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The ScienceBehindFall Color

Trees aren't just artistic — they're also smarter than you think

Even scientists see the beauty in autumn leaves. “I can still remember a hike when I was young," says Howard Neufeld, professor of plant ecophysiology at Appalachian State University. "The leaves were just a golden yellow, the air was clear, the sky was absolutely blue.”

“When you walk through something like that … ” he trails off. “There's nothing better.”

Which might do for the average person. The leaves change, the forest fills with bright yellows, vivid crimson. If you drive a mountain two-lane, scatterings of vibrant orange blow across the highway. Leaves pile around the base of split-rail fence posts, scatter against the sides of tobacco barns.

Again: enough for you and me to just see. Neufeld sees and asks: why? What do the trees get out of it? People get joy from looking, from kicking our feet through the crunching leaves on the ground, from smelling apple butter, pumpkins, smoke. Appalachian towns, adapting to that appreciation, get tens of millions of dollars offering food, drink, lodging — and history and experience. And people have also adapted to the timing of that sensory bonanza, planning festivals and harvest fairs around the peak of leaf color.

"If you had time lapse photography," Neufeld says, "you could watch the trees start to turn at the top of the mountain and then every week move down the side about a thousand feet." Mount Mitchell and other peaks tend to peak around late September, Boone peaks reliably in mid-October, Asheville a week later, and so on until you reach the Piedmont and Raleigh in November. If you could watch from space you could take climate into account and watch the color move south down the spine of the mountains, too. And though you'll see those bright yellows, reds, and oranges, Neufeld reminds you that fall has other colors too. Deciduous magnolia leaves turn a chocolate brown that makes a nice partner to the pale brown of corn sheaves and haybales. And color varies with a lot more than elevation.

"If you had time lapse photography," Neufeld says, "you could watch the trees start to turn at the top of the mountain and then every week move down the side about a thousand feet." Mount Mitchell and other peaks tend to peak around late September, Boone peaks reliably in mid-October, Asheville a week later, and so on until you reach the Piedmont and Raleigh in November. If you could watch from space you could take climate into account and watch the color move south down the spine of the mountains, too. And though you'll see those bright yellows, reds, and oranges, Neufeld reminds you that fall has other colors too. Deciduous magnolia leaves turn a chocolate brown that makes a nice partner to the pale brown of corn sheaves and haybales. And color varies with a lot more than elevation.

For one thing, whereas New England features overwhelmingly beeches, birches, and maples, making for large blotches of single colors, western North Carolina has an enormous variety of trees — the Smokies, in fact, peppered with microclimates, host some 120 species of tree, the greatest variety in the United States. That variety gives the southern Appalachians a broader spectrum – a patchwork quilt of mountainside color.

Color Progression by Elevation

But again: the trees provide all this wonder, free of charge. What's in it for the trees?

Start by looking at trees in a new way. Trees are capitalists; trees are in business. Trees do work, from which they try to profit, using a special process called photosynthesis.

They acquire raw materials — sunlight, water, carbon dioxide from the air — and they create a product: delicious carbohydrates, especially sugars like glucose, which they use for energy. They also bind that glucose up with nutrients from the soil and other molecules to create the raw materials for structural components like leaves and stems, just like a business using some of its income to invest in its factories and processes.

Like businesses, if trees make a profit — if they generate more energy than they use — they grow. Like businesses, if trees don't make a profit — if it costs more energy to keep going than they can get — they fail.

Which brings up leaves.

"I look at the leaf as sort of a photosynthetic machine," says Neufeld, the reigning North Carolina fall color guy, providing information and predictions for various agencies. "A leaf is the capitalist side of ecology. Leaves are the factories generating a product." Carbon, which the leaf gets from CO2, is of course the basis of all life. "So think of carbon as the money of the tree. If you invest so many units of carbon to build the leaf, in order to get a profit back, you have to first pay back the cost of the leaf."

Like a successful business, the tree designs factories to fit their environment. Canopy trees like oaks, maples, hickories, stretch their limbs towards the sun and so create thin, fluttering leaves for next to nothing. "They make back their initial investment in a few weeks," Neufeld says, "and then anything else is pure profit." Far below hunches the rhododendron. Unable to reach open sun, its leaves have to prosper in a shady environment. They'll never photosynthesize enough to earn back their investment in a single season, so the plant invests more resources and makes leaves that are rubbery and thick — and last several years before the plant finally finishes with them and they drop onto the forest floor.

Neufeld, whose decades in the Southern Appalachians followed a boyhood jumping in autumn leaf piles in Maryland, didn't come up with the theory of trees as profiteers, of leaves as factories or franchises. "There's a whole thing about leaf economics that's out there in the literature. We ecologists tend to borrow from the economists," he laughs. "It may be the dismal science, but we still like their theories."

Those theories raise a question about leaves that was until recent years almost completely misunderstood — even unasked.


We understood some things, of course. Neufeld specializes in plant stress, and autumn is a kind of preparation for the stress of winter: the tree preemptively drops the leaves that the frost would kill, getting its use out of them beforehand, and prepares for winter dormancy. Leaves are green because of the miraculous pigment chlorophyll, which absorbs photons from the sun, using that energy to spark the chemical reactions that turn water and carbon dioxide into sugars and complex carbohydrates — and provide the oxygen for our planet as a byproduct.

As autumn approaches, trees recognize both the shortening days and the cooling temperatures and begin doing the same thing we do: preparing for winter. We position the storm windows, insulate the pipes, and take in the porch furniture; trees disassemble the chlorophyll, reabsorb any useful nutrients into the twigs, and close the pathways to those now useless leaves. And, well, the leaves fall off.

In summer, the green chlorophyll absorbs energy for photosynthesis, says Neufeld, "but there are also accessory pigments that are yellow and orange. One of them is beta carotene, the same thing that makes carrots orange. And the xanthophylls are what make things yellow." The colors are present in the leaves all along, just concealed by the chlorophyll. But when the tree begins reprocessing chlorophyll, the green fades and these bright colors remain.

Again: just because? Or — maybe — is there an answer?


"The insect theory," says Neufeld, suggests there is. "The bright fall colors may be a warning to insects that this tree, to produce those bright colors, must be healthy. Don't lay your eggs on that tree, because it will also have chemical defenses against those insects." That is, a tree with less-vibrant colors would be more attractive to insects than a bright one. "It's kind of like the poison dart frogs in the Amazon."

But — red. Red is another story. Birches, tulip poplars, and some maples turn brilliant yellow; but trees like red maples and sourwoods turn that bright crimson that seems to define for people a good color season — and they do it not by passively revealing an existing pigment but by creating a new one.

Why Leaves Turn Red

As the trees were dismantling chlorophyll and reabsorbing it along with other nutrients like nitrogen, potassium, and phosphorous to store them for the winter, they also suddenly began expending energy to create anthocyanin. Unrepentant capitalists, trees expend energy only for some benefit. Researchers figured out that as the chlorophyll vanished, the energy from the sun could damage the mechanism by which the trees reabsorbed nutrients. Research proved it out: mutant trees that produced no anthocyanin turned out to absorb much less nutrients from leaves than those that turned red.

The red-is-a-light-shield theory is much more widely accepted than the yellow-protects-from-bugs theory; regardless, for the first time ever, scientists have begun unraveling not just the how of fall color but the why. The red color might provide pest control as well, of course — perhaps covering up the yellow so the aphids can't see it.

Plenty of questions remain, and Neufeld rejects the notion that the answers might remove the wonder. "Knowing about it tends to increase your appreciation," he says. "You can still go out and go 'Wow, what a great color season,' even if you know the pigment is protecting the leaf."


Going wow about fall in the mountains is something that comes naturally to fifth generation native Becky Anderson, whose family pioneered in the mountains in the late 1700s. "We were the first white family west of Asheville," she says with pride. "We were known as the Ferguson clan, off of Fine's Creek in Haywood County." Throughout the centuries, her people have farmed these mountains. So no less than the trees, Anderson knows how to prepare for autumn — and like the trees, to waste nothing, from the viewsheds themselves to "the landscape and culture that go with it. People need to taste it, to smell it, to make it."

As an example she cites the humble corn sheaf. Familiar to every suburban family as autumn decoration, Anderson remembers corn sheaves as something simpler: a way to keep harvested corn off the ground, "away from the critters who wanted to take your corn from you," especially before scientific breeding made corn stalks more rigid. The cornshucks that visitors to the mountains might see in farmyards and meadows as decorations also bear a hint of the Cherokee respect for crops. "Corn was very sacred to them," she says. "The preservation of the corn shucks was a continuation of that," just as it probably also continues the English tradition of Harvest Home, in which the last sheaves of corn were stored as a token for the next spring.

In any case, Anderson looks at things like corn sheaves the way Neufeld looks at leaves: there's a lot more there than meets the eye. Same, in fact, with something as simple as a haybale, which modern drivers of mountain roads might scarcely notice, scattered around broad bottomland meadows or standing along farm fences. Those too are symbols of fall, and they recall a time when hay wasn't laid in mechanical bales by tractors like a hen lays eggs.

"Fall was gathering time in two ways," she said. "It was the gathering of crops, but it was the gathering of people." Reaping hay for her grandmother, for example, involved a pitchfork, a wagon and some competition.

So Anderson sees those circular haystacks and thinks of their history — and notices the way people now decorate them, covering them and painting them as pumpkins or stacking them. "They're quite beautiful too. They're spaced in different ways. You can see how they figured out how to stack them. People get very artistic with them. It's a great combination of art and earth." Pumpkins and gourds now decorate porches and seem quaint — but once, they were simply the welcome decoration the land provided. They remind her: "how functional we are, how resourceful with the natural materials we have."

Fall in the mountains used to mean gathering only for locals; now it means millions of dollars to the mountain economy. Once the Blue Ridge Parkway came open, it wasn't just the trees who were capitalists of necessity, adapting to their ecosystem. "Communities along the parkway learned how to take events," she said, decorating to welcome visitors and share their fall traditions. Those traditions still declare themselves from every roadside, whether as the Appalachian style of church architecture — "the small white churches, the single steeple, the single door entry" — or the barns showing quilt patterns, encouraging local families to display a pattern from an heirloom quilt just the way an apple orchard allows people to sample heirloom apples. And whatever they share, locals still have special knowledge, though.

"You knew which leaves were going to turn when," she says, picnicking first atop Pisgah and working her way down, weekend after weekend, "all the different elevations, so we got the benefit of all the depth of fall color."


Color varies over larger geography, too. Autumn color is a temperate phenomenon, but oddly you don't find much red color in Europe — the result, Neufeld says, of the Alps' east-west orientation. When during the ice ages the glaciers came down, over the years trees moved down before them. In North America they never encountered a firm boundary, but in Europe many trees hit the Alps and were extinguished. The theory goes that after the ice age, when tree species recolonized back over the Alps, the pests that might have caused the red color — if that theory turns out to be correct — didn't make it back with them. Climate change will be the next big factor in leaf color, and new research indicates it may result in a brighter and longer leaf season.

But it doesn't take climate change or an Ice Age to affect leaf color; ordinary weather affects it too. Yellows and oranges, remember, exist all along, but the trees synthesize red anthocyanin from sugars they make in the fall. So in August and September, bright days — which encourage sugar production, which in turn spurs anthocyanin — and cool nights — which slows sap, keeping it in leaves instead of flowing back into the trees — mean plenty of bright reds to complement the other colors. A dry month helps — less water inhibits nitrogen uptake, which in turn spurs anthocyanin production. Poor soil can yield the same result.

So: mystery, in some ways, resolved. Howard Neufeld can with some accuracy predict when the leaves will change. He can tell you how they do it — and even why. But if the leaves change, one thing doesn't. "The neat thing about predicting fall color for various groups," he says, "is I have to go out and look at the fall colors. There's no better job." He drives the same roads the visitors do, only probably more slowly. Which, according to Anderson, is a good idea.

"You don't drive these scenic byways," she says. "You slow dance your way."