Autumn Leaves: The Search for Purpose

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As the autumn leaves turn handsomely, I’ve been wondering, why do trees bother? It’s a question scientists have been asking for the past few years, and for the first time, they’ve carried out an experiment to find out.


The color of an autumn leaf can actually take a lot of work. In the fall, the green chlorophyll in a tree fades away, while the tree actively builds new pigments to turn it red or yellow. It’s generally agreed that these colors must serve some function for trees. Otherwise, natural selection would favor drab trees that dropped their leaves without such bother. They could use the energy they didn’t waste on autumn colors to fight diseases, capture more sunlight, or some other essential task.

In 2001, the late biologist William Hamilton made a provocative proposal: he argued that leaves turned colors in the fall to warn off insects. Healthy trees could produce anti-insect toxins and have energy to spare for building red and yellow pigments. The stronger the tree, the brighter the signal. As I described here, Hamilton and Sam Brown of the University of Texas found support for his hypothesis by comparing different species of trees. Species with bright leaves tend to be attacked by more species of aphids in the fall than species with drab leaves. This correlation was consistent the idea that the evolution of bright leaves was driven by feasting bugs.

But skeptics have argued that autumn colors actually serve a different function. They propose that the colors act as a sunscreen. An autumn leaf actually bustles with molecular preparations for the winter. Leaves pump their nutrients into a tree’s branches, where the tree can use them to survive till spring. But the cold, short days of fall play havoc with leaf chemistry. The sun’s UV rays cause more damage to the leaf tissue, as to harmful charged molecules released by photosynthesis. Some scientists argue that autumn pigments absorb these dangerous rays and molecules, allowing the leaf to do its autumnal business. (For more on the debate, see my blog post and the article I wrote for the New York Times.)

Scientists have continued to come up with new ideas about autumn leaves and to make new observations to test them. But no one has carried out a full-blown experiment. It’s easier said than done. Trees, after all, will not be rushed. Scientists must wait patiently for their leaves to turn in the fall. If they haven’t gathered enough observation by winter, they have to wait another year for another chance. It’s also difficult to reduce the glory of an autumn leaf to a precisely controlled variable, in order to measure the strength of its connection to the behavior of insects.

H. Martin Schaefer and Gregor Rolshausen, two scientists at the University of Frieburg, recently put autumn leaves to the test by painting trees. They found a stand of ash trees, and painted the leaves of some of the trees red and some green. If the colors served as a signal to the insects, the paint should make a difference.

It didn’t.

Aphids did not alter their attacks on trees after they were painted. Nor did painted trees get attacked at a different rate as unpainted ones. In a paper published today in the journal Biology Letters, Schaefer and Rolshausen argue that their experiment doesn’t support Hamilton’s signal hypothesis.

Interestingly, the scientists found that the aphids did prefer to land on some trees over others. The most heavily attacked trees turned out to be the ones producing the most seeds. It’s possible that the trees that produce lots of seeds have to sacrifice some of their defenses against insects, making them good targets for aphids. The aphids would somehow have to detect the seed-heavy ash trees–either by seeing the seeds themselves or perhaps by smelling some compound given off by the trees. In either case, the aphids don’t care about the color of the leaves.

Schaefer and Rolshausen also tested the various theories for autumn leaves by comparing different populations of ash trees near Frieburg. The scientists found an association with cold weather and both bright colors and few aphids. Their findings, which are in a paper in press at Plant Ecology, may favor the sunscreen hypothesis over the signal hypothesis. But there’s a catch. Some of the trees grew on a mountainside at 1100 meters, and others grew at 360. If leaves need sunscreen to withstand harsh environments, the high-altitude trees might have been brighter. But they weren’t.

So for now, the fall foliage remains a mystery. The slow life of trees will mean we have a long time to wait for a resolution. At least we can enjoy the colors in the meantime.

References:

Aphids do not attend to leaf colour as visual signal, but to the handicap of reproductive investment. H. Martin Schaefer and Gregor Rolshausen. Biology Letters, 2006. DOI:10.1098/rsbl.2006.0548

Do aphids paint the tree red (or yellow)–can herbivore resistance or photoprotection explain colourful leaves in autumn? Gregor Rolshausen H. Martin Schaefer. Plant Ecology in press. DOI: 10.1007/s11258-006-9215-3