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 fabulous display. 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 colours were lurking in the leaves all along. Only when the chlorophyll disappeared did they become visible. 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 a rare exception. About 10 per cent of tree species in temperate regions turn red in the fall by making anthocyanins for a few weeks. The question is: Why?

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.

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