buffelgrass Threatens Giant Saguaros

TUCSON, Arizona, April 25, 2002 (ENS) - The saguaro cactus, a symbol of the American West, may be losing a battle against invasive plants. Exotic grasses like buffelgrass, an African native, fuel frequent wildfires that kill the centuries old giant cacti and many other native desert plants and animals, and disrupt the fragile ecosystem of this arid region.

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Saguaro cacti could disappear from their namesake national park in Arizona. (Photo courtesy National Park Service)
The distinctive profile of the giant saguaro cactus, with its upraised arms and ramrod straight central trunk, is a Western icon. Standing head and shoulders above most native desert trees, the saguaro dominates the skyline and provides a dramatic perch and home for Harris hawks, Gila woodpeckers, elf owls and other native species.

That image could fade into the sunset, if steps are not taken to control the invasion of fire loving exotic plants, warn researchers at the U.S. Geological Survey (USGS).

One of the worst threats to the complex ecosystem dominated by the saguaro is buffelgrass. Introduced from Africa into south Texas in the 1940s and throughout Sonora, Mexico since the 1960s, buffelgrass has now begun to spread rapidly in southern Arizona, invading even remote backcountry areas of national parks and wildlife refuges.

"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.

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Adult saguaro cacti growing in a moderately dense buffelgrass infestation at the Javelina Picnic Ground, Saguaro National Park, Pima County, Arizona. This photo was taken in October 1998. (Three photos by Todd Esque, courtesy USGS)
In 1994, Schwalbe and Todd Esque, a USGS ecologist in Las Vegas, Nevada, 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."

USGS scientists are now exploring new, cost effective techniques for removing buffelgrass and restoring the desert's native vegetation.

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Buffelgrass spreading from cultivated buffelgrass pastures into rocky hillsides and native desert scrub, 30 miles north of Hermosillo, Sonora, Mexico. The photo was taken in January 1997.
"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 Esque, who is lead the USGS study along 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."

Buffelgrass, which is grown as livestock forage in parts of Texas and in Mexico, has escaped rangelands and invaded the native desert ecosystems of Arizona, Esque said. 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 along Arizona's border with Sonora, Mexico.

In some places, buffelgrass has become established along roadsides, particularly where runoff rainwater collects.

Working 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. The team hopes 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."

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Buffelgrass plants surrounding a young saguaro cactus in Saguaro National Park in April 1998.
At study plots with buffelgrass infestation, the researchers will evaluate chemical removal methods and compare those with manual removal of plants at another study plot.

By studying the effectiveness of different methods for removing the largest, most continuous stands of buffelgrass in southern Arizona, the researchers also hope to identify techniques for smaller scale eradication efforts for roadsides, and for the smaller infestations that are sure to recur after most of the plants are removed from a site.

"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. Rutman has led efforts to manually remove buffelgrass on park lands.

"Pulling plants by hand is labor intensive," Rutman said. "For large areas, we need a more cost effective means to address this problem."

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.

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Centuries old saguaro cacti can grow 50 feet tall and support dozens of branching arms. (Photo courtesy Joe Pleggenkuhle)
Rutman noted that buffelgrass can reach four 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 exotic species. Estimated losses to the American economy due to exotics are as high as $123 billion each year.