CRESSMAN, Calif. — California’s drought and a bark beetle epidemic have caused the largest die-off of Sierra Nevada forests in modern history, raising fears that trees could come crashing down on people or fuel deadly wildfires that could wipe out mountain communities.

Aerial images show vast forests that have turned a rust-color. The epidemic has killed an estimated 40 million trees since 2010 in the central and southern Sierra, and it’s spreading north.

Officials who are cutting down and stacking the most dangerous trees in piles across six counties, however, say they are stumped by how to get rid of them all.

One solution is to fire up a fleet of 10 large, mechanized incinerators the state just bought. Promoters say they burn so hot that they spew little if any smoke.

Environmentalists contend the burners undercut an emergency order by Gov. Jerry Brown – considered a global leader in the fight against climate change – who called for sending the trees to biomass plants and converting them into energy.

Chief Ken Pimlott, who manages the state’s response to the die-off as director of the California Department of Forestry and Fire Protection, defended the air burners as one of many tools.

He acknowledged the burners will contribute to air pollution, as with any work in the forest, but much less than a large wildfire, which the air burners may prevent by removing dead trees.

“We could have a catastrophic wildfire in any of these communities,” he said. “We have to be aggressive in terms of protecting life and property.”

Called air-curtain burners, the 20-foot long, steel containers blast a sheet of air over the open top, disposing of up to eight trees an hour. The state bought them for roughly $1 million, part of a $5 million investment in equipment to meet the epidemic.

A lack of visible smoke, however, doesn’t mean the air burners are clean, said Chad Hanson, a forest ecologist at Earth Island Institute’s John Muir Project. He called them a misguided approach to managing forests.

“You’re still pumping a lot of greenhouse gases into the atmosphere,” Hanson said.

Hanson, who favors leaving the trees that need to be cut down on the forest floor to naturally decompose, urged Brown to withdraw the emergency declaration, fearing that it would increase logging of dead trees that forests need to replenish.

Last year alone 29 million trees died at the height of California’s drought now in its fifth year, the U.S. Forest Service reports.

Drought makes trees vulnerable to the insects’ attack, officials say.

A beetle epidemic in the Rocky Mountain states was blamed in 2013 for contributing to Colorado’s second largest wildfire, forcing entire communities to be evacuated, said Jeff Mai, aerial survey manager for the U.S. Forest Service based in Colorado.

Mai said that in California, bark beetles have killed five times more area of forests than in Colorado.

It’s unclear how many trees in the Sierra will be cut down. Officials say the first job is removing those that threaten motorists and mountain communities.