The Health of the Sierra Forests

The Health of the Sierra Forests

Cover photo from The Health of the Sierra Forests

Five years of drought have had an unprecedented effect on our region–forestry specialists lay out the facts and explain why they matter.

By Whip Villarreal
Photography by Paul Hamill
Photo of Plane Courtesy of Greg Asner
Photo of Fire Truck by Nash Rood

A high-winged twin engine airplane glides over the forest canopy, shooting laser beams into the woodlands. But this aircraft and the lasers beaming from it are not a scene in a science fiction film or even a military training exercise. They are on a mission to find life and death within the trees scattered across the forests of the Sierra Nevada.

Greg Asner, an ecologist with the Carnegie Institution for Science at Stanford University, maps sick and dead trees from the skies by using the Carnegie Airborne Observatory III, a plane that’s outfitted with high-powered aerial imaging technology. The CAO III uses a system called laser-guided spectroscopy, which allows Asner to create 3-D chemical models of a forest’s canopy, measuring water inside trees that are still alive and locating trees that have already died.

“We have the dead-tree part of the Sierra landscape and then we have the still-living part of the landscape, which we can analyze to see how the living trees are doing and if they are heading toward the dead side or if they will survive,” Asner says. “That’s really what our technology was built to do. That’s why everybody talks about us as the predictors of mortality rather than just the counters of the dead.”

Over the last five years, Asner’s team has performed this assessment so land managers and scientists can determine which trees in the Sierra Nevada will eventually perish due to drought or attacks from bark beetles—up to a year before they actually die.

Because of this tree-mapping data, there is a growing understanding among experts that the Sierra Nevada forests’ health is declining. In addition to historically dry conditions, these forests are overly dense, with compact under- and overgrowth. The combination of these factors means the forests are extremely vulnerable to swarms of attacks from an ever-growing bark beetle population, and large, damaging wildfires.

The Carnegie Airborne Observatory III
The Carnegie Airborne Observatory III. Photo courtesy of Greg Asner.

The Carnegie Airborne Observatory III. Photo courtesy of Greg Asner.

Why It Matters

Asner cites three reasons it’s important for Sierra forests to be in good shape. One is that losing trees changes the habitat for all species that occupy these forests, adversely affecting the ecosystem. The second issue is the impact on air quality: As trees continue to die, the forest loses its ability to store carbon, which eventually decomposes and goes back into the atmosphere as carbon dioxide, vital to life on Earth. And the third factor is water: The forests in the Sierra mediate and moderate water availability in a phenomenon the scientific community calls “the sponge effect”—when it rains and snows heavily, as it has this winter, moisture is held in the soil by the root system and is managed by the tree canopy.

“It’s an ecosystem that helps regulate how much water is coming out of the Sierra,” Asner says. “Without tree canopies or trees mediating that process, water runs off the Sierra much faster than it would otherwise. If you lose a lot of forest, you have a harder time regulating input into reservoirs and agricultural fields and ultimately into the faucets of Californians.”

In the 1970s, 12 million trees died during California’s drought. However, the current beetle infestation and drought conditions have killed nearly 10 times that amount, with approximately 85 percent of that mortality occurring in the Southern Sierra Nevada.

“Two hundred years ago, the forests in the Sierra generally wouldn’t look as forested as they are today because they had a natural fire season,” Asner says. “Today we now have a century of fire suppression (so) there are a lot more trees in the region than there would have been historically. It’s like too many straws in a cup; there is just not enough water for all these trees. We have lost more than 100 million trees during this five-year drought, and that’s a massive loss. It’s in nobody’s rule book on how ecosystems work, and it puts us in a totally different regime ecologically to try to figure out if this is too much loss or not. We simply don’t know, and I don’t think anyone knows.”

Asner is not the only forestry expert expressing concern. “To be clear, what is occurring in the Sierra Nevada is not natural or normal,” says Jim Branham, the Sierra Nevada Conservancy’s Executive Officer. “The tree mortality in the Southern Sierra Nevada is truly unprecedented. There is no historical record to suggest anything close to this level of mortality has ever occurred here.”

Branham says while irreversible change may not be unusual, the scale and rate of change have many alarmed about what the future might hold.

However, he believes there is still an opportunity to aid the forests. The forests could be healthy again if there is additional investment in maintaining the lands and if lawmakers revisit policies that improve the ability to get needed forestry work done. If not, Branham says, there could continue to be serious environmental repercussions throughout the Sierra Nevada region.

Beetle in Bark

Bark Beetle


In these overgrown forests, trees have to compete for limited resources of water, nutrients, light and space, which further weakens their natural defenses to bark beetles. In fact, during these recent drought conditions, trees had a harder time fighting off bark beetles because they made less sap, a low priority for trees trying to compensate for lost water. As Stanford ecologist Greg Asner explains, in a healthy tree, sap pushes beetles out of the bark and drowns them in its sticky substance.

In an unhealthy tree, without these sap defenses, bark beetles can quickly and easily attack it and kill it in two ways. The first is the beetles feed on the inner bark, which creates an internal tourniquet, according to scientists, and cuts off the flow of nutrients down from the leaves. The second is the introduction by the beetles of blue stain fungus, which prevents water moving up from the roots to the leaves.

Higher temperatures also promote faster beetle development, and the recent warmer winters have ensured more beetles survived to reproduce at high levels. This, combined with the ample food provided by stressed trees, has allowed the beetles to thrive and increase their population, which is estimated to be in the millions.

Forest Fires
Dead and dying trees clearly visible in the forests near Yosemite. Photograph by Paul Hamill.

Dead and dying trees clearly visible in the forests near Yosemite.

Fire–How It Hurts

The dead trees and overgrown forests in the Sierra have not only led to an increase of attacks from bark beetles, they have contributed to the massive wildfires of recent years.

According to the Sierra Nevada Conservancy, between 1984 and 2010, there was a significant increase in the number of acres within a wildfire that burned at high severity, from an average of 20 percent in fires in the mid-1980s to more than 30 percent by 2010.

When a wildfire burns at high severity, there is essentially no living vegetation left in its aftermath. These high severity burn areas have long-term consequences on the natural habitat, water quality, carbon levels and other ecosystem factors in the Sierra Nevada.

The Rim Fire burned more than 250,000 acres in 2013 and the King Fire scorched more than 97,000 acres of land in 2014. More recently, the Butte Fire in 2015 burned 70,868 acres and destroyed 921 structures, including 549 homes, 368 outbuildings and four commercial properties.

Forty percent of the Rim Fire and close to 50 percent of the King Fire burned at high severity. In some areas of the Sierra Nevada, forests that burned at high severity are not regrowing as forest. More and more areas are experiencing a change from forest to shrub or grasslands, which has the potential to burn at high severity in less than 10 years.

Landscapes that suffer from high severity fires also see dramatic increases in sediment that runs into streams and reservoirs, impacting water quality and infrastructure as well as displacing storage capacity in reservoirs.

With increasing size and costs of suppressing wildfires, the very efforts that would protect watersheds and restore forests to make them more resilient to fire in the future are being squeezed out of the budget. Last year, fire suppression alone consumed more than 50 percent of the Forest Service’s budget.

“The truth of the matter is there just aren’t enough resources we can employ or deploy to deal with fires of these magnitudes that spread so quickly,” says Kit Bailey, U.S. Forest Fire Chief for the Lake Tahoe Basin Unit. “There’s immense pressure to extinguish these fires when they’re small on an initial attack, and we do. We have a very high initial attack success rate of 99 percent. It’s the 1 percent that account for the biggest expenditure not only in firefighters and equipment, but in cost.”

Bailey says the strain on the fire suppression budget has been an ongoing problem for years, particularly because of the number, size and duration of these large wildfires. What could help, in his opinion: passing legislation to manage large fires from an emergency fund similar to the Disaster Relief Fund, which the Federal Emergency Management Agency uses to respond to natural disasters like hurricanes, earthquakes and floods.

“We have these large suppression costs and it ironically impacts some of the very programs—like timber management and fuel reduction—that we’re trying to implement to relieve some of these large fires,” Bailey says. “We want this legislation to move forward so we can fund the suppression activities a little differently than out of the Forest Service’s budget.”

Fire–How It Helps

Fire is a natural part of the ecosystem. Historically, fire played an important role in Sierra forests, burning slowly across the landscape and clearing out excess brush and small trees.

Over time, settlement in California led to significant changes, however, including logging of the larger, more fire-resistant trees; development of forest communities and infrastructure; and enhanced fire suppression. Today, the culmination of those changes has resulted in many forested areas that have more than five times the trees that existed historically.

Fire Damage
Massive wildfires can be catastrophically damaging, but managed fire practices can improve the forests’ health and offer protection for the future.

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Now it is up to land managers to thin the forests in order to mimic the role natural fires used to play as part of the cycle of a healthy forest.

Forests can be thinned in a couple of ways: Workers can use equipment to remove small trees and brush, or implement prescribed fires. How much and what kind of material gets removed depends on the location, but the desired result is a forest with more space between trees, so that the larger trees can thrive and fire can move safely and slowly along the forest floor.

However, many parts of the Sierra are too steep or inaccessible for thinning work to be conducted mechanically. For those areas, prescribed or managed natural fire practices are used.

Ecologically sound forest thinning can proactively reduce drought and disease-related tree mortality. In addition to alleviating the dense forest conditions, the use of managed and prescribed fire can also strengthen trees’ defenses against bark beetle attacks.

“Restoration can be expensive, but the benefits that can be gained from this type of work will ultimately outweigh the costs,” says SNC’s Executive Officer Jim Branham. “Restoring the forests to a more resilient state can greatly alleviate these problems and offer the best protection for the future. A significant increase in the pace and scale of ecologically sound mechanical treatments, and prescribed and managed fire, needs to occur to aid these forests.”

Forest Fires
Mount Danaher CAL FIRE station. Photograph by Nash Rood.


There are some ways residents and visitors in the Sierra Nevada can help protect their homes and the forest.

Homeowners can create a buffer zone between structures and grass, trees, shrubs, or any wild land areas that surround their property. Fire Chief Bailey says the reduction of fuels by creating defensible space in and around homes and property is the biggest help to the forests—and firefighters—when a wildfire breaks out.

CAL FIRE’s website also provides information and strategies that can help protect homes and the forests around them from large, damaging wildfires (

Finally, Bailey suggests that when you are out in the woods—building campfires, smoking or using off-road vehicles—it is critical to remember that many of the Sierra Nevada forests are not healthy, and are in need of restoration. He says that humans are the leading cause of wildfires, especially because of these drought conditions.

“Being mindful of the conditions when in these forests is important not just from a fire perspective, but from a tree mortality perspective,” Bailey says. “Trees are weak, branches are coming down, tops are coming out and trees are falling on their own, especially when there is any kind of wind conditions near or around a standing dead tree. It is important for visitors and property owners to be aware of any hazards associated with these weakened and drought-stressed trees.”

How you can help

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