EXPLAINER: Fighting fire with fire to protect sequoia trees

LOS ANGELES (AP) — With flames advancing toward the signature grove of ancient massive trees in Sequoia National Park, firefighters on Thursday fought fire with fire.

Using firing operations to burn out flammable vegetation and other matter before the wildfire arrives in the Giant Forest is one of several ways firefighters can use their nemesis as a tool to stop, slow or redirect fires.

The tactic comes with considerable risks if conditions change. But it is routinely used to protect communities, homes or valuable resources now under threat from fires, including the grove of about 2,000 massive sequoias, including the General Sherman Tree, the world's largest by volume.

Here's how it works:

IT'S ALL ABOUT THE FUEL

Three things influence how hot and fast a fire burns: the landscape, with fire burning faster up steep slopes; weather, with winds and dry conditions fanning flames; and fuel, the amount of material that can burn.

The first two can't be controlled, but there are ways to reduce fuels long before any fire breaks out — or even as one is approaching.

“Of all the things that affect fire behavior, the fuels is really where we can take action,” said Maureen Kennedy, a professor of wildfire ecology at the University of Washington.

Historically, low- to moderate-severity fires every five to 30 years burned out excess brush and timber before deadly fires in the early 20th century led to aggressive firefighting and a U.S. Forest Service policy to suppress all fires by 10 a.m. the day after they were reported.

That led to dense forests of dead trees, fallen logs and overgrown brush that accumulated over the past century, fueling more massive fires.

SLOWING FIRE BY CREATING FIRE

For centuries, Native Americans have used fire to thin out forests.

Prescribed burns set...