Fall Color near Asheville by Leslie Restivo

The Science Behind Fall Color

Article last updated 08/23/2024

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… There's nothing better.” 
- Howard Neufeld

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.

Fall on the Blue Ridge Parkway by Jared Kay
Blue Ridge Parkway

"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.

What's in it for the Trees?

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.

Tree in fall on the Blue Ridge Parkway / Photo: Jared Kay

"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.