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Flammable invasive grasses are increasing risk of devastating wildfires

A version of this story appeared in Science, Vol 377, Issue 6606.Download PDF

For decades, eastern Oregon’s scablands—rocky patches of open terrain—were a refuge for people fighting wildfires in the surrounding forests. The thin soil and sparse vegetation offered little fuel for the flames, creating an oasis from which firefighters could operate and a barrier that could help halt a fire’s spread.

That all changed in 2015. After lightning sparked a fire near a steep-walled canyon, the blaze unexpectedly raced across scablands so quickly that firefighters struggled to catch up. In the end, the Corner Creek Fire scorched more than 11,000 hectares. And Jeff Priest, who has spent more than 2 decades fighting fires in Oregon for the U.S. Forest Service (USFS), realized he had a new problem on his hands: the arrival of an invasive, shin-high grass known as Ventenata dubia. The plant created shaggy golden carpets of dry foliage, transforming once fire-resistant scablands into flame-friendly corridors.

“We knew it was coming,” Priest says about the annual commonly called wiregrass, which is native to countries surrounding the Mediterranean. “But all of a sudden, it was there.”

Ventenata’s spread into the forests of the northwestern United States is just the latest chapter in a phenomenon reshaping ecosystems—and wildfire—around the globe. In northern Australia, invasive gamba grass from Africa fuels intense blazes that rip through eucalyptus groves. In Brazil, molasses grass from Africa turns vast swaths of the savanna known as the Cerrado into fire-prone grassland. In the western United States, two Old World grasses are creating ecological mayhem: Buffelgrass feeds fires in the Sonoran Desert that torch iconic saguaro cacti, while blaze-tolerant cheatgrass crowds out native sagebrush in the high desert known as the Great Basin.

Even as catastrophic wildfires that roar through towering treetops capture the public’s attention, ecologists have been paying increasing attention to this less conspicuous trend: how seemingly modest nonnative grasses are allying with fire to eat away at dry forest and savanna ecosystems.

These invasive grasses can hijack fire to create a self-reinforcing cycle, explains Carla D’Antonio, an ecologist at the University of California, Santa Barbara, who has studied the phenomenon for more than 3 decades in Hawaii and California. Once established, the grasses help fuel blazes that kill and suppress less fire-tolerant native plants, opening up new territory for the invaders to colonize—catalyzing yet more fire. In a short time, land that was once shrubland, savanna, or dry forest is locked into being a grassland. “It’s that trigger of grass and fire that sets the system off in some undesirable direction,” D’Antonio says.

These grass invasions are now threatening native plants and the animals that rely on them, reshuffling nutrients in the soil and the ability of ecosystems to store planet-warming carbon, and disrupting efforts to use fire to benefit the native flora. And once the invaders take hold, ecologists say it’s tough to break their grip. In such places, the future promises to be hot, smoky, and full of grass.

Grasses and fire have been intertwined since before humans walked the planet. Millions of years ago in southwest Africa and Asia, a dramatic rise in wildfires went hand in hand with the emergence of vast grasslands, researchers say. There and elsewhere, changes in weather patterns—particularly the emergence of a dry season—helped grasses spread. Select species developed a new way to photosynthesize that gave them an advantage in hotter, drier environments.

Evidence suggests some of these grasses evolved to thrive with fire, says Allison Karp, a paleoecologist and postdoctoral researcher at Yale University. In ancient sediments pulled from the Bay of Bengal near India’s eastern coast, for example, she found carbon isotopes trapped in ancient plant waxes that indicate grasses became widespread on the subcontinent about 7 million years ago. During the same period, according to the sediments, molecules tied to wildfires increased 10-fold, and the chemical traces suggest grasses played an outsize role in fueling those blazes.

Today, some of the most problematic invasive grasses seem built to burn. They grow and dry quickly, creating abundant fuel each year. Certain species have leaves filled with oily tannins—chemicals that slow the decay of dead leaves, making it easier for them to ignite. One species, molasses grass, is coated with a residue that enables it to catch fire even while green.

Although fire can kill invasive grasses, they often bounce back quickly, enabling them to outcompete charred competitors, including native grasses that didn’t evolve with frequent, intense fires. The lack of woody trunks and branches means grass seedlings start to photosynthesize before trees or shrubs put out leaves. Some grasses resprout from rootlike stems that grow underground, insulated from the flames.

Invasion hot spots

Fire-friendly grasses have invaded new habitats around the world. Five species are considered among the most problematic grasses, threatening to transform entire ecosystems.

World map showing locations across North America, South America, and Australia where five grasses have invaded. All five grasses are native to either Eurasia or Africa. Details: Cheatgrass, Northwestern U.S.; Cogon grass, Southeastern U.S.; Gamba grass: Northern Australia; Molasses grass: Hawaii, Papua New Guinea, Northeastern Australia, and parts of Brazil and Central America; Buffelgrass: central Australia, Mexico, and Southwestern U.S.

Flammable invasive grasses are increasing risk of devastating wildfires

 

Cheatgrass

Taking over high-desert habitat that’s home to endangered sage grouse and sagebrush, a shrub sensitive to frequent, intense fires.

Flammable invasive grasses are increasing risk of devastating wildfires

 

Cogon grass

In the southeastern United States, this grass chokes out native plants and burns at high temperatures, stoking intense fires that could damage forests.

Flammable invasive grasses are increasing risk of devastating wildfires

 

Gamba grass

Planted to feed cattle, it creates dense walls of vegetation up to 4 meters tall. Some Australian states have banned plantings because of fire risks.

Flammable invasive grasses are increasing risk of devastating wildfires

 

Molasses grass

Taking over parts of Hawaii and Brazil’s tropical savanna, the Cerrado. Grows in dense mats, and is named after oily substance on leaves.

Flammable invasive grasses are increasing risk of devastating wildfires

 

Buffelgrass

Fuels fires threatening Sonoran desert habitat, including iconic saguaro cacti. Also spreading in central Australia.

Credits: (Graphic) N. Desai/Science; (Data) EDDMapS. 2022/UGA Center For Invasive Species and Ecosytem Health; EDDMapS. 2014/UGA Center For Invasive Species and Ecosytem Health; Melanie Brown/OSU/CBI; K.S. Kunth (1829), Revision des Graminees Part 10: 163; Department of Environment, Parks and Water Security; © 2022 CABI; V.M. Marshall et al., Journal of Arid Environments 78 (2012) 1–12; R. Van Klinken et al., Environmental Management 2011

Such adaptability has helped many grasses naturally expand their ranges. But in recent times, humans have accelerated that process by scattering grass seeds far from their native habitats, sometimes by accident and sometimes intentionally, to feed livestock, control erosion, and decorate gardens. “The [grass] invasions in the last 100 years or so are just a radical example of a speeded-up process that’s been happening over millennia,” says Dave Richardson, an ecologist and invasive plant expert at South Africa’s Stellenbosch University.

Southern Africa is a disproportionate source of the grasses that have invaded other parts of the world, Richardson has found. Grasses evolved there to take advantage of frequent disturbances, such as fire and grazing by herds of wildlife, making them tough competitors in new habitats. And once they gain a roothold outside Africa, fire often follows. In the United States, when scientists compared fire behavior in areas invaded by fire-prone grasses with nearby uninvaded areas, they found six different grasses were tied to as much as a 150% increase in fire frequency, according to a 2019 report in the Proceedings of the National Academy of Sciences.

In northern Australia, the arrival of gamba grass has provided a textbook example of this process. In the 1980s, the Australian government promoted planting the African grass as forage for cattle. But ecologists soon warned of the dangers it posed to the nation’s tropical savannas, a mix of sparse grasses and eucalyptus trees that cover one-quarter of the continent. The original ecosystem evolved with frequent, low-level fires, including ones set by Indigenous Aboriginal people to create open savannas that improved hunting and provided habitats for specific plants and animals.

But gamba grass formed dense walls of vegetation, reaching 4 meters tall, that transformed fire behavior. The grass burned four times more intensely than native vegetation during experimental burns, researchers found. The flames ran so high and hot that they had to abandon a common gauge of fire behavior—measuring the highest scorched leaves in a tree—because even the highest leaves in gamba-infested sites were singed.

In 2008, several Australian states reversed course, restricting the use of gamba grass. But in many places it was too late. By then the grass covered more than 15,000 square kilometers. Researchers fear it could ultimately spread through much of the country’s 2 million square kilometers of tropical savanna.

Oregon, meanwhile, shows how a single invasive grass species can alter both rangelands and forests. In 2015, reports of Ventenata-driven fires reached Becky Kerns, an ecologist at USFS’s Pacific Northwest Research Station. Kerns had given little thought to the plant, in part because other invasive grasses were already causing headaches in the region. Cheatgrass, for example, was spreading in parts of Oregon. But the harsh scablands had proved inhospitable to that invader. Ventenata, Kerns says, “was really a game changer.”

A year after the Corner Creek Fire, Kerns and a group of Oregon State University graduate students began to examine what wiregrass might mean for the region. Their findings were alarming. In scablands overtaken by Ventenata, they found oceans of grass that create fuel loads 50 times greater than in areas free of the species. Compared with cheatgrass, wiregrass can colonize cooler, higher elevation locations and take root in thinner soils.

In the process it appears to be crowding out native plants, such as fire-intolerant sagebrush, that support the local wildlife. And it threatens rare endemic plants found in rocky scablands, such as Spalding’s catchfly, a federally protected perennial with pale pink, trumpet-shaped flowers.

Unlike some invasive grasses, Ventenata doesn’t appear to need disturbances such as fire to spread in some parts of eastern Oregon. In study plots on native prairies, the grass advanced just as quickly over unburned land covered with native bunchgrass as it did through plots burned for a long-term experiment. In both cases, in 10 years Ventenata spread from approximately 10% of the test plots to nearly 60%.

Between two rows of pines, fires blaze in two small areas of tall brown grass, and a researcher takes notes on a clipboard. Behind them, two rows of ash piles from previous fires extend along the line of trees.
Experiments in plots of longleaf pine in Florida suggest a combination of drought and invasive cogon grass can amplify wildfires and kill the trees.Camila Guillen/UF/IFAS

In forests, by contrast, fires do appear to help spread the grass, Kerns says. An examination of a decade of fires in the region revealed that more severely burned areas were more likely to be invaded. One explanation for that pattern, researchers say, is that although Ventenata doesn’t fare well in dense shade, it can encroach on forest edges, where it fuels fires that clear parts of the overstory, letting in more sunlight. Computer simulations suggest that cycle could gradually shrink forests, Kerns says, like waves eroding a beach.

That scenario was a revelation, and not a welcome one. Historically, Kerns says, land managers in the western United States have thought of invasive grasses as a problem for wide-open rangelands. But modeling suggests wiregrass could creep into many forests in eastern Oregon, as well as those in dry, higher mountains as far south as Arizona and New Mexico. “Ventenata has really challenged a lot of our notions about invasive grasses in the West,” she says.

In many locations, scientists are just beginning to document the impact of invasive grasses. In Hawaii, however, D’Antonio has been closely observing the interlopers since the early 1990s. And her ringside seat has enabled her to see how the effects can echo through a landscape in complex and surprising ways.

In Hawaii’s Volcanoes National Park, for example, she has tracked how a handful of invasive grasses—many introduced by cattle ranchers—have altered environments once rarely touched by fire. Studying places that first burned in 1970, D’Antonio found that in forests dominated by the native ’Ōhi’a tree, relatively shade-tolerant beardgrass from South America took root first. But if the beardgrass later fuels a fire, she says, then a “second invader, molasses grass, pours in.” Native grasses simply can’t compete, she adds. Whereas invasive grasses produce large numbers of airborne seeds, for example, a native species called Kawelu, or love grass, produces just a few seeds that drop nearby. Compared with molasses grass, she says, “It’s a total wimp.”

Molasses grass can also alter soil nutrient cycles in ways that ultimately benefit invasive species, D’Antonio has found. As the grass leaves decomposed, for instance, they led to a bounty of soil nitrogen that helped the grass. After 16 years, however, nitrogen returned to preinvasion levels as the nutrient leached out of the soil. Rather than pave the way for a return of native plants, however, the leaching opened the door to an invasive tree known, appropriately, as the fire tree. That tree, in turn, again increased soil nitrogen levels, which revived the molasses grass. “The situation there gets more and more grim,” D’Antonio says.

These new grasses could also scramble how much carbon is stored in vegetation and soils. In Hawaii, D’Antonio found that in unburned areas where native ’Ōhi’a trees remained, the plants held twice the carbon of burned areas taken over by grass—even 25 years after a fire.

A worker wearing a cap and bandanna kneels and swings a pickaxe amid dense grass. A tall saguaro cactus nearby is partially obscured by the flare of the sun.
In Arizona’s Saguaro National Park, workers are trying to curb buffelgrass, which fuels blazes that threaten saguaro cacti.Norma Jean Gargasz/Alamy Stock Photo

Drought could heighten the impact of the grass-fire cycle, suggest studies by ecologist Luke Flory of the University of Florida. He has run field experiments that simulate how drought affects longleaf pine forests invaded by cogon grass from Asia, which can grow in dense, waist-high thickets topped with fluffy seed heads.

Flory’s team grew pines on small plots in Florida; many were infested with cogon grass whereas others only held native plants. Some of these miniforests were covered by canopies that limited rain to simulate drought.

After 6 years, trees in the dry plots averaged about 3 meters tall, nearly 1.5 meters shorter than those in wetter plots. The researchers then set fire to all of the plots. In those infested with cogon grass, the fires burned hotter and the flames rose higher than in plots without the grass. And in the dry, grassy plots, the combination of shorter trees and higher flames proved “highly problematic,” Flory says, with nearly half of the trees dying within 1 month of being burned. But only 10% of trees in the wetter sites died, regardless of whether cogon grass was present.

Flory is reluctant to sound the alarm too loudly, because there are still plenty of unknowns about how drought, fire, and cogon grass will interact in the wild. But the experimental results so far are worrying, he says. “None of it’s good. … It’s just how much evidence we have that it’s really bad.”

Such experiments have helped focus attention on a practical matter: What can land managers do about the tenacious invaders once they arrive?

Eradicating the grasses is often not possible once they have reached high densities, researchers say. But preventing them from completely transforming ecosystems might be an option in some places. In Volcanoes National Park, for example, park officials are trying to rehabilitate shrubby or forested habitats that have burned by replanting them with more fire-hardy native species such as māmane, which can grow to 15 meters tall. The māmane could restore lost habitats for birds and insects, while being less vulnerable to grass-fueled fires, says Sierra McDaniel, a botanist who leads the park’s natural resource program. “We are accepting these grasses are widespread,” she says, and instead focusing on “how do you live with them?”

In Oregon, USFS officials are weighing difficult choices in the fight against wiregrass. On one hand, they want to intentionally burn more forests, in order to clear out brush and saplings, and restore ecosystems that historically evolved with fire. But such controlled burns also risk opening new areas to grass invasions.

With that in mind, Kerns has obtained funding for an experiment that will use herbicides to kill Ventenata along forest edges before crews set controlled burns, to see whether that curbs later encroachment by the grass. In the scablands, Priest is trying a more aggressive approach: sending in heavy equipment such as bulldozers to help squelch fires, fearing that native plants will suffer if the open areas burn. But he worries the equipment could leave lasting marks on delicate ecosystems—and that the disturbance could ultimately benefit the grasses. Learning to live in a “Ventenata world” is challenging, Priest says, and he doesn’t know whether “I actually have the right answer.”

Kerns echoes that uncertainty. Trying to cope with invasive grasses poses a conundrum, she says. “We kind of want to go back and restore things, but it’s not the same world.”

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