Why do leaves turn color in the fall?

Scientists understand why golden tones emerge after green disappears. But they're still figuring out red.

There aren't many things that look their best right before they die, but the leaf of the sugar maple is one of them.

Briefly at the end of each growing season, maple leaves seem to want to imitate the sun, whose energy they've been dutifully collecting all summer. As their green-pigmented chlorophyll breaks down, they glow red and orange in a display more suitable to the exhibitionist tropics than the sober temperate zone. It doesn't last long. In a few weeks they're brown, dry and on the ground.

Until about a decade ago, the autumnal turning of the leaves was viewed by biologists as a pointless if appealing feature in the life history of many deciduous trees. The standard teaching was that the bright colors were lurking in the leaves all along. Only when the chlorophyll disappeared did they become visible, the colorful undergarments in a deathbed striptease.

It turns out, though, that's only half true.

Yellow and to some extent orange are the result of "unmasking" pigments already present in the leaves. But the red and magenta hues come from chemical compounds the tree makes just as it's preparing to go dormant for the winter.

Research going back 30 years suggested that compounds called anthocyanins are produced in a few red-turning trees late in the season. It's now clear that this isn't some rare exception. About 10 percent of tree species in temperate regions (and about 70 percent in New England mixed forests) turn red in the fall by making anthocyanins for a few weeks. The question is: Why?

"Plants don't tend to be cute; they aren't like pandas and koala bears. But one of the things they have is autumn foliage," said David W. Lee, a botanist at Florida International University. "Everyone can see it. It inspires poetry. Now we're trying to figure out the science of it."

Natural selection dictates that if a species -- and in this case, many species -- invests resources in some trait or activity, there's probably a good reason for it. Leaves don't turn red to entertain human leaf-peepers; it's got to help the trees some way.

What the survival advantage of red-turning leaves might be is now a topic of lively debate in certain scientific journals. In the past few years, data and experimental results have been added to what had mostly been a battle of theories.

There are two contending ideas.

One is that the red pigments are somehow involved in a Dunkirk-like operation mounted each fall in which the tree salvages useful chemical compounds from the dying leaf and transports them back into the wood for future use.

The alternative idea is that redness is a signal to leaf-sucking (and tree-damaging) insects called aphids, which are on the move in the fall and looking for places to lay their overwintering eggs. Aphids don't like the color red, and the message is: Don't lay them here; it's not a good place to raise young aphids.