The Semiotics of a Leaf

A new autumn has brought another burst of red and yellow leaves. And it has also brought an interesting new idea about why trees put on this show every year.

In recent years, scientists have been roughly divided into two camps when it comes to autumn leaves. One camp holds that autumn colors are just part of preparations for winter. The other holds that the colors are a warning to insects to stay away.

The warning hypothesis came from the late biologist William Hamilton. He pointed out that trees fight off insect larvae with toxins. A more vigorous tree could produce more toxins than a weaker one. It could also produce more vibrant colors in the fall by producing more pigment molecules. (The red in leaves is created by molecules called anthocyanins, for example). Perhaps a tree could send a message to insects looking for a place to lay their eggs: stay away from me or I’ll kill your kids in the spring.

Insects that could recognize this signal benefited by staying away, laying their eggs on weaker trees. And brighter leaves were in turn favored, since trees that produced them didn’t have to struggle so much with insects devouring their equipment for gathering sunlight. Hamilton and other scientists put his warning hypothesis to the tests, and they found some hints that he was right. The color of leaves, for example, did seem to influence where insects chose to lay their eggs.

But scientists who study the physiology of trees weren’t sold. They came at the question of autumn colors from another direction: they studied the molecules that create the colors. During autumn, leaves dismantle their green chlorophyll and pump the nutrients back into their branches. It’s a vulnerable time in a tree’s life, because it becomes vulnerable to damage from ultraviolet rays. Anthocyanins and other pigment molecules protect leaves by absorbing the damage caused by UV rays, so that the dwindling chlorphyll in a leaf can continue to generate the energy required for winter preparations. (For more details, see my article last fall in the Times.)

Everyone in the debate that I spoke to agreed that there was a lot left to learn about autumn leaves, and as I mentined here, it was even possible that they might be the result of several different forces working together. And in a paper in press at the journal Bioessays, University of Frieburg biologist H. Martin Schaefer and his student Gregor Rolshausen have come up with a way to account for a lot of the evidence offered up by both camps. The core of their argument is that autumn leaves are cues, not signals.

This may all sound more like semiotics than botany, but there’s some real science here. Schaefer and Rolhausen point out that the molecules that give leaves their fall colors and the molecules that defend leaves against insects have a lot in common. In order to make some toxins, a tree actually starts out using the same recipe for making anthocyanins. Only relatively late in the process does it alter the recipe to produce toxins (which are colorless, unlike anthocyaninins). Other toxins are made with a completely different recipe, but they have another link to anthocyanins. These molecules are very delicate, and are easily damaged by sunlight, and they need anthocyanins to protect them.

Schaefer and Rolshausen argue that the primary function of anthocyanins is not to ward off insects. Its main job is to shield leaves from radiation. It just so happens that a tree with a lot of colorful anthocyanin is probably also a tree with a lot of colorless toxins. One reason is that a tree that is physically capable of producing lots of anthocyanins can also produce lots of toxins that use an almost identical recipe. Another reason is that a lot of anthocyanins can protect other toxins, so that they can attack insects.

Insects can’t see toxins, but they can see reds and yellows produced in autumn leaves. And anthocyanins may be reliable cues about how dangerous a tree will be to an insect’s eggs. While it’s a cue, Schaefer and Rolshausen argue, it’s not a signal that’s directed to the bugs.

By linking autumn colors to insect defense, Schaefer and Rolshausen can account for a lot of the evidence amassed so far. But their new hypothesis won’t win the day until more experiments can find more support for it than for the previous ones. It may even turn out that other forces are also at play. I suspect that many more autumn leaves will fall to the ground before scientists have worked them all out.

Update, 12/2/05: The paper is now out.