Science says trees can send chemical warnings and SOS signals through the air, what plants do when danger strikes

A tree being attacked by insects cannot run away. But that does not mean it is chemically helpless. When leaves are damaged by herbivores, pathogens or mechanical injury, plants can release volatile organic compounds (VOCs) into the surrounding air. Nearby plants may detect some of these airborne chemicals and alter their own defensive chemistry.

Scientists increasingly describe this phenomenon as plant chemical signalling, although it is more precise to say that plants release chemical cues that other organisms can perceive. The process does not involve a nervous system or a brain. Instead, it relies on molecular recognition, signalling pathways and changes in gene expression.

Science says damaged plants release an invisible chemical alarm

When an insect begins feeding on a leaf, the plant does more than suffer physical damage. Herbivory can activate metabolic pathways that produce blends of airborne molecules known as herbivore-induced plant volatiles, or HIPVs.


These compounds include green leaf volatiles, terpenoids and other specialized metabolites. Their precise chemical mixture can depend on the plant species, the type of herbivore and the nature of the damage. The resulting “chemical fingerprint” can provide information about what is happening to the plant.


Some VOCs can also attract predators or parasitoids that attack the herbivores eating the plant. This creates an extraordinary ecological chain: an attacked plant changes its chemistry, predators detect the altered chemical environment and the herbivore can become easier to find.

How can another plant detect chemicals floating through the air?

Plants do not have noses, but their cells can interact with airborne molecules. Research suggests that VOCs can enter leaves through stomata, the microscopic pores that regulate gas exchange, and move into internal tissues depending on their chemical properties.

Once perceived, volatile exposure can influence intracellular signalling. Researchers have implicated processes involving calcium ions, protein phosphorylation, mitogen-activated protein kinases and transcription factors in plant responses to airborne chemical cues.

The important point is that the receiving plant does not necessarily immediately launch a full-scale defense. Instead, it can become primed.

Plant priming is like putting defenses on standby

One of the most important concepts in this field is defense priming. A plant exposed to an airborne cue can become physiologically prepared to respond more strongly when an actual attack occurs.

This is different from simply activating every defense mechanism immediately. Priming can allow the plant to conserve resources while remaining capable of mounting a faster or stronger response when danger arrives. Reviews of plant volatile signalling describe this as a major mechanism through which neighboring plants can adjust their defenses according to environmental threats.

A major pathway involved in herbivore defense is jasmonate signalling. Jasmonic acid and related jasmonates function as important regulators of plant responses to herbivory and other stresses. They can influence the expression of defense-related genes and production of protective compounds.

A famous experiment involved poplars and maple trees

The idea that damaged plants could influence their neighbors is not new. In a landmark 1983 Science study, Ian Baldwin and Jack Schultz examined damaged poplar and sugar maple plants.

They found that undamaged plants sharing an enclosure with damaged plants showed changes in defensive chemistry. Nearby sugar maple seedlings developed increased concentrations of phenolic compounds and tannins after exposure to damaged plants. The researchers proposed that an airborne cue from damaged tissues could be responsible.

The interpretation was controversial, partly because scientists questioned whether such chemical communication occurred under realistic natural conditions. Later field and laboratory research provided additional evidence for airborne plant interactions.

Sagebrush offers another striking example

Research by ecologist Richard Karban and colleagues examined interactions between sagebrush and wild tobacco. In field experiments, tobacco plants growing close to clipped sagebrush experienced less natural herbivore damage than plants whose neighboring sagebrush had not been clipped.

The exposed tobacco also showed increased activity of polyphenol oxidase, an enzyme associated with plant defense. The findings provided field evidence consistent with the idea that damaged sagebrush could influence nearby plants through airborne cues.

Plants can even call in the enemies of their enemies

Perhaps the most fascinating example comes from lima bean plants. When attacked by two-spotted spider mites, lima beans release VOCs that can attract predatory mites called Phytoseiulus persimilis.

In a 2000 Nature study, Gen-ichiro Arimura and colleagues found that undamaged lima bean leaves exposed to volatiles from mite-infested plants activated five defense genes. The response resembled one produced by jasmonic acid, and specific terpenoids in the airborne mixture contributed to gene activation.

This represents indirect defense: the plant does not necessarily attack the herbivore directly. Instead, its chemical response can recruit another organism that attacks the herbivore.

It is not quite a “forest internet”

The popular image of trees sending messages through a forest is compelling, but scientists urge caution. VOC signalling is highly dependent on species, environmental conditions, concentration, wind and the chemical properties of the compounds involved.

Researchers also distinguish between a signal, which has evolved to convey information and influence a receiver, and a cue, which provides information that another organism can exploit even if the emitter did not evolve specifically to communicate it.

So plants are not exchanging sentences. They are participating in a complex chemical environment in which molecules released by one organism can alter the physiology and behavior of another.

What looks like an invisible warning system is therefore better understood as a sophisticated network of chemical ecology, molecular signalling and evolutionary adaptation, one in which a damaged plant can change the chemistry of the air around it and potentially leave its neighbors better prepared for what comes next.

FAQs

Can trees really warn other trees about insects?

Research indicates that airborne VOCs released by damaged plants can influence neighboring plants, including changes in defense-related chemistry and gene expression. Scientists generally describe this as volatile-mediated signalling or cueing rather than communication in the human sense.

What chemicals do plants release when attacked?

Plants can release complex mixtures containing green leaf volatiles, terpenoids, methyl salicylate and other specialized metabolites. The exact blend varies according to plant species, herbivore and environmental conditions.