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Last Year's Santa Cruz Lightning Fires Still Causing Trouble

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Redwood trees after the CZU Lightning Complex wildfires burned much of the area at Big Basin Redwoods State Park on Sept. 10, 2020. (Beth LaBerge/KQED)

A small fire in the Santa Cruz Mountains started on May 2 because of a flare-up inside a tree that had burned in last year’s CZU Lightning Complex fires. Those fires raged for more than 40 days, consumed over 85,000 acres and destroyed about 7,000 buildings.

Sunday’s Basin Fire ignited from embers that had smoldered for roughly eight months inside a 30-foot tree with a 6-foot diameter. The fire burned through about 7 acres and was quickly contained by Cal Fire.

On Monday, the fire agency conducted an infrared-mapping flight that identified several similar hot spots within the CZU burn scar.

“Our crews have been hiking into each identified location,” said Jonathan Cox, deputy chief of the agency’s San Mateo division, in an email.

Coastal California around the Bay Area received less than half its average rainfall this past winter. “When you have that type of setup, these fuels can smolder,” said Scott Stephens, a fire scientist at UC Berkeley. “And they could smolder for many months.”


The incident is not the only flare-up this year. Last week, scientists with Sequoia National Park found a giant tree still beset by flameless burning in the remote Board Camp Grove, nine months after it was scorched during last summer’s Castle Fire.

Flare-ups are not uncommon in thickly forested places after wildfires have ripped through areas with heavy fuels. Coals remain warm and deep within big logs, root balls, and rotten material buried underground.

When the weather warms up and trees, brush and grasses dry out, these embers can flare up and ignite new fires. At times, they spread and burn dangerously out of control. That’s how the 1991 Oakland Hills Firestorm began, Stephens says, which eventually spread to consume 2.5 square miles of neighborhoods off Grizzly Peak Boulevard and Claremont Canyon, killing 25 people.

“Generally, agencies will go around the edges of big fires and do a mop-up operation, trying to get rid of all of those smoldering areas,” Stephens said. “Maybe 200 feet, 300 feet from the fire line. But the interior of those big giant fires are not going to be mopped up. They’re just too large.”

A lack of substantial winter rain has left much of the state parched in drought conditions, especially in the Bay Area.

Last weekend  the National Weather Service issued a rare spring red flag warning across the inland hills of the Bay Area.

UCLA climate scientist Daniel Swain says California is drier than it was at “any point during the last drought, which is kind of shocking, because things got pretty bad then.”

He says the dry conditions are a “very alarming signal” in terms of the health of forests and wildfire risk.

“You’re going to see increased forest mortality, and we’re going to reap the wildfire impacts, unfortunately, later in the season,” he said. 

More fires have ignited earlier this year than last year in the state, but spring fires tend to be smaller and less dangerous. Swain says conditions are primed for a particularly severe fire season, once again, in most of the state.

Last year’s season was so bad due to extreme heat and dryness, but it was August’s dry lightning storms that sparked hundreds of fires, including the summer’s largest and most catastrophic wildfires.

“I’m hopeful that we won’t have that simultaneous ignition of hundreds of fires in August coupled with one of the hottest heat waves on record,” Swain said. “My fingers are crossed. Statistically, we should be able to avoid it. The odds aren’t high. But that doesn’t mean we won’t see something similar.”

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