In an effort to prevent the spread of wildfires, the U.S. Forest Service is planning 18 controlled burns in the Angeles National Forest starting Monday and lasting through April, the agency said Thursday.

The Forest Service will set small fires in order to burn off piles of dry grasses and underbrush near Mt. Wilson, Glendora, Wrightwood, Chilao, Mt. Baldy, Santa Clarita and within the San Dimas Experimental Forest, the agency said.

This effort represents the largest number of controlled burns in the Angeles in 10 years, said Nathan Judy, a Forest Service spokesman.

While a few burns occurred last year after substantial rain, the practice was halted during the region’s recent five-year drought from 2011-2016 — brush was too dry and the risk of a burn getting out of hand was too high, Judy said.

By burning dead branches and twigs stacked up throughout the forest north of Los Angeles, as well as portions of the interior forest’s grassy understory made up of plants and small trees, the agency says it will help prevent catastrophic fires by limiting their reach.

“These projects are part of a continuing effort to reduce the threat of wildfire to people and communities across the Southland,” Judy wrote in a prepared statement.

The burned areas will act as fire breaks, preventing any potential wildfire from jumping fire lines and spreading into nearby communities, he said. Fires starting in thick brush killed 42 people in Napa Valley and the Thomas Fire in Ventura and Santa Barbara counties killed 32-year-old firefighter Cory Iverson and destroyed 1,063 structures in 2017. Mudslides later took the lives of at least 21 people in Montecito this month.

“It is fire prevention,” Judy said. “It is literally fighting fire with fire.”

Exactly what days the prescribed burns will take place is not yet known, he said. The fires can only be set when weather conditions, including temperature, relative humidity and winds make it safe for a burn. Also, the air quality must not be unhealthful.

Fire crews will be given a 24-hour window or less to start the burn. A burn can last 4 to 7 hours, Judy said. The Forest Service firefighters will be given the go-ahead only after fire management officials determine it is safe to do so.

“If I were the Forest Service, I would be incredibly cautious,” said John Monsen, co-chair of the Sierra Club Angeles Chapter’s Forest Committee. “It is tricky,” especially in a forest surrounded by a metropolitan area of 10 million people, he said.

A controlled burn is a recognized tool used to prevent future wildfires from getting out of control. Using burns instead of bulldozers to create fire breaks lessens the damage to the forest ecosystem, Judy said.

Chad Hanson, a forest ecologist with the nonprofit The John Muir Project, said they are useful but can also cause an imbalance in the ecosystem.

“It can be a useful tool to insure a slower, low-intensity fire closer to communities,” Hanson said in an interview. However, in vast, interior portions of the forest, burns can damage habitat for birds by preventing mixed intensity fires that are needed for forest regrowth.

In the past, plumes of white smoke could be seen from the burns by folks living in the Inland Empire or San Gabriel and Santa Clarita valleys, Judy said. In order to prevent 911 calls, the Forest Service will alert the public via its Twitter feed, which is @Angeles_NF, as soon as it learns a burn will take place.

Piles of collected branches will be burned in several controlled burns in the following locations:

• Mt. Wilson Observatory site (40 acres)

• Charlton-Chilao on the Angeles Crest Highway south of Three Points (100 acres)

• Crystal Lake on Highway 39 about 22 miles north of Azusa and at the Coldbrook Campground in San Gabriel Canyon (75 acres)

• Around the community of Mt. Baldy (85 acres)

• Clear Creek Fuelbreak on Angeles Crest Highway near the Clear Creek Fire Station (136 acres)

• Liebre Mountain, northwest of Lake Hughes (100 acres)

• Big Pines Highway, west of Wrightwood (74 acres)

• A larger controlled burn, totaling about 100 acres, will take place in the San Dimas Experimental Forest north of Glendora, San Dimas and La Verne (100 acres).