Forest noise usually involves the rustle of leaves or creaking branches, representing the tree’s interaction with its external environment. Trees also generate internal sounds related to biological processes, far beyond human hearing. Scientists use specialized equipment to listen to this hidden acoustic world, confirming that trees constantly emit high-frequency clicks and pops. These inaudible sounds are evidence of life-sustaining processes and reveal the tree’s physical state.
The Silent Sounds of Water Movement
The most fundamental source of internal tree sound comes from water transport within the xylem tissue. Water is pulled from the roots to the leaves through microscopic tubes using cohesive forces, a mechanism called the cohesion-tension theory. This process places the water columns under immense negative pressure.
When a tree experiences drought or severe water stress, the tension becomes too high. If the pressure exceeds the water’s molecular strength, the column breaks, and dissolved air rushes in to form a bubble, known as cavitation. This physical rupture releases a burst of energy that travels through the wood as a high-frequency acoustic wave.
The frequency of these acoustic bursts is typically above 20 kilohertz (kHz), making them inaudible to human ears. Detecting these mechanical pops allows scientists to non-invasively monitor the tree’s water status in real-time. A high number of cavitation events indicates severe dehydration, signaling that the tree is struggling to maintain hydraulic function.
Ultrasonic Distress Signals and Communication
Recent research suggests that plants emit distinct ultrasonic airborne sounds in response to external stressors. Scientists have recorded plants producing rapid bursts of clicks when physically damaged or deprived of water. These sounds are measurable sound waves traveling through the air, often in the 40 to 80 kHz range.
A healthy plant is relatively quiet, emitting less than one click per hour. This acoustic output dramatically increases when the plant is injured or subjected to severe drought, emitting dozens of popping sounds per hour. These acoustic bursts carry specific information, allowing machine learning algorithms to differentiate between a cut plant and one suffering from dehydration.
The function of these airborne signals is under investigation, suggesting the sounds may be utilized by other organisms. Female moths, for instance, prefer healthy plants, avoiding those emitting ultrasonic distress signals. The plant’s stress byproduct is a measurable signal that animals like moths, bats, and small rodents, which hear in the ultrasonic range, may exploit.
How Scientists Listen to Trees
To capture the tree’s hidden soundscape, researchers rely on highly sensitive technology to detect high-frequency vibrations. Acoustic Emission (AE) sensors are frequently used; these piezoelectric transducers attach directly to the bark or wood. AE sensors convert the mechanical energy of an internal pop or click into a measurable electrical signal.
For detecting airborne sounds, researchers use ultrasonic microphones designed to register frequencies beyond the human hearing limit of 20 kHz. These microphones often require controlled environments, such as soundproof chambers, to isolate faint plant sounds from background noise.
The collected acoustic data then undergoes complex signal processing. The raw data is analyzed using sophisticated software or machine learning algorithms. This computational analysis allows scientists to differentiate between a cavitation pop, wind vibration, or insect activity. Processing these patterns converts the tree’s inaudible language into data revealing its physiological state and stress level.