Trees call for water when they are thirsty

When we are dehydrated, it means that our bodies do not have enough water to carry out normal tasks. Then thirst is appearing as the main symptom. To the other symptoms of dehydration, we can include headache, dry mouth or feeling tired or lightheaded. These symptoms make us wanting water. Have you ever been thinking how trees behave when they are running out of water? Read more what scientists from France discovered.

Already in 1966, scientists were recording strange ‘clicks’ in the audible acoustic range coming from trees. Twenty years after, the ultrasonic emissions have been evidenced within wood structures, and it was established that ultrasonic emissions are more frequent under water stress conditions.  However, since then, there has not been any direct evidence of the exact origin of ultrasounds that are coming from trees.

Researchers from France decided to examine more accurately the question of origin of ultrasound emissions in their lab experiment. They developed a new method that allowed them to record simultaneously both acoustic emission and optical observations of thin slices of wood.

In order to monitor functional xylem conduits they used 50 micrometers thick slices from fresh xylem samples of Scots Pine (Pinus sylverstris L.). The samples were around 5 millimeters long (0.2 inches). Next, they placed the samples in a hydrogel, where they experienced negative pressure as water evaporated from the hydrogel. In addition, they installed a camera on optical microscope, which recorded images of bubble development (black conduits on image b) in water columns (grey on image b). Simultaneously, the microphone was recording the ultrasonic signals. The experiment set-up is presented below.

experimentFigure. Experiment set-up: (a) the water contained in the wood inclusion in hydrogel underwent negative pressure as water evaporated from the hydrogel. The camera installed on an optical microscope with magnification between 1.25 and 20 recorded images (b) of bubble development (black conduits on image) in the water columns (grey on image). (c) Simultaneously, the microphone monitored the ultrasonic signals. Source: Ponomarenko et al. (2014)

The results were astonishing. During the water stress, air bubbles appeared suddenly in conduits. They were detected by image analysis. A second major finding was the detection of all ‘acoustic events’ in the ultrasonic range. Recordings showed that a sound was always synchronized with a bubble appearing at the millisecond time scale. This result clearly indicated that the ultrasound emission was correlated in time with bubble nucleation.

The research leader Philippe Marmottant told the Huffington Post in 2013:

“We can track the articulation of bubbles, and what we found is the majority of the sounds that we hear are linked to bubbles. I say majority, because there may be other causes like cracks in the wood or insects. But the majority of sounds that occur during cavitations (tiny air bubbles that pop out in the water) are due to these bubbles.”

The most incredible for me is that when trees are suffering in drought conditions, they make “ultrasonic pops” that are 100 times faster than the human ear can hear. Therefore, walking in the woods, we are not able to hear fully all noises of nature.

Recently, I have found one good solution practiced in Estonia. Photo below presents a big megaphone located in forest, build out of wood, thanks to which people can hear better the sounds of nature. Only three of such megaphones can be found in Estonia. Maybe we will not hear air bubbles, but at least we can have better perception of other sounds of nature, that are within our human acoustic range.
Congratulations for great and innovative idea!


Source: Ponomarenko A, Vincent O, Pietriga A, Cochard H, Badel E´, Marmottant P. 2014. Ultrasonic emissions reveal individual cavitation bubbles in water-stressed wood. J. R. Soc. Interface 

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