90ways - Science

Well, it's that time of year again. The clouds are settling in, the air is turning cold and, if you get up early in the morning, you can see your breath. Here in the Northeast, that means only one thing: peepers. That's right, day-hikers and their foliage. To prepare you folks for those forays into the outdoors, we here at 90ways have decided to offer a brief tutorial on the science of changing colors and falling leaves. Along the way we'll meet a few enzymes, talk about the evolutionary advantages of investing in the hottest fall fashions, and perhaps strike a blow against iPods in the wilderness.

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Leaves have been around for a long time, and they've been changing colors for almost as long. The first deciduous trees appeared about 180 million years ago, but most of them were wiped out (along with most of everything else) when giant frozen mountains came grinding down from the poles during the last Ice Age. About 10,000 years ago life started to make something of a comeback, and deciduous forests returned to Northern Asia, the Eastern United States, and most of Europe. Birch trees did most of the early colonizing, followed by oaks, elms, and ashes. Between that time and the present, the land has seen its share of human colonizers, all of whom managed these ecosystems in sophisticated and significant ways.

The most extreme form of management, that which we call farming, reached its peak in the Northeastern United States during the early 19th century. When the likes of Thoreau and Emerson were walking the paths of the Concord woods, the deciduous trees of the Northeast were just beginning their most recent in a series of comebacks, and they now cover a much more significant portion of the land than they did 150 years ago.

During the summer months, the leaves in these deciduous forests are engaged in an activity much like the one farmers once carried out on these same lands: growing food. Leaves use the well-known molecule chlorophyll to capture light energy from the sun and, through a series of electron shuffles, use it to turn 6 molecules of carbon dioxide and 6 molecules of water into one molecule of sugar and 6 molecules of oxygen. As most everyone knows, chlorophyll can't use green light because its wavelength isn't exciting enough. As such, green light bounces back off of leaves and strikes our eyes, leaving us to think that leaves are green. Leaves contain other molecules, like carotene and xanthophyll, that cause light to bounce back into our eyes. These molecules look yellow or orange in the sunshine, but we can't see them in the texture of a leaf because their presence relative to that of chlorophyll is negligible.

Toward the end of the summer, as the days grow shorter and leaves start to realize that their food making days are numbered, the cells at the base of their stems start to divide rapidly without expanding. They pile up on top of each other and essentially gum up the works, blocking the transport of food and nutrients between the leaf and the rest of the tree. Quickly, the leaves run out of the minerals they need to support photosynthesis and molecules of chlorophyll start to degrade. Without all the green-light trampolines that these chlorophyll molecules provide, the light bouncing back from other pigments in the leaf becomes more apparent, and we are left to think that the leaves are orange or yellow or red.

For a long time, scientists thought the colorful nature of fall leaves was simply their default state. That is to say, color indicated the absence of function. In 2001 William Hamilton, a biologist every bit as engaging and lovable as E.O. Wilson or Stephen Jay Gould but without access to the same branding channels, published an influential paper in the Proceedings of the Royal Society of London in which he suggested that vibrant leaf color was an adaptation some trees had made over the millennia.

Hamilton's argument goes something like this: making colorful pigment is a huge energy investment. By devoting that energy to something that does not have immediate, direct bearing on its survival, the tree is showing that it has energy to spare. Bright displays of foliage during the fall are essentially a tree's way of showing off. Why does a tree care how healthy tourists from New York City think it is? It doesn't. But it does care how healthy parasites, birds and bugs think it is. By displaying its chromatographic vigor, the tree is dropping hints about its infection-fighting prowess.

To test his idea, Hamilton did something pretty straight forward: he looked at how many parasites trees with bright leaves had and compared it to how many parasites trees with dull leaves had. Sure enough, the bright-leaved trees had fewer.

But, like any good piece of scientific scholarship, that wasn't the end of the story. In a recent article, H. Martin Schaefer and David M. Wilkinson have argued that brightly colored leaves result from trees doing their best to batten down the hatches for winter. As leaves prepare to sever themselves from the tree, they ship as many nutrients as they can back into the trunk, branches and roots to help them survive the winter. To facilitate this process, leaves produce proteins that contain orange, red and yellow pigments. The trees that produce the most of these molecules have the brightest leaves and shuttle the most nutrients. In the game of evolution, that means they are the big winners.

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We sometimes talk about listening to Nature. We use the term in a vague, broad sense: by listening we mean absorbing the signals of the landscape and trying to figure out what processes they represent. In this week's New York Times Travel section, Denny Lee published a piece about podcasting your way to the best fall foliage. This industry that compels us to plug up our ears, follow our maps, and stare at the leaves, has, in the most fundamental way, led us to miss the point. If we are to believe Hamilton and his colleagues, the signal that the trees are sending when they put on their fall displays is anything but "come look at me." Rather, they are essentially telling us to buzz off.