Forests Are Warning Us About Drought, and the Signal Is Hidden in Their Trunks

Step outside on a summer afternoon and look at a tree in your yard or on your street. It seems still. Solid. Unmoving.

But inside that trunk, something is happening. Every single day, the tree is shrinking and swelling in a steady rhythm that scientists can now measure with remarkable precision.

Researchers have discovered that trees show a daily pattern that resembles a pulse. It is not a heartbeat. Trees have no hearts. But their trunks contract during the day and expand again at night, following the movement of water through their tissues. This pattern is measurable, repeatable, and grounded in more than a century of plant physiology research.


What Happens Inside a Tree When Morning Light Hits

The cycle begins at sunrise. As light reaches the leaves, tiny openings called stomata begin to open. They allow carbon dioxide to enter so the tree can perform photosynthesis. At the same time, water escapes into the air. This process is known as transpiration.

More than a hundred years ago, scientists Henry Dixon and John Joly described how this evaporation creates tension inside the tree’s water-conducting tubes, known as xylem. Their cohesion-tension theory explained that water molecules cling to each other and to the walls of these tubes, forming a continuous column pulled upward from roots to leaves.

As sunlight strengthens during the day, the pull intensifies. Water is drawn upward and also pulled from elastic storage tissues in the trunk. Because these tissues are flexible, the trunk narrows slightly. The change is tiny, often just a few micrometers, but it is consistent.

When night falls, the process slows. Stomata close. Transpiration drops. Roots continue absorbing water from the soil. The trunk refills and expands again.

Plant physiologist Ulrich Lüttge described plant water balance as a dynamic system that constantly adjusts to environmental demand. What we call a tree’s pulse is the visible expression of that adjustment.

How Scientists Measure Something So Small

Measuring these movements required extremely sensitive tools. That is where dendrometers come in. These devices are attached to trunks and record minute changes in diameter around the clock.

Research led by Andreas Zweifel showed that daily stem shrinkage reflects water withdrawal from storage tissues, while long-term expansion reflects growth. His studies demonstrated that trunk measurements can reveal internal water stress before any outward signs appear.

At the same time, scientists use sap flow sensors to measure how quickly water moves upward through the tree. Many of these sensors are based on thermal methods developed by Jean Paul Granier. By combining trunk measurements with sap flow data, researchers can track the full daily cycle from morning water pull to nighttime recovery.

Today, these tools are used across forest monitoring networks in North America and Europe.

When Shrinkage Becomes a Warning Sign

The most powerful insight from this research is what it reveals about drought.

As the soil dries, trees must generate stronger tension to lift water. This increases daytime shrinkage and reduces nighttime recovery. If the trunk does not fully expand by morning, it can signal mounting stress.

When water columns inside the xylem break and air bubbles form, a process called cavitation occurs. Research by William Anderegg has shown that vulnerability to hydraulic failure strongly predicts which species survive severe drought. His work connects water transport limits to large-scale forest dieback during extreme heat events.

A global study led by Brendan Choat found that many tree species operate close to their hydraulic safety margins. That means an intensified drought can quickly push them toward failure.

Because trunk diameter responds immediately to water imbalance, dendrometers act as early warning systems. They can signal trouble days or weeks before leaves begin to wilt.

Why This Matters as Heatwaves Become More Common

As temperatures rise and rainfall patterns shift, understanding tree water dynamics has become urgent. During heatwaves, the air demands more moisture. Trees respond with stronger daytime contraction. Repeated stress without full recovery can reduce growth and increase mortality risk.

Ecologist Nate McDowell has emphasized that hydraulic stress is a central link between drought and tree death. Continuous stem measurements allow scientists to quantify that stress in real time.

Long-term forest studies now use these measurements to predict forest vulnerability, guide orchard irrigation, and evaluate species suited for hotter climates. Urban planners can also use this knowledge when selecting trees for city streets that must withstand future heat.

A Quiet Daily Rhythm We Are Only Beginning to Hear

The idea of a tree having a pulse feels almost human. But what scientists are measuring is pure physics and biology. Water rises because evaporation pulls it upward. Elastic tissues respond to internal tension. The trunk narrows and expands in response.

Every sunrise triggers contraction. Every night allows renewal.

What appears still from the outside is actually a living system in constant motion. By listening carefully to these subtle shifts, researchers are learning how trees cope with stress and how forests may respond to a warming world.

The pulse is silent. But it tells a powerful story.