If the National Science Foundation and two scientists it supports with research money are to be believed, trees talk to each other.
At least they do in the woods near Seattle, where Drs. Gordon H. Orians and David F. Rhoades of the University of Washington have found that the willows and the alders (birch-like trees) warn each other when they are being attacked by leaf-eating insects.
"I know it sounds like something right out of Doonesbury, but it is definitely a form of communication we've witnessed in dozens and dozens of trees," Orians said in a telephone interview from his laboratory. "We cannot explain what happened without assuming that trees being damaged by insects release a chemical in the air that warns nearby undamaged trees to prepare a defense against these insects."
About four years ago, Orians and Rhoades set out to find how trees survive mass attacks by insects such as tent caterpillars and fall webworms, which are similar to tent caterpillars but which thrive in August and September. Tent caterpillars are most abundant in April and May.
The two ecologists placed swarms of as many as 700 tent caterpillars and webworms in the branches of dozens of willows and alders to see what kinds of defense mechanisms the trees might place against the attack.
What they found was that the trees being attacked began producing chemicals such as alkaloids and terpenoids, which show up in their leaves and make them unpalatable to the leaf-eating insects.
Not only did the leaves suddenly become unappetizing, but they also began to form the protein that the insects normally thrive on in a way that made them indigestable, causing the insects to starve and even die from protein deficiency.
"The insects would begin to lose all their vitality," Orians said. "They were suddenly unable to resist the cold at night or ordinary bacteria they could normally ward off with ease."
Even more remarkable was the ecologists' finding that nearby trees of the same species suddenly began to mount the same chemical defenses even though they had not been invaded by insects. At first, the ecologists thought that the chemical defenses were being transmitted by the damaged trees to the undamaged ones through their roots, but they discovered that this was not the case.
"All the undamaged trees we studied were 30 or 40 meters from the damaged trees . . . much too far away to have any root connections with the damaged trees," Orians said. "The only explanation has to be that there is some airborne chemical released by the trees being attacked to warn the nearby trees that an insect attack is under way."
Orians said he and Rhoades have been unable to identify the airborne chemical but are about to begin lab studies to look for it. With NSF support, they plan to plant trees in a large greenhouse, attack some of them with tent caterpillars and webworms, and run tubes from the trees under attack to those not being attacked in an attempt to capture the chemicals being released by the attacked trees as a warning to the others.
Orians thinks that all species of trees, not just willows and alders, are able to warn their brothers and sisters of an insect attack.
"There is already some evidence of it from studies of sugar maples in Vermont and New Hampshire," Orians said, "and if you ask me to make a guess, I would be surprised if this form of communication were unique to only three species of trees."