Tree mortality at Bass Lake, Sierra National Forest. Credit: U.S. Forest Service Region 5 Flickr (CC BY2.0)
The latest news release by the U.S. Forest Service reads ominously: the Sierra Nevada has over 100 million dead trees.
Swathes of standing dry trees infested with insects populate the Stanislaus, Sequoia, and Sierra National Forests. Climate change, a five-year drought and bark beetle attacks have together resulted in what the Forest Service has called “historic levels of tree die-off.” Many of these trees stand precariously close to roads, homes and power lines, posing a direct safety hazard.
While most scientists are troubled by the great amount of standing dead trees, a relatively small group of ecologists and conservationists contends the trees are not a problem, even in the face of new data that contradicts their opinion. These groups stand in the way of the Forest Service’s attempts to manage dead trees.
Scientists nearly all agree that the bleak sea of withering trees is an ecological problem. The general consensus is that human fire suppression since the 1930s has created a forest far more dense with trees than before. More trees means less water for each one, exacerbating the already dry conditions leading to tree death. Scientists assert that a healthy Sierra Nevada forest, resembling the pre Euro-American settlement condition, should have much fewer trees crowded together.
However, some environmental advocates argue that the dead trees do not present a problem.
“From an ecological standpoint it’s not a problem. It’s going to be a benefit from a biodiversity standpoint,” said Chad Hanson, an ecologist at the John Muir Project.
Concerned that the government warning will accelerate logging, conservationists have mobilized to tell a contrarian story of forest history. Hanson authored a letter to Gov. Jerry Brown in August claiming that the forest of dying trees is, in fact, returning to a healthy state.
Aerial view of tree mortality. Credit: U.S. Forest Service Region 5 Flickr (CC BY2.0)
Hanson and other conservationists are fighting a mountain of scientific evidence. The foundation of their arguments is that the pre-settlement Sierra Nevada forest looked dense and full of dead trees, like it does now. Hanson argues that about 15 percent of the trees in California’s “natural” forests were standing dead trees. In this interpretation, a variety of high and low intensity fires burned through the dense forest and created unique habitat. Hanson states that having many dead trees is important to animals like the black-backed woodpecker, which makes a home in these trees.
“This pulse of tree mortality has increased habitat to what it used to be. People are not used to seeing it so they think it represents a problem,” said Hanson.
Their interpretation of the historic Sierra Nevada is precariously supported by a handful of studies, one of which was just refuted by a team of researchers at UC Davis and UC Berkeley.
Historical data by the U.S. General Land Office largely comprises the foundation of the dissenting scientists’ beliefs. The General Land Office, a former federal agency, was responsible for mapping the western US in the late 1800s.
During the mapping process, the western U.S. was marked with a boxed grid, with the finest boxes covering an area of one square mile each. At every corner of the square-mile boxes, a surveyor recorded the nearest tree, its diameter, and the closest tree neighboring that tree. In total, this method recorded about eight trees per square mile across the western U.S. in the late 1860s.
In 2011, an ecology professor at the University of Wyoming, William Baker, used the recorded tree points to infer tree density during the historic surveys. Starting with eight trees per square mile, he used a computer model to estimate from these few points the total amount of trees in every box. Baker concluded tree density in the late 1800s was two to five times greater than other studies had estimated.
Since then, environmental advocacy groups like the John Muir Project have used this historical construction to prop up their arguments against the Forest Service, publishing letters and editorials and starting legal battles. These groups rush to litigate Forest Service proposals to permit tree thinning, such as in the case of their decision to allow logging of damaged trees following the Rim Fire in Stanislaus National Forest.
The problem is, Baker’s use of historical data was never re-tested by other scientists, UC Berkeley ecologist Carrie Levine found. Levine and the other researchers in their study “Reconstructing the Historical Forests of California” re-evaluated the historical data using three different models, including the one used by Baker. Levine and her team judged how effective each method was at taking the historical data and predicting the tree density in real-life, modern forests. They found that the Baker method consistently overestimated tree density.
“It’s important in science for new approaches to be independently vetted by different people to demonstrate that they work well and should be applied across a range of places,” said Levine. “That’s our goal: not to be a voice in the argument but to do good science.”
Levine hopes that her work will have an immediate impact of demonstrating the shortcomings of using the historic data.
Supporting Levine’s findings are a number of studies using historic maps, timber inventories and tree ring data that have lead to the same conclusion: forests are denser and have fewer large trees than they did before fire suppression took off in the 1930s. Even John Muir described the forests of Yosemite as having lots of large, spaced-out trees and expansive open clearings and meadows.
“The consensus is that there are too many trees,” said Christina Restaino, postdoctoral scholar and co-author of the new study. “Even if we for a minute suspend our belief in science, that still doesn’t deal with the current problem at hand, which is that trees are being stressed from drought and climate change.”
The implications of this skewed debate are important. The conservationists that view the forest through this lens are affecting public opinion and policy. Through their letters and litigation, they complicate the ability of the Forest Service to perform even basic safety management. Meanwhile, those on the ground still struggle day-to-day to keep up with the proliferating dead trees that loom close to power lines and homes.
“We can’t deal with them all. We’re stretched as thin as we can get,” said Jim Junette, Groveland district ranger at Stanislaus National Forest.
Dead trees near roads pose a safety hazard. Credit: U.S. Forest Service Region 5 Flickr (CC BY2.0)
To contend with the notion that the dead tree scare is a way to increase logging, forest ecologists and rangers assert that the vast majority of forest land is wildness with no roads and steep, inaccessible slopes. In the southern Sierra Nevada, only 20 to 30 percent of forest lands are accessible to logging. Most wilderness would be unaffected by tree thinning projects.
“The work that’s going on federal lands is almost entirely protecting human lives. We are not going be able to cut ourselves out of the problem,” said Hugh Safford, regional ecologist for the Forest Service and researcher at UC Davis.
Safford contends that a greater part of future management by the Forest Service would consist of re-introducing fire to long-deprived forests, bringing them closer to what most scientists think the forest used to look like.
A final consensus is needed on this matter — not only to mobilize efforts to fell and remove hazardous trees, but to pave the way for future forest management. Currently, several National Forests are updating their Forest Plans, which are long-range planning documents that dictate forest management. These plans may include a shift from suppressing fires to prescribing fires, which could return forests to a less-dense state.
“If we come to a consensus on what the forest looked like, it will be easier to have a starting point for future management,” said Levine.
These decisions will truly shape the future health of the forest.
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