California has just emerged from two back-to-back years of record-setting wildfires, including the Camp Fire, the state’s single most deadly and destructive blaze on record, which killed at least 86 people in October 2018.

On Monday, the state received a fresh warning sign of why the risks of massive, devastating blazes like it are growing.

According to the US Forest Service’s latest aerial survey of federal, state, and private land in California, 18 million trees throughout the state died in 2018, bringing the state’s total number of dead trees to more than 147 million. The concern is these trees could be matchsticks for another conflagration, or that the decaying timber could maim a hiker, a ranger, or a firefighter.

The 2018 results actually represent a decrease in tree deaths compared to 2017 and 2016. But they’re still far above what’s considered typical. “Normal background levels of tree mortality for California, what we would typically see through both insects and diseases, is well less than a million trees per year,” said Sheri Smith, a regional entomologist at the US Forest Service.

So why are so many Ponderosa pines, Douglas firs, and quaking aspen in California’s forests dying? There’s no single reason — but the combination of years of drought, extreme heat, and bark beetle infestations are causing trees to splinter and wither. You can see how this wave of tree deaths has spread since 2014 through private, state, and national forests in California in this GIF:

And if high temperatures and dry weather — the weather conditions that ramped up fire risk in 2018 — recur this year, the state could face an even larger forest fire than any seen before.

“Everybody is kind of on the edge of the chair see what comes next,” said Alicia Reiner, a fire ecologist at the USDA Forest Service Enterprise Program.

Trees have been dying en masse for years in the West

Several trends have converged to kill off these trees. More than a century of suppression of natural wildfires has allowed forests to grow more crowded with trees, brush, and plants than they would if they were allowed to burn.

Between 2010 and 2017, the state of California was parched in an epic drought. As a result, this densely packed vegetation quickly dried out. Smith explained that the drought was especially devastating for trees standing close together. “There’s not enough water to go around when you have really dense forests,” she said. The summer of 2018 also turned out to be a dry season as temperatures surged into triple digits in many parts of the state.

And the end of a drought doesn’t necessarily reduce fire risks either. Rainfall can’t resurrect a dead tree, but it can lead to more fast-growing grasses and shrubs, which can then dry out in the summer heat, becoming tinder for wildfires.

“It is a trade-off,” Reiner said. “In the wetter years, you can get an extremely high loading of grasses in the lower elevations. However, it does kind of dampen the fire behavior in higher elevations because it makes the season effectively shorter.”

Though many trees can withstand drought to an extent, it makes them vulnerable to other threats. In California’s case, it was bark beetles. Trees defend themselves against these pests by producing a resin that traps and kills the critters as they try to burrow through the bark. “The resin production is almost completely based on water availability,” said Smith. Little water means little resin, which means little resistance against beetles, which are also appearing in larger numbers due to climate change. Warmer winters mean that more beetles survive the cooler season, leading to more abundant numbers in the spring.

But more dead trees doesn’t always lead to more or larger fires. Several studies have shown that dead trees have not increased fire size or frequency in some western forests. That’s because the mechanisms that drive fires are different depending on the region and can vary depending on the altitude in a given area. “In ecology and fire, nothing is one-to-one,” said Reiner. “Ecology is diverse in California, and fire is too.”

So it’s important to monitor the backdrop of other factors that affect fire risk, like temperature, moisture, elevation, and ignition potential. All these variables were optimal for massive fires over the past two years.

People are driving up the risks from wildfires, but they can reduce those risks too

All in all, it’s clear that deliberate decisions and actions by people have made wildfires not just in California but throughout the West larger and more dangerous.

One reason forest managers have suppressed natural fires is that people are increasingly living closer, if not directly in, fire-prone areas. This has increased the likelihood of a power line, a barbecue grill, or an errant spark triggering an inferno. Proximity to forests is also what’s driving the increasing damage tallies from these fires. The Camp Fire, for instance, racked up nearly $13 billion in losses. The Thomas Fire in 2017 torched the densely populated Ventura and Santa Barbara counties, leading to at least $1.8 billion in damages.

Growing populations have also made it more difficult to conduct prescribed burns. Wildfires are a major air quality hazard and have led to some of the worst breathing conditions in the world for millions of people. That’s why deliberately ignited fires have to be managed carefully to mitigate pollution that can span hundreds of miles.

The rising concentration of greenhouse gases in the atmosphere is also leading forests to dry out in the West and has helped stretch California’s wildfire season almost all the way through the year.

But humans also absolutely have the ability to reduce fire risk. Active forest management through deliberately starting fires, removing trees, and strategically planting new ones can drastically reduce fire risks in some areas. Cutting down 149 million trees is an expensive proposition, but some of the higher-risk husks can be strategically removed to protect homes. Codes that limit construction in fire-prone regions can reduce the amount of real estate in harm’s way.

Last year, the Forest Service conducted restoration work across 313,000 acres, including 63,000 acres of prescribed burns. In California, state officials convened a tree mortality task force in 2015 to try to get a handle on all the dead trees. California Gov. Gavin Newsom last month proposed $105 million in new funding to cope with past wildfires and prevent new ones, on top of $200 million that state lawmakers approved last year.

Right now, forest researchers say the snowpack in the Sierra Nevada mountains looks abundant, meaning that there will likely be plenty of water to go around during the spring and summer. So the number of trees that will die this year is likely to drop further.

“The trend is declining, and as long as we have good winters and continue to increase the pace and scale on active [forest] management, I would expect our tree mortality numbers to continue to decline,” Smith said.

Nonetheless, California’s forests are in a period of intense change as they recover from fire, drought, and human development. It presents something of a blank slate for forest managers to cultivate a healthy mix of trees with different ages, species, and densities to accommodate recreation and timber harvesting while reducing wildfire risk. The question is whether enough money and time is allocated to manage them appropriately.

“What those forests are going to look like in the future, I don’t think any of us are sure,” said Smith.