GEORGETOWN, Calif. — This patch of woodland in the northern Sierra Nevada looks idyllic. Dappled sunlight filters through the lush branches of towering pines. Spaces between their slender trunks are thick with saplings.
But forest ecologist Robert York sees danger lurking in this portion of the UC Berkeley research forest.
The control plot has grown unmolested since the 1930s, when a timber company turned the land over to the university. It is now teeming with young cedars and firs — conifer species that tolerate crowded and shady conditions, but are more vulnerable to fire and drought.
Pockets of dead pines are forming as trees compete for water, weakening their resistance to pathogens and bark beetles.
“We have different metrics for describing why the fire hazard is high here,” York said. “But basically the gist is: There’s a lot of fuel on the ground and there’s a lot of trees here.”
Most of California’s forests — especially the 57% managed by the federal government — look somewhat like this, said Berkeley Forests co-director Scott Stephens.
It’s a key point of comparison to the other parts of the 4,200-acre research forest, where scientists have been experimenting with different ways of managing the land for nearly nine decades.
Now, as the U.S. government embarks on an ambitious, multibillion-dollar plan to make Western forests more resilient to wildfire, research here at Blodgett Forest Research Station could help determine what actions are needed. It could also help inform a bitter debate among some environmental activists over forest thinning.
Blodgett Forest Research Station
While York and many other scientists see forest management as critical to combating the increasing destructiveness of wildfires, a smaller but vocal group of environmental advocates says it is an open invitation to commercial logging and threatens the ability of forests to remove planet-warming carbon dioxide from the atmosphere. They argue that the money is better spent on making communities and homes less vulnerable to wildfire, and that forests should be left in their natural state.
But given these forests’ histories, that term is a misnomer, said John Battles, another Blodgett scientist who studies how and why forests change.
“Nature hasn’t taken its course without a lot of human intervention since the last glaciation,” he said.
One major link between the Blodgett control plot and other forests in the Sierra Nevada is a conspicuous lack of fire.
Controlled burns help reduce impact of wildfire
When the Mosquito fire burned through an area that had been intensely managed with prescribed burns, researchers found more trees survived than in other, denser areas.
©2022 Google, ©2022 Landsat / Copernicus, ©2022 NASA, TerraMetrics
Until roughly 100 years ago, lightning-sparked fires regularly rippled across the landscape.
Many other fires were purposely set by humans.
In the valleys, mountain meadows and up into the foothills, Indigenous people set fires to clear out brush and stimulate the production of foods, fiber and medicines. The forests, researchers say, adapted to the routine introduction of fire.
Then European settlers arrived and made the forests their commodities. They forced Native people from their lands and outlawed their burning practices. They cut down the biggest, oldest, most fire-resistant trees to supply lumber to towns and mines.
They let their sheep and cattle graze, disrupting soil and vegetation.
The desire to preserve commercial timber also caused forest managers to aggressively fight wildfires.
Deprived of regular low- and moderate-intensity fire, experts say, forests grew denser and less diverse as trees in logged areas grew back at uniform ages and heights. That created a continuous canopy that helps fire spread more quickly and makes it harder for certain plants to thrive.
Species like the ponderosa pine that had adapted to live with fire by developing thick bark, tall trunks and plentiful seeds were in some places replaced by shade-loving cedar and firs, which are more likely to succumb to flames.
As the mix of species became less varied, it increased the likelihood that an outbreak of disease, fungus or insects, which are usually species-specific, could wipe out a stand. Scientists believe the competition for water has also made pines more vulnerable to bark beetle attacks by decreasing their ability to fight off the pests by saturating them with resin. Mass tree die-offs ensued.
How bark beetles attack a tree
After boring through the outer bark, females carve galleries to lay eggs, and spread fungus as they burrow. Larvae continue to burrow as far as the cambium layer, spreading more fungus. The fungus cuts the veins that carry water to the branches, and the tree dies.
Pitch tubes appear when a healthy tree can produce enough resin to repel boring beetles.
Pitch tubes appear when a healthy tree can produce enough resin to repel boring beetles.
These changes — compounded by global warming and severe drought — have combined to fuel larger, faster-moving fires that are burning more intensely through forestlands that have grown less resilient to their effects, experts say.
The control plot isn’t the part of Blodgett Forest where fire danger is highest.
That distinction probably belongs to a patch of land where scientists took an active approach to management. For the last four decades, they held commercial tree harvests every 10 years or so using a method called single-tree selection. Foresters chose which trees to take out, and loggers cut them down, dragged them to a landing and loaded them on a truck bound for a sawmill.
These decisions penciled out for the university financially because they were able to sell the trees, York said. But they ended up increasing fire danger.
Smaller trees were left in place, and logs and sticks accumulated on the ground.
That combined with the regular harvests to create a vertical distribution of vegetation heights that could act as a staircase ...
... carrying fire from the forest floor into the treetops.
Such vertical continuity increases the risk of a crown fire — one that burns across the canopy — which can be exceptionally intense, fast-moving and difficult to control, the scientists say.
Their modeling predicts that if a wildfire were to reach this land, it would burn more severely than the control plot, probably killing most of the trees.
The plot serves as a reminder that not all forest management techniques are created equal — and not all of them reduce the risk of fire damage, York said.
Not long ago, this stretch represented the ideal that foresters were striving for, said Brandon Collins, who conducts research at Blodgett in partnership with the Forest Service’s Pacific Southwest Research Station. All the growing space is being used for trees, maximizing the potential future revenue.
But over the last decade or so, foresters have had to rethink their goals. More land is burning under increasingly dire conditions — a trend that’s expected to continue as the planet warms.
A study by researchers at UC Irvine and UC Davis found that solely considering changes in summer temperatures, the amount of land burned in the Sierra Nevada will increase by 26% to 92% by the 2040s.
More forestland in the Sierra is burning at high severity
High-severity flames kill most or all of the vegetation in an area.
Total area burned
1.2 million acres
Larger patches of forest are also burning in so-called stand replacing fires, in which high-severity flames kill most or all of the vegetation, according to UC Berkeley research. This can mean that remaining seed sources are too far away for conifer forests to regenerate on their own in these areas.
In light of such projections, land managers say it’s increasingly important to prepare the landscape so that it will burn at lower intensity — namely by manipulating vegetation so that when a fire does reach it, there’s less fuel to burn and a better chance the forest will be able to survive.
“It just can’t be wall-to-wall trees; it just can’t be,” Stephens said. “It’s not sustainable.”
There are trade-offs, however.
A dense forest is better for some purposes — more trees means more carbon being removed from the atmosphere and stored, which helps reduce the greenhouse gasses that are fueling climate change. This type of forest structure also provides better habitat for some wildlife, such as the near-threatened California spotted owl, which likes to nest in areas with high canopy cover and plentiful standing dead trees, Stephens said.
But increasingly, he and the other scientists say, fire risk must be taken into account. A forest can’t store carbon or provide shelter for owls if a wildfire has burned vast swaths at high intensity and converted the land to a brush field — something these scientists say they see happening with increasing frequency.
“That’s one of the giant conundrums of the whole state,” Stephens said. “How do you work with endangered species, rare species, knowing their population may be at risk at current levels but also the forest is basically falling apart right in front of your eyes?”
A short drive from the control plot is a stretch of land where the trees are green at the top, but bare from the waist down, their bases blackened and charred. Pines, cedars, oaks and firs grow in clumps. The gaps between them are wide enough to see the gentle slope of the hill on which they sit.
This most closely resembles the ideal forest structure, York said.
Scientists have been working to re-create historical conditions here by reintroducing controlled fire at regular intervals. Every couple of years, they use drip torches to paint stripes of flames across the forest floor, clearing out sticks and brush that could otherwise carry flames up into the tree canopy.
If a wildfire were to burn through this area, the flames would probably stay slow-moving and low to the ground, giving the trees a good chance of surviving, they say.
The low-intensity burning would give firefighters an opportunity to directly attack the blaze, protecting the university research offices that are nearby.
But it wasn’t easy to get to this point. Before they reintroduced fire, the land managers held two timber harvests starting about 25 years ago. They used the proceeds to fund follow-up treatments. Brush and branches were piled and burned. Small trees were knocked over and ground up. That removed some canopy, letting more sunlight hit the ground. It also created a fuel bed of wood chips so controlled burning could be more easily conducted.
It would have been possible to achieve the same result using only prescribed fire, Collins said, but it would have taken longer. The scientists would have had to wait for a specific set of conditions to align: enough precipitation to dampen the risk of the fire escaping control, but not so much that the land was too wet to burn. Also, enough sun and heat to dry the ground even through the heavy canopy, but not so much that dry plants would stoke a high-severity fire.
Even then, the first burn would have been riskier because of the buildup of dense vegetation, Collins said.
Other than timber harvests, which generate money, these treatments tend to cost at least $1,000 per acre and require multiple passes, York said. Land managers must budget about $3,000 per acre and expect that it will take three to seven years of work to achieve a forest that is ready for low-intensity, ecologically beneficial “good fire,” he said.
“The challenge to get there, to get to the forest structure that we want, is it takes a lot of time,” York said. “And it also takes a lot of investment.”
It also takes maintenance. In order to keep this structure, land managers will need to continue performing controlled burns every two or three years, he said.
And although the scientists have tried to mimic the pre-colonial conditions, when low- and mixed-severity fires burned through this patch on average less than every 10 years, they face limits.
“When I look at this stand here and think about what it looked like 100 years ago, there were probably more really big trees,” York said. “They’re just not here because they were logged 100 years ago. And we can’t, despite our best efforts as managers, we can’t replace those immediately.”
Recently, Blodgett Forest underwent a real-world test of management techniques when the Mosquito fire burned through about 300 acres of research plots. The results appear to have confirmed the researchers’ findings.
In dense areas that had not been managed to reduce fire severity, flames spread quickly and killed most or all of the trees. But when the fire reached a plot that had been thinned and treated with multiple prescribed burns, its heat and severity dramatically lessened. Most of the trees survived and firefighters were able to get close to the blaze and stop it from spreading farther south.
“It’s very clear that fire behavior changed and the treatments did accomplish that function of reducing fire severity,” York said. “So what it means is that on a bigger scale, in forests in California, if we can accomplish the level of fuel reduction and thinning that we did, then it’s very likely that we can also accomplish reduced fire spread at that scale.”