Alien grasses are making wildfires more frequent in the U.S., study finds

Invasive grass species
Cheatgrass, right, and invading shrubs, left, are shown in a 2005 photo taken near Lovelock, Nev.
(Bethany Bradley/University of Massachusetts)

For much of the United States, invasive grass species are making wildfires more frequent, especially in fire-prone California, a new study finds.

Twelve non-native species act as “little arsonist grasses,” said study co-author Bethany Bradley, a University of Massachusetts professor of environmental conservation.

Wherever the common Mediterranean grass invades, including California’s southern desert, fires flare up three times more often. And cheatgrass, which covers about one-third of the Intermountain West region of the U.S., is a big-time fire promoter, Bradley said.

“I would not be surprised at all if invasive grasses are playing a role in the current fires,” Bradley said, “but I don’t think we can attribute to them directly.”


University of Utah fire expert Philip Dennison wasn’t part of the study, but he said it made sense. “In a lot of ways, California was ground zero for invasive grasses,” he said. “Much of California’s native perennial grassland was replaced by Mediterranean annual grasses over a century ago. This study doesn’t look at invasive grasses in the areas that are burning in California, but invasive grasses are contributing to the fires there.”

Experts say the areas burning now in California are more shrubs and grasses than forests, despite what President Trump tweeted over the weekend.

“This is a global problem,” said University of Alberta fire expert Mike Flannigan, who wasn’t part of the study. “I think with climate change and human assistance, we are moving to a grass world. One region they should have mentioned is Hawaii, where wildfires are increasing in large part due to invasive grasses.”

Invasive species are spreading more because of climate change as warmer weather moves into new areas, said the study’s lead author, Emily Fusco, also of the University of Massachusetts. New England and the Mid-Atlantic are seeing new invasive and more flammable grasses, Bradley said.


The study in Monday’s journal Proceedings of the National Academy of Sciences looks at the connections between a dozen species of invasive grasses and fires nationwide, finding fires occur more often in places with the non-native grasses. But the study did not find a link between invasive grasses and the size of the fires.

Four of these species, including cheatgrass and common Mediterranean grass, are in California. When these grasses get dry, watch out, Fusco said.

“When you start a fire normally you want kindling,” Fusco said. “The grasses are, more or less, like kindling”

If someone lights a match and throws it in the middle of a forest, it is unlikely a fire will start, but throw it in a field of cheatgrass “and odds are that it’s going to catch,” Bradley said.


“We are the reason that invasive species are here. We are the reason that they get spread around,” Bradley said.

Flannigan noted that invasive plants that are not grasses also feed the wildfire problem.

Most outside experts said the study was important. But wildfire expert LeRoy Westerling of UC Merced said that with wildfires the size of the blaze was key, so he felt this study was less valuable because it measured frequency.

Although size matters in forest fires, study author Bradley said, mid- to small-size fires are the ones “in everybody’s backyard” and affect people and their buildings more.