You Can Hear When Trees Are Thirsty, Because They Tell Us

We've all heard the sound of the the wind blowing through the trees. But now, scientists have discovered a way to tell when trees are thirsty...because they tell us.

According to an article in Popsci, at the recent meeting of the American Physical Society, scientists from Grenoble University in France presented research that not only were they able to determine that drought-stressed trees make noise, they were also able to show exactly which process created the sound.

First, lets understand how trees transport water. Trees draw ground water up through specialized tubes relying on forces between water molecules and themselves,  to create a single column of unbroken water in each tube. But as groundwater dries up, the trees must pull harder on the remaining water; if the pressure is greater than the strength of the tubes, the column of water breaks and an air bubble forms. Too many air bubbles can mean death for the tree.

Scientists tried a test "drought" -- and recorded video and sound of the cavitation in the xylem. The researchers discovered that about half of the sounds made by a tree are due to cavitation, and  In the future, the researchers say, forest managers could use a hand-held acoustic device to identify water-stressed trees before permanent damage sets in.