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Climate and Environment

Smog Is Contributing To Pine Tree Deaths -- And That's A Set Up For Huge Fires

A stand of dead trees in the Sierra Nevada in 2016. (Courtesy USDA Forest Service Region 5)
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Back in 1953, high up in the San Bernardino Mountains, foresters noticed something strange. The needles of ponderosa pines were yellowing and dropping, tree growth was slowing, and some were dying. They called the condition "X-disease" at the time, but soon figured out that it had to do with Southern California's notoriously toxic air.

Emissions from cars, trucks, shipping, agriculture, manufacturing, and construction were interacting with sunlight and heat to create toxic clouds of smog that washed over the region, causing not only our lungs to ache, but our pines to suffer.

In the decades since then, we've made big strides in tackling our air pollution problems, but both Southern and Central California still regularly violate federal air quality standards for ozone roughly a third of the year.

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So it should come as no surprise that trees in some of our favorite places -- from the San Bernardino mountains to the southwestern part of the Sierra Nevada -- are still getting sick. They're being made more vulnerable to life-ending conditions, including those brought on by climate change. And in turn, becoming fuel for fires.

"It's something that should be a co-factor in any of the wildfire risk models and it's not," said Nancy Grulke, a research forest ecologist with the U.S. Forest Service, who's been studying the impacts of air pollution on our forests for decades.

Air pollution isn't the sole or primary reason why so many of California's trees have died, but it appears to be an often overlooked contributing factor.


During and shortly after the 2011-2017 California drought -- our worst drought in 1,200 years -- roughly 150 million trees perished, mostly across the Sierra Nevada and especially within Yosemite, Kings Canyon, and Sequoia national park, potentially setting the stage for some of the most destructive and deadly fires in state history. Our mountains in Southern California saw tree deaths as well.

The die-off was linked to a number of factors, including extreme drought conditions and hotter temperatures associated with climate change. Bark beetles also had a field day, spreading between densely packed stands of trees, which were an unnatural phenomenon of their own, a result of successful fire suppression techniques deployed over the past century.


Air pollution also played a role. While it's the drought, bark beetles, and viruses that ultimately do the trees in, it's the smog -- or, more specifically ozone -- that can weaken the immune systems of ponderosa and jeffrey pines, and leave them susceptible to all sorts of issues.


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When excess amounts of ozone flood an area, tiny little holes in the pine needles, the stomata, suck in not only carbon dioxide, but pollution as well, kicking off a cascading series of negative events.

Needles begin to turn yellow, making them less efficient at using the sun's rays to convert the CO2 and water into food.

Healthy pine needles can be seen in the right third. Those injured by ozone are on the left. (Courtesy USDA Forest Service)

The opening and the closing of the stomata becomes more sluggish, making the tree more prone to moisture loss through the tiny holes.

Over time, the tree ends up dropping damaged needles prematurely and rushes to create more. But creating those needles takes energy and water, finite resources. Resources which could instead have gone to the creation of robust root systems that might have made them more resilient to drought conditions.

Their ability to withstand pests also takes a hit.

Healthy, unstressed trees normally respond to beetle attacks by releasing a resin that stops the bug's progress by both poisoning them and pushing them out. Weakened trees can't put up as much of a fight, all but welcoming the beetles to kick up their feet and release pheromones that tell their friends to come and crash.


The trees dying as a result of beetle attacks are often the larger, more fire resilient ones, not the smaller interstitial ones that would naturally be thinned by frequent, low-level burns. Losing them can make it harder for us to cultivate the naturally fire resilient landscapes that we're striving for.

Ozone also makes that mass of excess needles that fall to the ground tougher than they'd normally be, meaning they're slower to decompose, and, in turn, become more of a fire hazard at the bases of trees.

USFS Smith River Hotshots hike past dead trees in Sequoia National Park. (Courtesy USDA Forest Service Region 5)

Excess nitrogen in the soil -- also due to air pollution -- also results in weakened root systems, as it promotes vigorous needle growth and densely packed tree canopies that can make it easier for fire to spread.

When asked if more trees would have survived the rough conditions thrown at them over the past decade if air quality had been better, Grulke said, "That's right."


All we have to do is clean up our air pollution problems and we'll be all good.

Pretty simple. Right?

Forests that haven't been irreparably harmed will recover, at least from the ozone damage, though it'll take some time. However, the issue of nitrogen deposition isn't something that air quality agencies seem to be specifically tackling. So it's unclear if and when we'll get that under control.

In the meantime, forests near areas with high amounts of pollution, like the southwestern Sierra Nevada, will continue to suffer.

"These areas are heavily impacted by air pollution as we speak," said Grulke. "It's getting really terrible there."

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