Talking to Insects

Autumnal forest in Charlevoix, Quebec.  From the series The Earth from Above by Yann Arthus-Bertrand.

The painter and experimental filmmaker Jeff Scher has a wonderful video in today's NY Times entitled Leaf and Death. Scher collected fallen leaves from around his neighborhood, dried them, and placed them on a light table to photograph. The result is stunningly beautiful, no less because there is something haunting and melancholy about its subject matter. 

As the summer begins to wane, leaves turn to brilliant colors before they are shed and fall to the ground. “Entire landscapes are transformed into a state of agitated Technicolor,” Scher writes in a statement that accompanies his piece. “It’s nature’s color-coded warning of the approaching longer nights and colder days sneaking up the calendar.”

But why do autumn leaves really change their color?

It turns out that this question has become something of a controversy among botanists. (For a review of the relevant arguments, check out this recent paper in the aptly named journal TREE.) The issues at stake in the debate are fascinating, and they will ensure you will never look at an autumnal forest the same way again!

For a long time, botanists did not trouble themselves with the question of why leaves turn from green to red and yellow. There was a shared consensus that color change was simply a byproduct of leaf senescence. As temperatures begin to drop and days grow shorter, the energetic benefits of maintaining leaves capable of photosynthesis are gradually outweighed by the physiological costs associated with doing so.  For this reason, treed begin to shed their leaves. As they do so, chlorophyll is broken down into more basic pigments of lower molecular weight, which causes leaves to loose their green color.

Although this story has a certain poetry to it—trees die a little bit each year, only to be renewed again come springtime—it has recently been found wanting. First, although there is no question that its central premise is correct, it only explains why trees shed their leaves in the autumn, not why they change their color. Second, botanists have found that the color change cannot be explained by the breakdown of chlorophyll alone. The bright red and yellow colors we see are also a product of de novo pigment synthesis. Leaf color change is therefore an active process, not just a byproduct of death!

But why would a tree go to the trouble of changing the color of its leaves? Two competing but not entirely incompatible hypotheses have recently been proposed, both of which suggest leaf color change is an adaptation, not just a spandrel of sorts. The first actually dates back to the 19th century, but it has only recently been revived.  It holds that changes in pigmentation act as a kind of sunscreen, protecting leaves against light and low temperatures. What’s the point of screening a leaf that’s about to fall off and die? The tree is not just protecting its autumnal leaves because they continue to engage in some low-level photosynthetic activity, but mostly because functional leaves enable a better reabsorption of nutrients.

But it’s the second hypothesis that really changed the way I look at autumnal trees. To my knowledge, it was first proposed by Marco Archetti, an evolutionary geneticist and theoretical biologists at Oxford. Archetti argues that changes in leaf coloration may be a tree’s way of talking to insects.

At the end of the summer, a number of insects, including aphids, migrate to trees where they lay their eggs. This is bad news for the tree, because when the next generation of insects hatches, they can cause considerable damage to their host. Of course, the tree has ways of defending itself against parasites. Among other things, it can produce Jasmonic Acid to ward off a pest. However, defending themselves against parasites is metabolically costly for trees. Hence, vigorous, healthy, plants can devote more of their resources to fighting parasites than sick ones do.

Red leaves are also costly to produce, since metabolic resources have to be diverted for pigment synthesis. Hence, leaf color change serves as a signal of a tree’s health. The redder the leaves, the healthier the tree. And, by extension, the better it will be able to ward off insect parasites. Over time, we would therefore expect that insects evolved a preference for trees that don’t have red leaves.  Leaf color and insect preference, the hypothesis states, have coevolved over time. Differently put: autumnal changes in leaf coloration act as an honest signal of a tree’s health, and insects have evolved the ability to interpret this signal correctly.

(Economists make similar arguments about education, advertising, money-back-guarantees, and a host of other practices in which we humans engage.  The idea is always the same: because they are expensive to produce, they serve as an honest signal of product quality.)

If Archetti’s hypothesis is correct, what we are seeing when we walk through an autumnal forest is a none too cordial conversation between trees and their insect parasites.