Support Provided ByLearn More
Planet EarthPlanet Earth

Invasive, flammable grasses now blanket much of the United States

New research quantifies the fire risks of eight species of invasive grass.

ByKatherine J. WuNOVA NextNOVA Next

The 2015 Soda Fire, which burned nearly 280,000 acres in southwest Idaho and southeast Oregon, was fueled in part by invasive species like cheatgrass. Image Credit: Hugo Sindelar, BLMIdaho, flickr

In the conversation around wildfire risk, a couple factors tend to steal the spotlight.

Climate change is a big one, as droughts, rising temperatures, and powerful winds drive the intensity and spread of fires worldwide. Then there’s human disturbance: People and their homes continue to push into the wilderness, adding flames and fuels to places they’re not typically found.

Those two players can’t be ignored, says Bethany Bradley, a biogeographer and spatial ecologist at the University of Massachusetts, Amherst. But there’s more to the story than just how the burning begins, she says. It also matters what’s burning—which plants are around to catch a stray spark.

Some of the most worrisome culprits are nonnative grasses invading ecosystems across the country. Given the chance, these hardy, fast-growing plants can easily outcompete their native kin, blanketing entire landscapes with their thin, ignitable leaves. From there, a single trigger—a cigarette butt, a spark from a railroad track, a rogue firework—can lead to disaster.

Invasive grass isn’t your average kindling, Bradley says. There’s always more where it came from. It’s kindling that lives and breathes. It’s kindling that reproduces and spreads.

Now, Bradley and her team, led by ecologist Emily Fusco, have intertwined decades of satellite and ground-based records to create a nationwide map of where eight of these incendiary species have taken hold. Their findings, published today in the journal PNAS, suggest that some of these grasses can more than triple a region’s likelihood of succumbing to wildfire, imperiling entire swaths of land from the shrublands of the West to the pine savannas of the southeast.

“This paper sheds light on an often-overlooked issue in the United States,” says Marina LaForgia, a grassland ecologist at the University of California, Davis who wasn’t involved in the study. “So much of the focus, especially in California, is on forest fires, and for good reason. But during fire season, it’s not just the forests that burn.”

The link between invasive grasses and wildfire isn’t new. Researchers have been aware of the connection since at least the early 1990s, when Californian ecologists Carla D’Antonio and Peter Vitousek raised the alarm in what’s considered one of the field’s landmark studies.

With their speedy growth cycles, invasive grasses outpace other plants, transforming landscapes into conduits for fire during dry summer months. They also tend to be remarkably resistant to disturbances like drought—and even when felled by fire, they’re some of the first species to spring back.


Cheatgrass, an invasive grass species, covers 100 million acres of the western United States. In the summer, it dries out into a continuous, easily ignitable carpet. Image Credit: .deeneg, flickr

“These grasses perpetuate this feedback loop between more fire, more grass, more fire, more grass,” says Lisa Ellsworth, a fire ecologist at Oregon State University who wasn’t involved in the study. “We see this across ecosystems, all over the world.”

Most infamous, perhaps, are the fire-starting feats of cheatgrass (Bromus tectorum)—a droopy, buff-colored species that, since its accidental introduction in the 1800s, has infiltrated some 100 million acres of the country’s western flank, throttling local plants out of existence. Last year, more than 400,000 acres of cheatgrass provided ample fodder for Nevada’s Martin Fire, birthing a plume of smoke so big it was visible from space.

It’s long been clear that where invasive grass goes, flames follow. But putting numbers to fire risk has been tough—in part because the data on both grass distribution and fire frequency has, until recently, been pretty spotty.

“In the early ‘90s, we didn’t have access to this kind of big-scale data...we had to dig and dig and dig,” says D’Antonio, who wasn’t involved in the new study. That’s changed in the past few years, she says, with the advent of widely-used digital repositories that are being constantly updated by researchers around the world. “Finally,” she says, “we’re seeing that data being used to address these questions.”


A controlled buffelgrass fire in Avra Valley, Arizona in May 2008. Image Credit: Daina Dajevskis, flickr

At least 176 grass species have been tenuously tied to wildfires in the United States. Fusco and her colleagues combed through records on all of them, scouring studies and databases for numbers that could help them pinpoint places the plants had infiltrated. In the end, there was only enough to map out 12 species. The team then overlaid information from 16 years of recent fire records, comparing the number, size, and likelihood of burns occurring in both invaded and uninvaded places.

Of the dozen grassy interlopers analyzed, eight were associated with increased fire risk in the areas they inhabited. Though cheatgrass was among them, it wasn’t the most problematic firestarter of the bunch. That title went instead to common Mediterranean grass (Schismus barbatus), a grass that sprouts in thin, threadlike clumps across the American southwest—and more than tripled the chance a region would burn. Similarly potent was buffelgrass (Cenchrus ciliaris), a particularly stubborn plant that’s plagued Arizona since the 1930s. Where its squat, recalcitrant tufts sprang up, the team found, the land was more than twice as likely to ignite.

Support Provided ByLearn More

But it’s not only the West at risk. The team’s map of invaded areas covers land from coast to coast, with patches of plants like Japanese stiltgrass (Microstegium vimineum) and Chinese silvergrass (Miscanthus sinensis) extending as far north as Pennsylvania and New Jersey. “It’s just not where you expect to see fire,” D’Antonio says. “But it’s worth appreciating...this isn’t just a problem for the deserts.”

The picture painted isn’t exactly a cheery one, Bradley says. And the eight species highlighted in the paper are probably just a drop in the bucket—the ones the team could scrounge up enough data for. “My guess is there’s a heck of a lot more fire-prone grasses that we don’t have the numbers to back up yet,” she says.

Invasive grasses come in all shapes, sizes, and spark-promoting potential. But they have at least one crucial thing in common: they’re still being spread by humans, often as they’re pushing into wild environments. Though many introductions have been accidental, all it takes is a couple seeds stuck to the bottom of a shoe or embedded in a bale of hay to breach a barrier, Bradley says.

pnas.201908253 copy.jpg

Study areas for 12 invasive grass species. Colored polygons show where the researchers analyzed data for invasive grasses; yellow regions denote places with demonstrated fire impact. Image Credit: Fusco et al., PNAS, 2019

Complicating matters is the increasing influence of climate change. Droughts, heat waves, and other extreme weather events wipe out fragile local plants, clearing the way for invasive grasses to grow. Wetter years favor foreigners, too, allowing these species to bloom out of control. In a single season, an infiltrated ecosystem can become completely unrecognizable. As the world continues to shift around us, D’Antonio says, there are probably more grasslands—and more fires—in our future.

Once they take root, these pest-like plants are tough to exterminate. Grass-specific herbicides, carefully timed grazing by livestock, and firebreaks—natural or human-made gaps in flammable vegetation—can all help, Ellsworth says. But for the most part, where landscapes have already been overrun, “it’s not looking great,” she says.

Where researchers can still make a big difference are intact environments, says Lauren Porensky, a restoration ecologist at the United States Department of Agriculture who wasn’t involved in the study. A combination of monitoring and early interventions can purge habitats of invaders before they have a chance to settle in. And even before their arrival, efforts can be made to boost the resilience of native plants. “We need to get more proactive on invasion,” she says, “and not just be reactive.”

All this will require raising awareness, and plenty of collaboration, Fusco says. But putting the focus on prevention means there’s hope, she says. Just because the kindling’s there doesn’t mean it has to burn.

Receive emails about upcoming NOVA programs and related content, as well as featured reporting about current events through a science lens.

Funding for NOVA Next is provided by the Eleanor and Howard Morgan Family Foundation.

Major funding for NOVA is provided by the David H. Koch Fund for Science, the Corporation for Public Broadcasting, and PBS viewers. Additional funding is provided by the NOVA Science Trust.