The chemical may be a sort of immune response that helps protect the plants, the scientists speculated.
According to the researchers, the finding raises the possibility that farmers, forest managers and others may eventually be able to start monitoring plants for early signs of a disease, an insect infestation or other types of stress.
Currently they often do not know if an ecosystem is unhealthy until there are visible indicators, such as dead leaves.
"Unlike humans, who are advised to take aspirin as a fever suppressant, plants have the ability to produce their own mix of aspirin-like chemicals, triggering the formation of proteins that boost their biochemical defenses and reduce injury," NCAR scientist Thomas Karl, the lead researcher, said in a statement.
"Our measurements show that significant amounts of the chemical can be detected in the atmosphere as plants respond to drought, unseasonable temperatures or other stresses."
While researchers had known that plants in the laboratory produce a form of aspirin known as methyl salicylate, they had never looked for it in the forest.
But when they set up measuring devices in a walnut grove near Davis, Calif., to monitor plant emissions that can affect pollution, they discovered measurable amounts of methyl salicylate.
Previous studies have shown that plants being eaten by animals also produce chemicals that can be sensed by other plants nearby.
The new findings, announced Thursday by NCAR in Boulder, Colo., were published in the journal Biogeosciences. The research was funded by the National Science Foundation, NCAR's sponsor.
Measuring instruments 100 feet above the ground measured methyl salicylate from plants that were stressed by a local drought and unseasonably cool nighttime temperatures followed by large daytime temperature increases.
In addition to having an immune-like function, the chemical may be a means for plants to communicate to neighboring plants, warning them of the threat.
"These findings show tangible proof that plant-to-plant communication occurs on the ecosystem level," says NCAR scientist Alex Guenther, a co-author of the study. "It appears that plants have the ability to communicate through the atmosphere."
Karl added: "If you have a sensitive warning signal that you can measure in the air, you can take action much sooner, such as applying pesticides. The earlier you detect that something's going on, the more you can benefit in terms of using fewer pesticides and managing crops better."
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National Center for Atmospheric Research: http://www.ucar.edu