Climate Change Stress Killing Forests, and Why it Matters

By Michael D. Lemonick

Forests cover some 30 percent of Earth’s surface, and it’s hard to overestimate how crucial they are to the functioning of the planet. Forests provide shelter for uncountable numbers of species, hold soil in place that would otherwise wash away, pull excess carbon out of the atmosphere, absorb and re-emit water at such a rate that they literally control the weather, and serve as an economically vital natural resource.

All of those functions have long been endangered by human activities such as excessive logging and clear-cutting to open new agricultural land. But another factor, increasingly, is the stress of climate change — in particular, the higher temperatures and more frequent and intense droughts that human-generated greenhouse gases have begun to trigger. Now a new paper, released Sunday in Nature Climate Change, has attempted to lay out just how climate stress affects forests, and how serious the consequences of could be.

Credit: Paul Hennon/USFS

“This is the first snapshot of how these things fit together,” said lead author William Anderegg, “So we don’t have a lot of final answers yet.”

What they do know is that tree deaths go up during periods of excessive heat and drought. Some of the biggest of these die-offs have happened in the western U.S. in recent years, among pinon pines forests in the Southwest, for example, and trembling aspens in both the U.S. and Canada, and lodgepole pines and spruces in the Northwest — but they’ve also been documented on every continent except Antarctica.

“It’s a very important type of forest mortality,” Anderegg said, “And we expect it to become more common.” 

Until recently, however, ecologists hadn’t really focused on drought-induced die-offs as a discrete category of forest trauma. That began to change, Anderegg said, after a meeting in Austin, Texas, last year. “Several of us decided to sit down, put our heads together and begin to look at the possible effects.”

They looked at dozens of individual studies, and found plenty. The loss of a forest’s dominant tree species has a ripple effect on all the other species that live there, by changing the amount of sunlight that reaches the forest floor; changing the mix of nutrients that enter the soil as leaves or needles decompose; allowing soil to wash away, especially on steep slopes, and — in some cases, at least — encouraging more fires.

Forest die-offs also impose an economic hit on loggers and those who depend on income from hikers, campers and others who use forests for recreation.

“One of the most interesting studies we looked at,” Anderegg said, “looked at the negative effect on real estate values in areas with forest loss.” 

Forests are not only affected by climate; they also affect it. A living tree absorbs carbon from the air; kill the tree, and the carbon stays in circulation to trap heat. Not only that, as the tree decays, the carbon locked inside is gradually released back into the atmosphere. “It’s a double whammy,” Anderegg said.

On the flip side, leaves and needles are relatively dark, so they absorb solar energy. When a tree dies, they fall, exposing the ground below — and if that ground is light in color, more energy gets reflected. In a case like this, tree death can actually work to counteract global warming. “In a northerly forest where there’s snow on the ground,” said Anderegg, “this can be a big deal. In temperate forests, it’s not so much.” 

The loss of trees also changes the amount of moisture in the air, which could potentially affect rainfall, although Anderegg and his co-authors write, “so far no studies have examined this.”

That’s not entirely true: a study just published in Nature argues that projected loss of tropical forests by 2050 could reduce rainfall in the Amazon basin by up to 21 percent — but Anderegg and his co-authors couldn’t have known this was coming. Broadly speaking, there are huge gaps in what scientists know. “This whole area has been fairly under-studied until now,” Anderegg said. “We need more research with a really wide net.” 

To try and coordinate it, he and several colleagues have created a collaborative website to share knowledge about drought and tree mortality. The urgency of such research is only underscored by the 2012 drought, the worst to hit the U.S. in more than 50 years.

“The droughts of the early 2000’s caught us by surprise,” Anderegg said. “This one is our chance to pay attention as it unfolds.”