National Geographic News
Single tree on hillside, late afternoon.

A tree stands alone in the drought-stricken Salmon-Challis National Forest, Idaho, in an undated picture.

Photograph by Pete Ryan, National Geographic

Gabe Popkin

National Geographic News

Published April 15, 2013

When drought hits, trees can suffer—a process that makes sounds. Now, scientists may have found the key to understanding these cries for help.

In the lab, a team of French scientists has captured the ultrasonic noise made by bubbles forming inside water-stressed trees. Because trees also make noises that aren't related to drought impacts, scientists hadn't before been able to discern which sounds are most worrisome. (Watch a video: Drought 101.)

"With this experiment we start to understand the origin of acoustic events in trees," said Alexandre Ponomarenko, a physicist at Grenoble University in France, whose team conducted the research.

This discovery could help scientists figure out when trees are parched and need emergency watering, added Ponomarenko, who presented his team's results last month at an American Physical Society meeting in Baltimore, Maryland.

Listening to Trees

To figure out how to listen to trees, the French scientists drew on their knowledge of how trees take in water—essentially by drinking from a really long "straw."

Inside tree trunks are bundles of specialized tubes called xylem, which rely on the attractive forces between water molecules as well as those between water and plant cells to lift liquid to the highest leaves and branches. (See National Geographic's tree pictures.)

Because trees are so tall, the liquid in the xylem can be under intense pressure—many times that of the atmosphere around us—but the attractive forces between neighboring water molecules keep the water column intact.

Imagine using a straw to slurp the last few drops from the bottom of your glass: You have to increase the pressure even more. In drought-stricken trees, this increased pressure can cause the water column to break, allowing dissolved air to form bubbles that block water flow.

These events are called cavitations, and while trees can withstand some, too many can be deadly.

Since cavitations can kill trees, scientists and forest managers want to know when they are increasing. (Also see "Pictures: Saving and Studying Tasmania's Giant Trees.")

Scientists have known for decades that microphones can pick up the noises that cavitations make. But because they couldn't see inside the tree, they weren't certain of the origins of these sounds, which could have resulted from wood creaking or breaking or xylem cells collapsing.

To answer the question, the team put a thin slice of pine wood into a liquid-filled gel capsule to mimic conditions inside a living tree.

The scientists then evaporated water from the gel, simulating a drought. As the wood began cavitating, the scientists filmed bubbles forming while recording with a microphone.

The scientists found that around half the sounds they picked up were associated with cavitations. The rest were from other processes, such as bubbles invading neighboring cells. Most important, the sound waves from each type of event made a distinct pattern. All of them are above the range of human hearing.

The researchers think they can compare sounds from living trees with these patterns, and determine which processes are creating the sounds.

Helping Thirsty Trees

According to Ponomarenko, the findings could lead to the design of a handheld device that allows people to diagnose stressed trees using only microphones.

Such a device may be particularly important if droughts become more common and more severe, as many global warming models predict they will. (Read "The New Dust Bowl" in National Geographic magazine.)

In fact, a study published in Nature last fall suggested that trees in many places—from tropical rain forests in South America to arid woodlands in the U.S. West—already "live on the edge," meaning their cavitation rate is almost as high as they can sustain.

Ponomarenko's method could provide an early warning that cavitations are increasing.

For instance, he envisions a device that would attach to a tree and constantly listen for sounds of thirst. If needed, the device could then trigger an emergency-watering system.

Ponomarenko's research is promising, added Cornell University's Abe Stroock, whose lab designed the gel capsule the French team used. He said the result "opens a new mode of observation" into cavitation. (See pictures of the 2012 drought that parched much of the United States.)

But he also noted that the wood samples used in the team's study had to be "excised and abused," so they don't necessarily behave exactly like wood in a living tree.

"Translating [these findings] to a living plant and into different species is a lot of work, potentially," he said.

D Berry
D Berry

spectacular, i am currently looking into Bioacoustical controls for the potential of invasive pest control

Nursery Manager

The Huntington Botanical Gardens

San Marino, California

Gail Zawacki
Gail Zawacki

It would be really nice if they could figure out what sounds the trees make when they are forced to inhale air pollution, which is what is killing them.

The drought is the secondary impact.  Trees are dying in places that have become wetter from climate change, as well.  The primary reason is that they are extremely sensitive to air pollution.  You can't see ozone, because it's invisible just like oxygen, nitrogen and CO2.  But the persistant background concentration is inexorably rising, as precursors from burning fuel and agricultural fertilizers traverse across oceans and continents.

The level is now so high that virtually all species of trees, of all ages, in even remote locations are dying prematurely at an exponential rate.  Decades of research have demonstrated that the first impact is shrunken roots, making trees more vulnerable drought and wind.  Lowered immunity to biotic pathogens - diseases, insects, and fungus - are the more obvious sources for tree death, but the are opportunistic, following upon air pollution damage.

The USDA knows that ozone reduces the quality and yield of annual agricultural crops by billions of dollars every year.  Imagine what cumulative exposure does to trees, season after season.

check for links to scientific research.

Samantha Cooke
Samantha Cooke

It would be interesting to listen for sounds in the TickleMe Plant which will close its leaves and lower its branches when tickled by reducing the osmotic pressure in its cells. I have grown these  plants from online kits for years and it is easy to see this plant calling for water as shuts itself down like an animal going to sleep when the soil is dry.

Charles Greenlaw
Charles Greenlaw

gives a whole new meaning to " If a tree falls in the woods when no ones around does it make a sound?" 


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