Invasive cheatgrass, reviled by Western ranchers and conservationists, has long since earned a reputation as a firestarter, making wildfires worse and more common. Same with climate change: It's well understood that climate warming is making western wildfires worse. But it’s not just cheatgrass anymore, or just a warming West: a new analysis finds at least seven other non-native grasses can increase wildfire risk in places across the country, some doubling or even tripling the likelihood of fires in grass-invaded areas. 

“There’s bufflegrass in the desert southwest and Japanese stiltgrass in eastern temperate deciduous forests, medusahead in the Great Basin… and the spread of these invasive grasses makes fire more likely across the United States,” said Chelsea Nagy of the University of Colorado Boulder’s Earth Lab and CIRES.  When flammable invasive grasses are present, they can impact wildfire about as much as climate warming does, said Nagy, who is co-author of the new work, published today in Proceedings of the National Academy of Sciences. 

She and several colleagues, led by University of Massachusetts Amherst postdoctoral researcher Emily Fusco, conducted what they believe is a first-of-a-kind analysis, combining several big datasets into what they’re calling a “pyrogeographic” study.

“Most of the work on this question has been on a small scale, studying fuel loads or fire intensity in one plot versus another plot of a few acres, or in one ecosystem in one national park,” Fusco said. That’s in part because until recently, the big data, tools, and computational power needed to run these kinds of analyses haven’t been widely available, she explained.

Her team started by compiling a list of fire-starting suspects, relying on the Invasive Plant Atlas of the U.S., other scientific literature, and a database called the Fire Effects Information System. They identified 12 invasive grass species for which there was enough data to map out “invaded” and “uninvaded” areas in pixels of 500 x 500 meters (roughly 62 acres). Then, they used fire records to compare fire occurrence, size and frequency between invaded and uninvaded areas, from 2000–2015.

Eight of the grasses, including cheatgrass (Bromus tectorum), significantly altered something about the region fire regimes of ecosystems they invaded—increasing the frequency of fires, for example.  

Where Common Mediterranean grass (Schismus barbatus) invaded, fire occurrence more than tripled. Invasion by silk reed (Neyraudia reynaudiana), buffelgrass (Pennisetum ciliare) and cogon grass (Imperata cylindrica) all increased fire frequency, the researchers found, and the presence of the flammable invaders increased fire occurrence by between 27 and 230 percent. 

“This work shows that invasive species are one of the big three ways that people are changing fire regimes,” said senior author Bethany Bradley, also from Amherst. “Climate change more than doubles the likelihood of fire, human ignitions triple the fire season, and now we can add invasive species fueling fires.”

She and her colleagues said the paper does suggest  another possible tool that could be used to mitigate wildfire in some places: controlling invasive species. People have helped invasive grasses take root in the United States both deliberately and accidentally: promoting their use for forage or as ornamentals, or transporting hitchiker seeds in hay or seed mixes. Human-caused disturbances to native ecosystems have also promoted the spread of the grasses, the authors said. 

“I think one of the most important messages is that in the places impacted, fire management and invasive species management need to be done together, and where these managers are distinct groups, they would benefit from closer collaboration,” Fusco said. “And, looking at future fire risk modeling, we should definitely be including invasive grasses, and their likely spread, in the mix.”