You can actually hear a tree dying.
No, it doesn’t scream in pain as a denim-clad lumberjack joyfully chops its trunk. However, during the increasingly common periods of extreme drought and heat, a tree’s slow desiccation becomes audible through a microphone pressed to its trunk.
“It sounds a little like popcorn popping—little cracks and pops,” says William Anderegg, a biologist at Princeton University.
The process that leads to the crackling noise is one of several that scientists are studying to better understand how trees react to drought and heat. With the loss of millions of trees as global temperatures continue their upward march, this information could help scientists more accurately predict which trees are most in danger, leading to improved climate models as well as better management of forests during periods of drought.
“In just the past several decades, we started to see a lot more of these widespread, drought-driven, tree mortality events,” says Anderegg “That has prompted a lot more concern from scientists to try to figure out what's happening.”
So what makes the snaps and pops? The sounds are the result of a failure in the plant’s xylem, the bundles of tubes that draw water and nutrients from the roots out to the limbs, similar to how arteries replenish the human body with oxygenated blood.
When it’s dry, trees have to suck harder to draw water up from the soil. And if the tension inside these pipes reaches a certain point, the sides give way, allowing in tiny air bubbles.
What scientists’ microphones are picking up are the air bubbles “violently expanding” as they enter the cells, says Anderegg.
Like a human heart attack, these air bubbles blocks the plant’s watery life force from spreading to its drying limbs. But the blockage of a single vein isn’t a death-knell for the tree, explains Louis Santiago, an ecologist at the University of California, Riverside and the Smithsonian Tropical Research Institute.
“Just like we have many veins and arteries, [plants] have many vessels,” he says. “So if a few cavitate under drought, that's probably not a big deal. But if more than half cavitate or more, then you could be heading down dangerous roads."
Embolism is thought to be one of the leading causes of tree death under dry conditions. But plants have a variety of adaptations to prevent them from reaching that critical zone, says Santiago. This slew of adaptations is what scientists are still trying to tease through to determine why some trees cope with drought better than others.
To look at a global picture of these adaptations, Anderegg and his colleagues compiled data on tree mortality from 33 studies of droughts around the world and examined 10 physiologic properties of the affected trees. The study, published this week in the Proceedings of the National Academy of Sciences, suggests that how plants manage water is a telling factor in survival rates.
Part of this boils down to the brute strength of the tree’s pipes. Some trees, like the Utah juniper, have much more hardy xylem and can withstand greater internal tensions than others.
The other important factor is how the trees balance photosynthesizing—taking in carbon dioxide to produce sugar—with drinking. While trees breathe in carbon dioxide, water evaporates through the pores in their leaves, called stomata. When the water dries up, trees close down their pores to prevent water loss. The “cautious” trees that shut down their stomata more rapidly after embolisms start tend to do better in drought, says Anderegg.
The predictive powers for these factors are moderate, but this is not necessarily surprising, considering the diverse group of trees and the range of environments the team was studying. “Ecology is a noisy world—there's a lot of things going on,” says Anderegg. Competition for water, soil type or even characteristics of the drought can all muddy the waters.
There are also many other potentially important factors that can affect tree survival on a local scale, such as root depth. Long roots, for instance, might be able to sip from deep water stores that linger out of the reach of stubby roots.
Trees can also deal with drier conditions by developing green stems, says Santiago. Plants will often lose their leaves when they dry out, halting photosynthesis and growth. But with a green stem, they can continue photosynthesizing even without leaves. Flowering trees in the genus Parkinsonia, which goes by the common name of palo verde or “green stems” in Spanish, are known for having evolved this type of adaptation.
Being able to accurately predict global tree mortality is extremely important for climate models. Trees act like air filters, drawing down roughly a quarter of the carbon dioxide people pump into the sky, storing that carbon in their thick trunks and luscious foliage.
But trees are in trouble. 2015 was the hottest year in over a century—the 39th consecutive year of abnormally hot temperatures. In recent years, droughts have struck parts of Australia, India, Europe, the United States and elsewhere and are expected to become more frequent and severe.
U.S. Forest Service surveys suggest that nearly 12.5 million trees in California alone died from drought in 2014. Such losses are a blow for the planet, because when the trees die, the stored carbon escapes back into the atmosphere. The release perpetuates our problems with greenhouse gasses, ushering in more droughts and more tree deaths, continuing the deadly cycle.