Invasive grass a wildfire threat to Sonoran Desert




National Parks Service crews spray herbicide on buffelgrass in Saguaro National Park. Photo by National Parks Service.

Wildfires throughout Arizona’s desert ecosystems are caused in part by invasive species of grass, and experts say the problem is only getting worse as the grass spreads. 

Researchers at the University of Massachusetts Amherst last week published a study in the Proceedings of the National Academy of Sciences examining how invasive species of grass affect where and how often wildfires occur in the U.S. The study was national in scope, but two findings particularly stood out for Arizona. 

First, the presence of a particular species of nonnative grass, called buffelgrass, significantly increased the likelihood a fire would happen.

Second, buffelgrass is linked to an increase in fire frequency. In other words, where there’s buffelgrass, fires will happen more often. The researchers found that fire frequency more than doubled in areas where there was buffelgrass. 

The invasive species was introduced to the U.S. from its native Africa in the 1930s for cattle forage and erosion control. The grass evolved over millions of years in dry savannah ecosystems, where water is scarce and fire is frequent.

“One of the interesting things about these grasses is that they like to burn. If you do get a fire in one of these systems, it will often kill all the native vegetation, but the invasive grass comes back really strong, and it will spread. So, you end up in this cycle where you get more fire and more grass and continue to burn out your native vegetation,” said Emily Fusco, the lead study author and a PhD candidate at Amherst.

The grass is a nuisance in Arizona, where the state’s Department of Agriculture lists buffelgrass as a noxious weed under the sub-category “regulated pest,” which means it “may be controlled to prevent further infestation or contamination.”

Kim Franklin, a researcher at the Arizona-Sonora Desert Museum in Tucson, said the effect buffelgrass and the fires have on Sonoran desert ecosystems is profound. Franklin said the invasive grass is transforming parts of the Sonoran Desert into grasslands. 

“Fire causes the desert to turn into a grassland rapidly, but even without fire, buffelgrass is causing the desert to turn into grassland anyways. It just takes longer. … There shouldn’t be fire in the desert,” Franklin said. 

The Arizona-Sonora Desert Museum thinks the threat posed by buffelgrass and invasive grasses generally is so profound the Museum declared that it “rivals climate change and water scarcity as our region’s most pressing environmental issue.”

The Woodbury Fire, which burned over 120,000 acres and 88% of the land in Tonto National Monument, was fueled in part by invasive grasses, particularly the Red Brome and Mediterranean species.

Mary Lata, a fire ecologist for Tonto National Forest who authored the fire ecology report on Woodbury, said that the U.S. Forest Service has not tried to give an accurate estimate of how much of Tonto’s ground area is infected with invasive grass. But she said she is confident that the majority of the Sonoran desert region within Tonto is infected. 

The Woodbury Fire was particularly bad, she said, because of the wet season that preceded it, which led to near-total ground cover of invasive grass, seeds of which from the previous season flourished in the moist soil. 

“In years like this one, it’s changed hugely. It’s a very different system in a year like this when the grasses grow. These invasive grasses come in and provide a contiguous fuel load so that fire can go pretty far,” Lata said.

She said she is hopeful about ways to manage the changes invasive grasses have wrought on the desert, but she is not optimistic about eradicating them. 

“The genie’s out of the bottle. They’re here to say. Anybody who manages lands — we’re going to have to decide what purpose we’re going to manage for, because we’ve got fire all over the Sonoran Desert. Even then, I don’t know if we can stop it,” Lata said. 

Meanwhile, buffelgrass itself has already made its debut as culprit for a wildfire in the Sonoran Desert. 

This summer saw the first documented fire fueled primarily by buffelgrass when 25 acres burned in the Santa Catalina Mountains north of Tucson. Though tiny compared to the Woodbury Fire, the blaze was significant in that researchers who’ve monitored the invasive grass have been predicting a fire fueled primarily by buffelgrass would happen. 

Fusco said the small acreage of the buffelgrass fire in Tucson is no cause for complacency. She mentioned the Martin Fire in Nevada; at its largest, the fire was more than 400,000 acres, 57 miles long and the largest fire in the U.S. during its burn. 

“Grass fires generally can be really big,” Fusco said. 

As for how to deal with buffelgrass, the only treatments currently used are pulling and herbicide. The former, Franklin said, is cost-prohibitive for the acreage of buffelgrass currently on public land. Meanwhile, the latter has low public support as the only herbicide that can be used to kill buffelgrass at scale is glyphosate, the active ingredient in Roundup. 

The Arizona-Sonora Desert Museum says glyphosate is the only herbicide that can kill buffelgrass while minimizing the effect on native vegetation and wildlife. The Forest Service considered another herbicide, imazapyr, as late as 2014, but a report drafted by the Service claims that chemical can get into root systems and damage or kill native plants. 

No matter how buffelgrass is reduced, Franklin said, the need to do so is urgent. She said that even though the problem continues to grow, it is one of many ecological catastrophes facing Arizona that can still be dealt with at a large scale. 

“We have to accept that these ecosystems are going to change. But there’s some invasive species that are so detrimental that the novel ecosystems they create really have no value to us or to biodiversity. It’s worth our effort to try to fight them if there’s a chance we can succeed,” Franklin said.

“Buffelgrass is one of those species.”