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First Pines, Now Firs: California's Trees Are Still Dying

Some of the most severe tree mortality was observed in and around Sequoia National Park. (Photo courtesy of National Park Service)
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Remember three years ago when California had its worst drought on record? We might have more water than we did back then, but trees are still suffering. It's just a different kind of tree.

When it all started, it was the pines that were suffering. That's because they live at lower elevations, where the drought hit the hardest. This year, California red and white firs are the biggest losers, and they live much higher up.

That's according to the latest aerial detection survey of the Southern Sierra, which the U.S. Forest Service released this month. Researchers flew over all the national parks and forests from Eldorado down to Sequoia and recorded how many of the 6.5 million acres contain dead or dying trees. Turns out it's 11 percent. If you're counting just the dead firs, that number would be nearly 10 percent.

Jeff Moore manages the regional aerial survey program. He said tree mortality has two main causes: drought and bark beetles. And, yes, the drought is over, but the beetles are still here.

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"Trees at lower elevations have already been heavily impacted by the drought," he said. "Now the beetles are just trying to find whatever's leftover, and often times that's in higher elevations."

Bark beetles love droughts, because it weakens the trees. That makes their job of killing trees and reproducing inside of them much easier. So the population grows.

"Then those populations of beetle continue to grow and have more of an effect with trees higher up. And when you have lots and lots of beetles, then they can still take out even healthy trees."

That's why firs at higher elevations where the drought didn't hit as hard are now suffering. Tree mortality could be getting better, but the beetles can make the effects of the drought last after drought conditions are gone, said ecologist Adrian Das of the U.S. Geological Survey.

"There might be some indications that if we continue to get more wet winters, that the mortality might be petering out. But it's not uncommon to expect lags," he said. "This drought was one of the most severe in at least a couple centuries."

Das calls this period after the drought an outbreak phase for the bark beetles, where they are abundant enough to start killing trees that are otherwise healthy.

But Moore said there's a silver lining in the thinning forests.

"The drought has ended," he said. "So that has allowed the surviving trees to become healthier, probably healthier than they were before the drought because they have less competition."

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