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California's expansive forests have experienced a profound tree die-off since 2010, exacerbated by a long drought between 2012 and 2015. These pine trees are tough, though, and have evolved to withstand parched years in the drought-prone Golden State. But not drought like this, which was amplified by the planet's relentless, accelerating warming.

"The rules are changing," said Nathan Stephenson, a U.S. Geological Survey forest ecologist who monitors trees in California's Sierra Nevada mountain range.

"It wasn't just dry -- it was warmer," added Craig Allen, a research ecologist with the U.S. Geological Survey.

New research, published this week in the journal Nature Geoscience, illustrates how trees in the state's sprawling Sierra Nevada mountains hung on to life before succumbing to drought. The great forest die-off is tied to a drying up of the deep, deep soil, up to some 50 feet below ground. California trees withstood a lack of rain for two to three years by drawing on this deep-seated water. But, unfortunately for many trees, those last reserves were eventually exhausted, too.

"It was only after this moisture was depleted that the forest became extremely stressed and ultimately began to die off," said Mike Goulden, a lead author of the study who researches terrestrial ecosystems at the University of California, Irvine.

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After the soil dried out, the trees labored through "an increasing stress" phase, which lasted another one or two years, explained Goulden. The trees essentially became feeble. They grew progressively dehydrated, failed to photosynthesize, and couldn't fend off the voracious appetites of bark beetles, which consume the helpless trees' inner tissues.

"The rules are changing."

Eventually, tens of millions of trees died. "The most dramatic die-off occurred four to five years after the drought had begun," said Goulden. A warming climate and unnaturally dense forests (wrought by forest mismanagement) have the potential to amplify this die-off, he noted.

The drought is now technically over (though drought will undoubtedly return). But still, trees are dying. Earlier this year, the Forest Service announced that 18 million more trees met their demise since the fall of 2017.

What's coming...

California isn't out of the woods. In fact, the great tree die-offs may have just begun.

"If it keeps getting warmer, droughts like the one we had can become more frequent and severe," said the USGS's Stephenson, who had no involvement in the study.

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And climate scientists globally expect it to keep getting warmer. For the last 40 years, Earth has experienced an accelerated warming trend. Eighteen of the 19 warmest years on record have occurred since 2000, while all of the planet's coldest-recorded years happened over 90 years ago. Meanwhile, the main driver of Earth's warming, the potent heat-trapping gas carbon dioxide, is now accumulating in the atmosphere at rates that are unprecedented in both the historic and geologicrecord.

Droughts will inevitably return. And they'll inevitably be more potent. Everything will feel the heat. "The warming temperatures amplify drought stress on the plants and the soil," emphasized Allen, who also had no role in the study.

Original image has been replaced. Credit: Mashable

To fend off dehydration, sometimes trees will essentially stop breathing. They'll close off the pores on their leaves, called stomata, to keep the dry California air from sucking out water. "When stomata are closed they cannot do photosynthesis," explained Allen. "They basically start holding their breath," added Stephenson. This means the trees have to start eating up their reserves of energy, until there's almost nothing left.

"They can literally starve to death," said Allen, noting that these trees fall into profoundly vulnerable conditions.

"They can literally starve to death."

And that's when the rice-sized beetles attack.

"Drought sets the foundation for bark beetles," said Stephenson. Even with terrible drought, amplified by climate change, many trees could have still likely persisted under dehydrated, stressed conditions, he said. But with no energy to spend on fighting native bark beetle invasions by releasing toxic resins, the trees are defenseless.

"These trees are really, really resilient," said Stephenson. "But they have their threshold.

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Goulden and his team began observing trees and deep soil at specific sites across the Sierra Nevada a few years before the drought began, allowing the researchers the golden opportunity to follow the trees' mounting stress as the drought progressed. They combined this on-the-ground work with satellites that detected evaporation over the region, which subsequently dried out the area. The picture was clear. Water stored in the deep ground could not outlast the drought.

This bodes poorly for the future. Goulden estimated that for every degree Celsius (1.8 degrees Fahrenheit) of warming, 15 to 20 percent more trees will die during droughts. Already, Earth has warmed more than 1 degrees C since the late 19th century, and with booming fossil fuel use there is no longer any realistic chance that modern civilization will curb warming at even 2 degrees C.

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There are, however, some critical ways to fend off some tree deaths. Like many U.S. forests, California's woodlands are notoriously overgrown, as decades-old forest mismanagement has smothered nearly all wildfires. But not all wildfire is bad. It naturally thins the forest. And fewer trees result in less competition for water.

In practice, this means, in certain swathes of forest, the Forest Service can either intentionally set controlled fires (known as prescribed burning) or use chainsaws and bulldozers to "mechanically" thin the forest. "Fewer straws in the ground" will increase resilience in the forest, explained the USGS's Stephenson.

But the droughts can't be stopped. And humanity is already locked in for continued, and perhaps severe, heating this century and beyond. This about assures a rough road ahead for California's expansive Sierra Nevada forests.

"This drought was potentially a preview of the future," said Stephenson.

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