Forests taking longer to recover from drought due to hotter temperatures, study says

Gov. Jerry Brown may have declared the drought emergency over in April, but don’t tell that to California's plants and trees. According to a new study, not only do the effects of drought on the environment linger after it starts raining, but this “drought recovery” period is lasting longer than ever before, likely because of climate change.

Scientists with Woods Hole Research Center, University of Utah and other institutions found that trees often don’t return to their pre-drought growth rates -- even after rain and snow return.

“Just because the rains come back doesn’t mean the ecosystem is functioning,” said lead author Christopher Schwalm with the Woods Hole Research Center in Massachusetts. His new study was published in Nature on August 10.

As global temperatures rise because of  climate change, plants and trees will take longer to recover from drought because they can't store as much water. 

When trees have sufficient water, they perform photosynthesis, turning sunlight and carbon dioxide into fuel for themselves. When water is limited, trees do one of two things: they slow photosynthesis and go into a sort of “starvation mode,” or they try desperately to continue sucking water up through their roots. Sometimes, trees suck so hard they can pull in air bubbles, which damages their plumbing.

It can take a while to repair that damage, even after it starts raining again, said ecologist Malcolm North with the US Forest Service, who was not involved with the study.

And the hotter it is immediately following a drought, the longer it can take trees to recover. That’s because photosynthesis slows down in warmer temperatures, according to the study. 

In addition, the warmer it is, the more water trees need, said Adrian Das, an ecologist with the US Geological Survey in Sequoia and Kings Canyon national parks. He was not involved with the study.

“Warmer temperatures mean for the same lack of precipitation, you’re getting a more severe drought effect," Das said. "So it’s not surprising that you’re going to have longer recovery times.”

If another drought hits while trees are still recovering from the first one, they may not be able to handle it.

“If you’ve been sick, and you get hit right again with another illness, the effects of that second illness might be more severe than they would have been if you hadn’t been sick before,” Das said.

Researchers are already starting to see waves of droughts hit the Amazon in short succession, according to the paper. And more of the world is in "drought recovery" than ever before.

“I hate to say it, but it’s a very scary paper,” said ecologist Malcolm North with the US Forest Service and UC Davis who was not involved with the study. “Anytime you have disturbances, but you accelerate frequency of them so there are very short intervals between them, that’s a very ripe situation for [the conversion of forest to shrubland].”

North said he’s already starting to see this in Southern California, where wildfires in quick succession are accelerating the transition from lowland forest to chaparral. He said the Woods Hole study shows multiple droughts could also cause the eventual loss of forest in places like the Southern Sierra Nevada, where millions of trees have already died because of the recent drought.

Schwalm said droughts are often accompanied by a host of other stress factors for forests. Bark beetle, wildfire, air pollution and logging also stress  trees, especially when they occur while a forest is still recovering from or experiencing, drought.

With so many stressors happening all at once, the study authors predict that “a chronic state of incomplete [drought] recovery may become established over the remainder of the twenty-first century” leading to even more tree mortality and loss of forests.