The fires that have ignited in California, leading to mass evacuations and seemingly otherworldly scenes, may have gotten their start from a surprising source.
Invasive grasses, or grasses that have encroached from other regions, can make wildfires more frequent, not just in California but nationwide, according to a study published Monday in the journal PNAS. The study looked at grasses like cane grass, which is native to Myanmar but is damaging ecosystems in Florida, and buffelgrass, which is helping to ignite fires in the Southwest and has origins outside of North America.
“These grasses can act as kindling,” said Emily J. Fusco, a postdoctoral researcher at the University of Massachusetts, Amherst, and lead author of the study.
The species add to fire risk because they are growing in regions that once had far fewer grasses and because of their physical structure. Absent rainfall, the long, thin blades dry out quickly. This creates potential problems in places like California where most rainfall happens in the fall and winter, leaving the grasses to dry out in summer.
“It’s like having thin strips of papers standing all through the dry season in whatever habitat it happens to be in,” said Carla D’Antonio, a professor in the department of ecology, evolution and marine biology at the University of California, Santa Barbara.
The grasses are more prone to burning than the native shrub land typically found in much of the West, including parts of California’s Ventura County, where the Maria Fire broke out late last week. After a fire, these grasses are able to grow back faster, leading to concern that they may replace less flammable shrubs.
“If we turn this place into a grassland, we’re going to be seeing fire every other year in individual locations,” said Dr. D’Antonio.
For the study, the researchers took data on the locations of 12 invasive grass species that were known to promote at least some fires locally, and combined it with data on wildfire occurrences. They found that in the case of at least eight species, there was a relationship between how often wildfires broke out and the presence of invasive grasses.
“It’s an impressive project,” said Dr. D’Antonio, who was not involved in the study. “It’s impressive in its scope, geographically speaking. It’s impressive in the number of different variables and number of species they are able to address.”
Humans introduce the invasive species onto the landscape in a variety of ways. Arundo donax, or giant cane, was deliberately brought to the United States as roofing material and to make reeds for instruments like the clarinet. But more commonly, the introduction is accidental.
“Roads are certainly a corridor for invasive species,” said Dr. Fusco, the study author. Seeds and plant fragments can grab an easy ride into new ecosystems on car tires or on people’s boots and clothing.
One concern regarding the Trump administration’s plan to put an end to road-building restrictions in Alaska’s Tongass National Forest, one of the world’s largest intact temperate rain forests, is that it would allow for the spread of invasive grasses that would heighten wildfire risk in a part of the country where climate change will likely increase the size and intensity of wildfires, according to the National Climate Assessment. In 2018 the forest experienced warm, dry weather that fueled a busier than normal wildfire season.
The concerns over encroaching grasslands aren’t limited to the West.
Earlier this year, Dr. D’Antonio walked through a park in Washington, D.C., while the city was in drought and spotted one of the invasive grasses on the study’s list.
“There was a 30-centimeter deep layer of dead grass,” she said. “This is an environment you would never imagine a fire occurring. And I walked through there thinking, ‘Wow, somebody could throw out a cigarette and this understory would start to burn immediately.’ ”
Invasive-plant managers are working to reduce the risk, “but it’s pretty time- and labor-intensive,” said Dr. Fusco.
“If we can be vigilant about keeping these grasses out of systems before they start to take over, you’re going to be way more effective at controlling them,” Dr. Fusco said. “And in the places where they already are, it’s prioritizing what kinds of systems we want to try to protect and restore.”
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