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The invasive, flammable plants making California's fires worse

The Thomas Fire broke out Monday near Santa Paula and grew wildly to more than 48 square miles in the hours that followed, burning at least 150 structures. More than 27,000 people have been evacuated.
Ventura County Fire Department
The Thomas Fire broke out last week near Santa Paula.

Scan the lower elevations of mountains across Southern California, and you'll see that they're covered in bunches of grass, bursting from hillsides like small tufts of wayward hair. Dense amounts of grasses have squeezed in between the native coastal sage brush and chaparral. The grasses are invasive and have provided the perfect fuel to start the fires that have torn through the of mountains of Ventura, Los Angeles and Santa Barbara counties, destroying habitats and homes.

"The invasive grasses have had a major role in most of the fires this year," said Richard Minnich, professor in the department of earth sciences at UC Riverside. "The fires have largely been at low elevations where exotic annual grassland is most abundant. And the amount of grass and biomass was unusually high this year because of the heavy rains last winter."

The Thomas fire began in grass and rapidly spread to shrubs and old chaparral that hadn't burned in years. The Creek fire saw a similar scenario play out as well, Minnick said.

Following the heavy rains in January and February, the grasses are taller and a lot more dense than years prior. Since then, Californian's lived through an exceptionally hot and dry year, to the point that nary a drop of rain has fallen during the traditional rainy season that started Oct. 1. What that means is that the dryness of those grasses might've been tempered by precipitation in December, remain parched and ready to burn.

"The grass is definitely predominant in a lot of these fires," said Scott McLean, deputy chief with the California Department of Forestry and Fire Protect. Although Cal Fire has not officially determined how the blazes started, McLean said all it would only take a spark to set them the grasses aflame.

The most common invasive grasses are wild oats, red brome and foxtail. When there's a disturbance to the local environment – be it from development, grazing, the clearing of area to prevent fires or fires themselves – the invasive plants are there to fill the void.

"Invasive annuals and perennial grasses make it more likely for fire ignition to occur," wrote Philip Rundel who studies California's fire ecology at UC Los Angeles. "Once we have a fire, however, these generally contribute little to fire spread or intensity."

Fires across the Southern California landscape have never been rare, and are part of a natural process. Chaparral and California sage scrub are resilient when they have 10 to 20 years to recover in between burns. However, invasive grass establish more quickly, upping the chance that the same environment might catch fire earlier, inhibiting the recuperation of native species. The grasses also burn at a lower temperature than the shrubs, meaning their seeds can survive, giving them a better chance to propagate.