Trees Share Some Survival Secrets Through Ultrasound

By Lynne Lescott

Trees give us shade on hot days and tasty fruit to eat and they help the planet, but did you know trees can also eat, drink, and get stressed out like we do? Environmental scientists have long studied how trees grow and survive. As the effects of climate change become more intense, scientists are focused on how trees use their resources to survive drought and other environmental conditions.

How It Works

Over the years scientists have learned that trees have channels like plumbing pipes that help them eat and drink. These phloem channels send nutrients created by photosynthesis from a tree’s leaves down to its storage organs and new growths. The xylem channel “sips” water and minerals from soil, then pushes it to the tree’s roots and upward.

Dry spells change how trees drink. During dry spells the xylem starts to gulp water instead of sipping it, like we gulp water on hot, dry days. When trees gulp instead of sip, embolisms, or air bubbles, can flood their channels. If a tree’s environment stays dry for too long the embolisms can build up, plug the channels, and prevent the delivery of water and nutrients to different parts of the tree.

Environmental scientists call this drought stress, and it can be bad news for trees when dry conditions persist.

Studying Drought Stress

A team of ecophysiologists at the University of Innsbruck in Austria set out to learn how mature trees manage resources during a drought, and if they bounce back after. The team also wanted to compare trees under drought stress to trees experiencing natural conditions. They couldn’t bring fully grown trees into their lab or wait for a drought, so the team set-up an in vivo experiment, a study of whole living organisms, in Germany’s Kranzberg Forest.

The team designed a five-year study that simulated a long drought in nature. They created drought conditions by building stands with removable roofs over some of the trees, which prevented rain from falling directly on the trees and soil beneath. The roofs were removed each winter and added back each spring, but the stands stood steadily for five years.

All the trees in the experiment were monitored for five years. One data point measured was the embolism level of each tree. The team placed ultrasound sensors on tree trunks to capture signals from drought-condition and natural-condition trees. The signals were created by waves bouncing off of the embolisms. The data revealed that the drought-condition trees generated more signals than natural-condition trees, indicating they had more embolisms. Increased embolism levels meant the trees’ overall resources were stressed.

Protecting Trees and the Planet

The good news? The trees bounced back when the research team re-irrigated the dried soil to help restore natural conditions. They monitored the trees’ photosynthesis rates, embolism levels, and soil health for several post-drought seasons.

As climate change produces effects faster than the environment can adapt, the Kranzberg Forest experiment and other research can help environmental scientists learn, and inform strategies that help protect the world’s resources.