Forest managers have never seen anything like it. Across California, an astounding 102 million trees have died over the past six years from drought and disease — including 62 million trees in 2016 alone, the US Forest Service estimates. Once-mighty oaks and pines have faded into ghastly hues of brown and gray.

The biggest worry is that these dead, dry forests will become highly combustible when California’s annual fire season rolls around next summer. The south and central Sierra Nevada regions, where most of the dead trees are located, are at particular risk of severe wildfires:

So how did we get to this point? “When you’re talking about tree mortality, it’s a whole bunch of things linked together,” says David Rizzo, chair of the plant pathology department at the University of California Davis. “The drought is important, but you also have to look at land-use and management decisions that go back a long time.”

Why California’s trees are now dying in record numbers

Let’s start at the beginning. Centuries ago, before California became thickly populated, small wildfires used to course periodically through the Sierra Nevada region, thinning out the pine and conifer forests and rejuvenating the ecosystems.

Over the past 100 years, however, humans have largely suppressed those small fires to protect homes and cities that have been built nearby. That means modern-day forests are much thicker and denser than they ever were historically. And that’s terrible news when drought comes along, Rizzo explains, because it means there are more and more trees and plants competing for scarce water.*

Today, California is in the midst of a historically brutal six-year drought — exacerbated by unusually warm summer temperatures. Those dense forests are particularly water-starved. They’ve become weak, susceptible to disease and other afflictions.

And now along comes the bark beetle, which loves to prey on species like oak and pine.

In normal years, a healthy tree can fight off bark beetles trying to squirm into its bark by producing pitch that drives them out. But drought-weakened trees can’t muster a proper defense. Once the beetles drill in, they start hatching larvae that eat away the tree’s system for transporting nutrients, and spread fungus that inhibits sap production. This beetle problem is made worse by global warming: As temperatures rise, more bark beetles can survive the winter, driving population growth.

And there’s more: As Rizzo and his colleagues discovered earlier this year, a separate fungal pathogen introduced into California in the 1990s has also started killing millions of oak trees along the coast, from Monterey County on north. The disease, known as “sudden oak death,” creates ugly cankers on the tree trunk that bleed out sap — a fatal condition for some trees.

Put that all together, and you have an ugly situation across the state. “The scale of die-off in California is unprecedented in our modern history,” Randy Moore, a forester for the US Forest Service, told the Los Angeles Times. The agency notes that tree mortality will remain high in 2017, particularly in dense forests affected by root disease and bark beetles.

The biggest near-term danger from dead trees: wildfire

There are plenty of reasons to worry about dead trees, but the most pressing is wildfire. California is just emerging from an especially brutal fire season, with the Blue Cut fire in San Bernardino Country alone forcing 85,000 people to evacuate. And these millions of dry tree corpses are an ominous sign for next summer, as well.

The extent and severity of California’s fire season is driven by many factors: wind, rain, temperature. But mass tree die-offs can be a problem for two reasons, says Brandon Collins, a research scientist with UC Berkeley's Fire Science Laboratory. The dead trees lower the moisture content of the forest, and when their leaves and needles fall to the ground, that adds to the surface fuel that usually drives wildfires in places like the Sierra Nevada. Not only can wildfires spread faster, but there’s a greater chance that entire trees will go up in flames (known as “torching”) — making fires more severe.**

This is particularly a problem in the south and central Sierra Nevada regions, which include populated areas like Fresno and Kern Counties. In some patches, more than 50 to 80 percent of trees have died. “It’s just eye-catching,” says Collins, describing the scene. “If it was just one out of every five trees dead, then okay, a few trees might torch, but you wouldn’t get really big runs. But the problem is when those dead trees are concentrated in large patches.”

Collins says that federal and state agencies typically manage to suppress about 97 percent of all ignitions. But the small handful that get out of control are the ones to worry about. In August, the Blue Cut fire in Southern California erupted on a 100 degree Fahrenheit day that featured 30 mph winds — it quickly spread and ended up burning 37,000 acres and destroying 105 homes. That’s the situation everyone’s worried about next year.

So what can California do about all these dead trees?

Now, one way to combat the immediate fire risk would be to go in and remove many of those dead trees. But that’s much harder to do than it sounds. Private companies are unlikely to go in and grab those trees: There are simply no longer enough mills in the region to drive demand for all that extra wood, says Collins. (And the trees would have to be grabbed quickly, before they start rotting.)

Meanwhile, state and federal budget agencies don’t have the funds for tree removal at this massive scale. Each year, wildfires keep getting bigger and costlier to tackle (in part because more people live near fire zones, which means more houses to protect). So, each year, the US Department of Agriculture is devoting more and more of its limited budget to fighting existing fires rather than preventing them in the first place.

“Until Congress passes a permanent fix to the fire budget,” said Agriculture Secretary Tom Vilsack in a statement on November 18, “we can't break this cycle of diverting funds away from restoration work to fight the immediate threat of the large unpredictable fires caused by the fuel buildups themselves." (See more on that budget battle here.)

A more lasting solution, meanwhile, would be to prevent so many trees from dying in the first place. Rizzo points out that this would require coming up with a comprehensive plan to deal with everything from fire to bark beetles to disease.

But to take just one aspect here: Remember, one of the underlying causes of this die-off is that California’s forests have become incredibly dense — when drought hits, there are too many trees in need of already scarce water. So one idea, Collins says, would be for forest managers to go in and manually thin out the forests. In California, though, this runs the risk of adversely affecting key endangered (and legally protected) species like the Pacific fisher and California spotted owl.

A more elegant — but also more controversial — idea would be to bring back the periodic small wildfires that were a mainstay of the forest landscape centuries ago. Carefully managed prescribed fires could thin out the forests and be good for wildlife. We’d be restoring forests to something closer to their earlier, healthier state.

The hitch here, says Collins, is that prescribed fires are incredibly tricky to pull off in dense forests that have been allowed to build up for many decades. If you want to intentionally set a small fire that doesn’t spiral out control, you need perfect weather conditions — not too windy, not too hot, neither too wet nor dry — that don’t come along very often. Second, Southern California already has awful air pollution problems, and the smoke from prescribed fires could cause areas like the San Joaquin basin to run afoul of federal pollution limits.

Still, in recent years, more and more fire ecologists have argued that prescribed fires will eventually have to become part of the solution in dealing with wildfires. (See here for more on the complexities of that.) The western forests we’ve shaped over the past 100 years have become eerily warped, as this year’s tree die-offs show. And, particularly as global warming makes the region hotter and drier, the price we’re paying will become increasingly untenable.

Further reading

Note that I focused on California in this piece, but dead trees and wildfires are a problem throughout the West. Here’s a recent piece on house bark beetles are decimating forests in the Rocky Mountains.

A more in-depth look at the ins and outs of using prescribed fires as a way of restoring forest health and preventing large wildfires.

Here’s a look at how budget constraints have made the US unprepared to deal with wildfires.

Footnotes

*Back in the 1990s, Rizzo and Patricia Maloney compared the Sierra Nevadas with the comparable fir and pine forests of Baja Mexico — which, at the time, had never endured the same fire suppression tactics. During drought, the Mexican forests were far more resilient.

**Interestingly, Collins points out, dead trees don’t always mean worse wildfires. Up in the subalpine forests in the Rockies, there’s little evidence that tree deaths driven by bark-beetle infestations increase wildfire risk. That’s likely because this region has a different fire regime than the Sierra Nevada region does — with more infrequent, high-severity fires that spread from tree crown to tree crown. In that situation, when a tree dies and its crown falls to the ground, there’s actually less fuel for these particular fires.