Those Brilliant Fall Outfits May Be Saving Trees

The New York Times > Science: "The scientists do agree on one thing: the colors are for something. That represents a major shift in thinking. For decades, textbooks claimed that autumn colors were just a byproduct of dying leaves. During spring and summer, leaves get their green cast from chlorophyll, the pigment that plays a major role in capturing sunlight. But the leaves also contain other pigments whose colors are masked during the growing season. In autumn, trees break down their chlorophyll and draw some of the components back into their tissues. Conventional wisdom regards autumn colors as the product of the remaining pigments, which were finally unmasked. In other words, autumn leaves were a tree's gray hair.

But in recent years, scientists have recognized that autumn colors probably play an important role in the life of many trees. Yellow leaves get their color from a class of pigments called carotenoids. Another group of molecules, anthocyanins, produce oranges and reds. Trees need energy to make carotenoids and anthocyanins, but they cannot reclaim that energy because the pigments stay in a leaf when it dies. If the pigments did not help the tree survive, they would be a waste. What's more, leaves actually start producing a lot of new anthocyanin when autumn arrives.

Evolutionary biologists and plant physiologists offer two different explanations for why natural selection has made autumn colors so widespread, despite their cost. Dr. William Hamilton, an evolutionary biologist at Oxford University, proposed that bright autumn leaves contain a message: they warn insects to leave them alone.

In the case of trees, Dr. Hamilton proposed that the visual message was sent to insects. In the fall, aphids and other insects choose trees where they will lay their eggs. When the eggs hatch the next spring, the larvae feed on the tree, often with devastating results. A tree can ward off these pests with poisons. Dr. Hamilton speculated that trees with strong defenses might be able to protect themselves even further by letting egg-laying insects know what was in store for their eggs. By producing brilliant autumn colors, the trees advertised their lethality. As insects evolved to avoid the brightest leaves, natural selection favored trees that could become even brighter.

The leaf-signal hypothesis has also drawn criticism, most recently from Dr. Wilkinson and Dr. H. Martin Schaefer, an evolutionary biologist at the University of Freiburg in Germany. In a paper to be published in Trends in Ecology & Evolution, Dr. Schaefer and Dr. Wilkinson argue that autumn colors are not sending messages to insects. It's wrong, but compelling, Dr. Wilkinson said. Their recent work suggests that fall colors serve mainly as a sunscreen.

The interior of an autumn leaf is a frenzy of activity. Much of the chlorophyll and other equipment necessary for photosynthesis is being carefully dismantled, while the nutrients it contains, like nitrogen and phosphorus, are shipped into the tissue of the tree. The tree will need those nutrients to grow and reproduce in the spring. The leaves need energy to send these reserves into the tree, which they can only get through photosynthesis. But because they have dismantled much of their light-harnessing equipment, it no longer works efficiently. Autumn leaves cannot capture all the sunlight striking them, and the leftover energy can build up in the leaf and cause damage to its tissue. "Sunlight in October isn't near as intense as in July," Dr. Hoch said, "but it can do more harm to a leaf."

Anthocyanins, the pigments that produce red and orange colors, appear to protect autumn leaves by blocking some of the sunlight. Dr. Hoch and his colleagues have found the most compelling evidence for this role. They raised normal trees along with mutants that could not produce anthocyanins. While the mutants thrived in a greenhouse, they could not ship nutrients out of their leaves in autumn sunlight.

Still, the plant physiologists have more work to do. Some trees, like birch, produce no anthocyanins. Their yellow leaves are produced by carotenoids. During the growing season, carotenoids help chlorophyll absorb sunlight, but in the fall they do not shield the leaves. Dr. Hoch suspects that trees with yellow leaves must have some other way to protect their leaves in autumn, which he is now trying to find."