Someday soon, you'll be able to ask your trees whether they need more water... and they'll answer you.
They won't use human voices, rather the bubbles that form insides of water-stressed trees create a unique ultrasonic sound that is detectable with the a carefully tuned microphone.
To figure out how to listen to trees, the French scientists drew on their knowledge of how trees take in waterâ€”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 pressureâ€”many times that of the atmosphere around usâ€”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.