War in the Woods
When winter covers the aspen forests of interior Alaska, it forces a truce in one of Earth's oldest wars: the battle between trees and insects.
The invaders are the large aspen tortrix, a moth whose larvae devour aspen foliage. Every summer their victories are recorded by brown and barren patches of defoliated trees, sometimes sprawling for hundreds of acres up and down over interior hillsides. It would seem as if their victories are inevitable, for the trees cannot evade the insects. The aspens have to stand there and take it. When they leaf out again, they look weakened, sprouting smaller leaves after the caterpillar-like tortrix larvae have grown up and flown away.
Appearances can be deceiving. The aspen trees are actually mustering a very sophisticated arsenal to defend themselves, With chemical weapons, they will force the invaders back--every time.
A team of University of Alaska Fairbanks researchers were interested in what defenses aspen trees have that allow them to prevail each time a massive tortrix outbreak occurs. John Bryant, a plant ecologist at the Institute of Arctic biology, Richard Werner, an entomologist with the federal Institute of Northern Forestry on the UAF campus, and chemists Paul Reichardt and I joined forces to tackle the question. Our research was supported by the National Science Foundation.
After three years of work, we understand the trees' strategy and tactics fairly well. When a tortrix bites a leaf, the tree reacts by sending two chemicals from twigs to the attacked leaf. The chemicals, salicortin and tremulacin, take a few hours to reach the leaf. In themselves they are not hazardous to insects (or plants), but in leaf tissue crushed by chewing, plant enzymes convert the two chemicals into a new and toxic compound.
The compound, known chemically as 6-hydroxycyclohex-2-ene-l-one, is probably harmful to any living organism. It reacts with most biological substances and forms other toxins when it decomposes. An aspen leaf painted with the stuff quickly turns black. It is easy to see why the tree delivers two harmless chemicals to its leaves that only combine into a hazardous one when the leaves are damaged.
This first line of defense is enough to repel many kinds of insects, but the tortrix have been adapting to the aspen's chemical weapons for thousands of years. They can tolerate moderate levels of the toxin and keep on chewing. For tortrix, the aspens have to bring out their big guns.
They do this by growing smaller leaves. In effect, they're pulling in their defense perimeter. The same amount of raw materials sent from the twigs can combine into a higher concentration of defensive chemical in a smaller leaf, and there's less leaf area to defend.
If the tortrix larvae can still eat and digest the poisoned leaves, the tree will try again with still smaller leaves. Eventually the tortrix larvae will have food of such poor quality that they cannot survive by eating it, even if they are not poisoned outright. The insect population will crash, and the aspens can cover themselves with full-sized leaves again.
The trees have evolved a truly winning strategy in biological warfare, with many interacting features. By storing their arsenal in their twigs, they keep their ammunition close to the action. If it were any closer--in the leaves themselves--it would be lost each autumn, and newly unfurled young leaves would be undefended until they produced chemical stores. By keeping their chemical weaponry in an innocuous two-part form until they are attacked, the trees keep from poisoning themselves. By decreasing leaf size after each major defoliation, the trees fine-tune the amount of chemical required to defend themselves and spare the energy cost of maintaining too large an arsenal.
Studies such as this one are convincing researchers worldwide that trees are not helpless in the face of attacking hordes. They respond actively, at a chemical level, when they must. When it comes to the warfare in the woods, it is indeed true that they also serve who only stand and wait.
