Bone Dry Bay Area Forests Portend Fierce Fire Season

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Sampson Spence of the Fremont Fire Department uses a water hose to douse flames in the Santa Cruz Mountains near Loma Prieta, Calif., on Sept. 28, 2016.  (Josh Edelson/AFP via Getty Images)

Blackberry Hill has a panoramic vista of the South Bay, the stuff of bucolic, real estate fantasies. Serene, nothing but birds and chaparral and oaks.

Alan Huston appreciates the view, but he’s much more interested in the chamise bushes that dot the sun-drenched hillside above Los Gatos.

Twice a month, Huston, a meteorology grad student at San Jose State University, comes here, snipping new growth off of the native chamise plant. On a sunny afternoon, he took part in a data collection project to measure how much water plants have lost to heat and drought.

“Up here at Blackberry Hill, you can tell the plants are struggling,” Huston said,  pruning twigs off the bushes and placing them in tin cans. “A ton of chaparral, chamise plants that are just dead.”

San Jose State's Alan Huston, a grad student studying meteorology, snips new growth from a chamise bush on a hill outside of Los Gatos on May 14, 2021.  (Raquel Maria Dillon/KQED)

In mid-May, the chamise had taken advantage of a late spring sprinkle of rain to send out tender new shoots in between gray, dead branches — a valiant, last-ditch attempt to grow during a dry winter.


“Seeing the plants slowly creep out of winter when we didn't have a lot of rain — it’s the most hopeful time because you see the new growth come out. This is supposed to be the brightest point of the year in terms of fuel moisture.”

But not this year. Researchers started tracking vegetation moisture at the Blackberry Hill site eight years ago. This year, fuel moisture levels in the forest and chaparral of the Santa Cruz Mountains are lower than they’ve ever seen. That means trees and shrubs of the Santa Cruz Mountains are drier than the peak of the last major California drought, and any other moment since researchers began collecting records about a decade ago.

Research suggests that climate change is making weather patterns responsible for drought conditions in California more likely. Warming is also a key driver of more frequent wildfires in the state. One study found the number of extreme fire-weather days has already doubled due to rising temperatures and will continue to increase.

Huston collects enough tiny twigs to add up to about 30 grams. Later, he’ll analyze the samples in the lab by baking them in tin containers for 24 hours in an oven and measuring how much moisture is lost to determine the precise moisture content of the chamise.

Huston’s boss, professor Craig Clements, summed up these findings in April, when he posted a line graph of the fuel moisture content at this monitoring site, along with a rough prediction about this fire season.

The tweet caught on well beyond his usual followers in the fire and weather research community.

Clements regularly posts wonky, data-rich tweets about vegetation moisture, as part of his job as the director of San Jose State’s Wildfire Interdisciplinary Research Center. But until this spring, only scientists and firefighters seemed to notice. 

The “fuel” in what academics like Clements call “fuel moisture” is basically just plants or buildings that could burn in a wildfire. Fuel moisture content is a measure of how much moisture is in the vegetation, which influences how fast it can ignite and burn. (It’s expressed as percentage, which indicates the ratio of moisture to combustible plant material.)

“It was shocking because usually when we go up in April to these sites, it's full of new growth and the plants are growing,” Clements said. “I tweet out fuel moistures all the time, nobody really cared until this year.”

This year is different because Northern California is in the second year of a punishing drought that’s hurt even native plants like chamise and manzanita, which are adapted to thrive in the Bay Area with winter rain and almost no moisture during summer months. But the low vegetation moisture is another data point reminding researchers and the public that the forests around the Bay Area are bone dry. These graphs indicate precisely how dangerous wildfire conditions are right now along the coastal mountains in the South Bay.

Measurements are collected at more than a hundred sites across California. San Jose State’s Fire Weather Research Lab, which is part of the Wildfire Interdisciplinary Research Center, manages three sampling sites in the hills around the South Bay. Cal Fire and other agencies and researchers manage others. The data informs a national database that helps scientists understand how wildfires burn, along with topography and weather, the other two factors that determine fire behavior.

The chamise and other native chaparral plants have evolved to go without water for months through hot summers, but they still are ignitable, Clements said. Later in the season as the soil dries out, the plant moisture content falls, and the fuel becomes easier to burn.

Low fuel moisture content “sets the stage for fires being able to spread a lot easier and earlier in the season than [is typical],” Clements said. “These data are really critical [to our understanding of wildfire] because of the new modeling and forecasting tools that we're developing at San Jose State, where we're actually developing very high-resolution prediction systems.”

Vegetation moisture is one parameter that goes into the fire danger rating system. Think of those Smokey the Bear signs, with colors that indicate fire danger. Predictions based on this vegetation moisture data allow fire officials to pre-position crews and allocate resources to respond quickly and put out fires while they’re still small. The information also helps park officials and land managers decide whether or not to keep a park open or close it during hot, windy weather.

Patrick McIntyre, a fire captain with the East Bay Regional Park District, is one of those decision-makers. The park district samples for fuel moisture at four remote weather stations around the East Bay.

Patrick McIntyre, a fire captain with the East Bay Regional Parks District, points out signs of drought stress in a cluster of Monterey pines in Tilden Park on May 17, 2021. (Raquel Maria Dillon/KQED)

But for him — a trained forester, outdoorsman, and veteran firefighter — vegetation moisture isn’t just a number. His senses provide other data points that tell the story without graphs or charts. The foliage in the parks he patrolled last spring looked more washed out instead of brilliant green.

Brush crunched underfoot earlier this year because it dried out sooner. As he drives through Tilden Park, he points out trees — eucalyptus from Australia, Monterey pines from just across the bay, Ponderosa pines from the Sierra — native and non-native alike, all showing signs of drought stress.

Leaning out his department-issued pickup truck, he points across a grassy swale at a copse of majestic redwoods. “The health of these giant sequoias is definitely declining," he said. "Some of them might not even make it. The plants that look the healthiest are the native trees: coast live oaks, California bays, poison oak.”

He sees signs of drought and the ominous wildfire season all over: a pine tree “flagging” a branch of dead needles, eucalyptus bark peeling more than usual, a hardy bay laurel dropping yellow leaves, or the puffs of dry dirt and brittleness of the brush underfoot. He says those symptoms are shocking.

“Everywhere — freeway, roadside trees, up in Oakland Hills there’s just massive swaths [of forest] that’re just looking peakish,” he said, describing it as a  “visible kind of sadness.”

“It doesn't seem as though there's enough soil moisture to keep the trees happy and alive right now,” he said. “It's worrisome.”

The district is working to clear the brush, especially the fallen bark and leaves in eucalyptus groves near buildings and infrastructure. McIntyre surveyed a crew from the Conservation Corps clearing a small grove of trees.

A work crew builds a fire break by clearing overgrown eucalyptus saplings from along a trail in Tilden Park on May 17, 2021. (Raquel Maria Dillon/KQED)

“A lot of the smaller diameter eucalyptus, less than 8 to 12 inches, we’ll remove and get out of there and then we'll leave the larger diameter to still provide shade habitat,” he said.

The winding trail by Tilden Park’s public golf course will still be shady and cool, but there will be less dry fuels — vegetation that could catch and spread fire.

These symptoms of drought won’t go away until rain returns in the fall — if it returns. Those record low vegetation moisture readings aren’t likely to get any better until then either.


Firefighters and scientists agree: this kind of dryness sets the table for earlier, bigger wildfires this season. One errant firecracker, one spark from an unlucky vehicle, is all it takes. The data and the look of the landscape point in the same, grim direction.