How do trees change color


Science of Fall Colors | US Forest Service

Aspen leafs: fall colors. Beaver Ranger District, Fishlake National Forest. (Forest Service Photo by Scott Bell)

For years, scientists have worked to understand the changes that occur in trees and shrubs during autumn. Although we don't know all the details, we do know enough to explain the basics to help you enjoy nature's multicolored display. Three factors influence autumn leaf color:

- leaf pigments

- length of night

- weather

The timing of color changes and the onset of falling leaves is primarily regulated by the calendar as nights become longer. None of the other environmental influences – such as temperature, rainfall, food supply – are as unvarying as the steadily increasing length of night during autumn. As days grow shorter, and nights grow longer and cooler, biochemical processes in the leaf begin to paint the landscape with Nature's autumn palette.

 
Leaf Pigments

A color palette needs pigments, and there are three types that are involved in autumn color:

- Carotenoids: Produces yellow, orange, and brown colors in such things as corn, carrots, and daffodils, as well as rutabagas, buttercups, and bananas.

Anthocyanin: Gives color to such familiar things as cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums. They are water soluble and appear in the watery liquid of leaf cells.

Chlorophyll: Gives leaves a basic green color. It is necessary for photosynthesis, the chemical reaction that enables plants to use sunlight to manufacture sugars for food.

 

Trees in the temperate zones store these sugars for the winter dormant period.

Both chlorophyll and carotenoids are present in the chloroplasts of leaf cells throughout the growing season. Most anthocyanins are produced in the autumn, in response to bright light and excess plant sugars within leaf cells.

During the growing season, chlorophyll is continually being produced and broken down and leaves appear green. As night length increases in the autumn, chlorophyll production slows down and then stops and eventually all the chlorophyll is destroyed. The carotenoids and anthocyanin that are present in the leaf are then unmasked and show their colors.

Brilliant Fall leaves on the Superior National Forest. (Forest Service photo)

Certain colors are characteristic of particular species:

- Oaks: red, brown, or russet

- Hickories: golden bronze

- Aspen and yellow-poplar: golden yellow

- Dogwood: purplish red

- Beech: light tan

- Sourwood and black tupelo: crimson

The color of maples leaves differ species by species:

- Red maple: brilliant scarlet

- Sugar maple: orange-red

- Black maple: glowing yellow

- Striped maple: almost colorless

Some leaves of some species, such as the elms simply shrivel up and fall, exhibiting little color other than drab brown.

The timing of the color change also varies by species. For example, sourwood in southern forests can become vividly colorful in late summer while all other species are still vigorously green. Oaks put on their colors long after other species have already shed their leaves.

These differences in timing among species seem to be genetically inherited, for a particular species at the same latitude will show the same coloration in the cool temperatures of high mountain elevations at about the same time as it does in warmer lowlands.

Length of Night

In early autumn, in response to the shortening days and declining intensity of sunlight, leaves begin the processes leading up to their fall. The veins that carry fluids into and out of the leaf gradually close off as a layer of cells forms at the base of each leaf. These clogged veins trap sugars in the leaf and promote production of anthocyanin. Once this separation layer is complete and the connecting tissues are sealed off, the leaf is ready to fall.

How does weather affect autumn color?

The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is dwindling. Temperature and moisture are the main influences.

A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions – lots of sugar and light – spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.

The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring, or a severe summer drought, can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn colors.

What does all this do for the tree?

Fall showing at the Norway Beach Recreation area on the Chippewa National Forest. (Forest Service photo)

Winter is a certainty that all vegetation in the temperate zones must face each year. Perennial plants, including trees, must have some sort of protection to survive freezing temperatures and other harsh wintertime influences. Stems, twigs, and buds are equipped to survive extreme cold so that they can reawaken when spring heralds the start of another growing season. Tender leaf tissues, however, would freeze in winter, so plants must either toughen up and protect their leaves or dispose of them.

Evergreens: pines, spruces, cedars, firs, and so on are able to survive winter because they have toughened up. Their needle-like or scale-like foliage is covered with a heavy wax coating and the fluid inside their cells contains substances that resist freezing. Thus the foliage of evergreens can safely withstand all but the severest winter conditions, such as those in the Arctic. Evergreen needles survive for some years but eventually fall because of old age.

Broad-leaved trees: These are trees that do not have needles or scale-like leaves. They are tender and vulnerable to damage, are typically broad and thin and are not protected by any thick coverings. The fluid in the cells of these leaves is usually a thin, watery sap that freezes readily, which makes them vulnerable in the winter when temperatures fall below freezing. Tissues unable to overwinter must be sealed off and shed to ensure the plant's continued survival.

What happens to all those fallen leaves?

Needles and leaves that fall are not wasted. They decompose and restock the soil with nutrients and make up part of the spongy humus layer of the forest floor that absorbs and holds rainfall. Fallen leaves also become food for numerous soil organisms vital to the forest ecosystem.

It is quite easy to see the benefit to the tree of its annual leaf fall, but the advantage to the entire forest is more subtle. It could well be that the forest could no more survive without its annual replenishment from leaves than the individual tree could survive without shedding these leaves. The many beautiful interrelationships in the forest community leave us with myriad fascinating puzzles still to solve.

Where can I see autumn color in the United States?

You can find autumn color in parks and woodlands, in the cities, countryside, and mountains - anywhere you find deciduous broadleaved trees, the ones that drop their leaves in the autumn. New England is rightly famous for the spectacular autumn colors painted on the trees of its mountains and countryside, but the Adirondack, Appalachian, Smoky, and Rocky Mountains are also clad with colorful displays. In the East, we can see the reds, oranges, golds, and bronzes of the mixed deciduous woodlands; in the West, we see the bright yellows of aspen stands and larches contrasting with the dark greens of the evergreen conifers.

Many of the Forest Service's 100 plus National Scenic Byways were planned with autumn color in mind. Almost every one of them offers a beautiful, colorful drive sometime in the autumn.

When is the best time to see autumn color?

Unfortunately, autumn color is not very predictable, especially in the long term. Half the fun is trying to outguess nature! But it generally starts in late September in New England and moves southward, reaching the Smoky Mountains by early November. It also appears about this time in the high-elevation mountains of the West. Remember that cooler high elevations will color up before the valleys.

What Causes Leaves To Change Color?

Fall provides us with a brilliant show worth taking in every year.

But what causes the trees and shrubs to change to such brilliant colors? We explain.

by Glenn Morris Updated: October 3, 2022

Ever wonder why leaves change color each fall?

We checked in with an expert “fall guy,” Appalachian State University Biology professor, Howard Neufeld, who has been studying fall color for much of his career, to get the answer. His scientific interest in what occurs inside a leaf has naturally progressed to what shows outside, and when. Neufeld‘s research and sociability have made him a go-to guy for fall color science and forecasting. So what does he say about why leaves change color?

What Signals A Leaf To Change Color?

The vivid, often simple colors on the outside are the products of the complex chemistry of growth inside a leaf. Take the yellows and oranges, for example—the dominant colors of aspen, ash, birch, beech, hickories, maples, some oaks, tulip poplar, and sassafras—generally, these colors come from compounds called carotenoids (also responsible for the color of carrots) which are present in the leaf during the growing season.

The green chlorophyll, the workhorse of photosynthesis, dominates and covers up those carotenoids in summer. As the days grow shorter and the temperatures cool, chlorophyll degrades and goes from green to colorless, allowing the oranges and the yellows to show up. These colors are present in the leaf during its growing season.

What About The Reds – How Does That Happen?

Those gorgeous scarlet, crimson, and ruby hues of the red maples, black gums, dogwoods, sourwood, and oaks are what makes fall the most breathtaking. “Red pigments are not present in the leaf during summer,” Neufeld says. “Trees that turn red actually produce this pigment, called anthocyanin, in the autumn.” However, though we might like to think so, plants don’t make this red pigment for our appreciation. These pigments play a key role in readying the tree for the next spring. Researchers discovered that anthocyanins act as a sunscreen, protecting leaves (especially evergreen ones) from bright seasonal light when it is cold outside. Other researchers have discovered that the sunscreening effect protects leaves from too much light, which can interfere with late-season transport of nutrients from the leaf back to the twigs, something trees do as a conservation mechanism.

Still, other scientists believe the red color serves to ward off insect pests. A healthy, strong plant has lots of anthocyanins; certain insects laying eggs in the fall may seek other, weaker host plants for their offspring.

While anthocyanins may ward off insects, there is no doubt that they are magnets for “leaf peepers” for fall color tourism. A little bit of red goes a long way—and more is even better.

Leaves change color as they slow their production of chlorophyll.

What Conditions Make For Good Fall Color?

Neufeld offers a “recipe” for good autumn color: “Starting in August, days must be sunny and the nights steadily cooling. This allows the trees to manufacture sugars, and sugars stimulate the leaves to make anthocyanins. The cold helps keep the sugars in the leaves producing anthocyanins.”

The calendar needs an August footnote to remind us to watch the weather during that month for fall color later.  Too many clouds, too much rain, or too much heat in the eighth month can make for a duller fall. September, though generally cooler, follows suit. “Drought is the other enemy of a good fall.” Neufeld adds, “The trees have to be in a healthy state —not water-stressed—heading into the season.”

Neufeld does his best to provide such guidance with continuous observation. The good news for would-be travelers is that “peak color” is a peculiarly local condition depending on the local weather, the mix of trees, the elevation above sea level, and the distance from the equator.

When Do Leaves Change?

Fall color starts earlier at higher altitudes and in northern regions than it does in lower elevations or farther south. Apply these rules to a map of North America and it is possible to zigzag north to south, higher to a lower altitude to extend fall color viewing for nearly six weeks, maybe longer. Fall color is about location, location, location—and good timing!

What About Trees Down South—Do They Change Color?

Where palm trees, live oaks, and cactus grow, there is little to “rake home about” in autumn. The Gulf Coast and the arid southwest are mostly fall-colorless.

What Can I Plant For Fall Color?

Want a burst of color each fall? Consider planting these small trees with reliable fall color:

Red –  Flowering Dogwood
Yellow – Redbud
Orange/Crimsom – Japanese Maple

Excellent fall color featured in large trees includes October Glory red maple (uniformly red), sugar maple (orange), and katsura (yellow).

Keep Learning

Where to see the leaves change color

When to expect peak foliage in your state.

6 More popular leaf peeping destinations!

The First Day Of Fall (Vernal Equinox)

Join The Discussion!

Are you a leaf peeper?

Where are some of your favorite destinations to peep?

Are changing leaves your favorite thing about fall? Or what is?

Let us know in the comments below!

We are looking forward to hearing from you.

Glenn Morris

North Carolina native Glenn Morris is a freelance Travel and Garden writer. He is the author of Taylor's Weekend Gardening Guide to Small Gardens, and North Carolina Beaches. His article What in the World is Workamping? appears in the 2021 Farmers' Almanac.

The secret of autumn colors. Why Leaves Change Color

The reason is that in temperate regions winters are quite cold and there is not much sunlight that trees use for energy. The leaves are tender and cannot survive the winter, so the tree prepares for the cold by taking all the nutrients from them before they fall.

This is the process of preparing for winter, and this is what makes the leaves show us their amazing autumn colors. However, there is a good reason why different trees have different colored leaves.

Winterizing

Most trees have green leaves in summer because they contain the pigment chlorophyll. This pigment is also used to convert sunlight into energy. In the summer, chlorophyll is constantly in the leaves, but when it gets cold, the plants stop producing chlorophyll, and it breaks down into other molecules. Trees can reuse the nitrogen found in the chlorophyll molecule.

That's why the leaves change color, and before they fall off the tree, important reusable nutrients are removed from the leaf. The time when the leaves begin to change color is more dependent on light than temperature, so the leaves begin to change color around the same time each year. When deciduous trees reach this light threshold, carbohydrates are transferred from the leaves to the branches and new minerals are no longer available to the foliage. The trees are preparing to part with them.

The growth and fall of foliage is more dependent on the amount of light than temperature. Longitude of the day in the region of 55 latitude.

A Rainbow of Autumn Colors

The green color of chlorophyll is so strong that it masks any other pigment. The lack of green in autumn allows other colors to dominate. The leaves also contain pigments called carotenoids; xanthophylls are yellow (like in corn) and carotenes are orange (like in carrots). Anthocyanins (also found in blueberries and cherries) are pigments that are only produced in autumn when the cold days begin. Because the trees cut off most contact with their leaves at this stage, the trapped sugar in the leaf veins promotes the production of anthocyanins, which are used in plant protection and create a reddish color.

This is interesting: How huge guns were used in the 19th century, firing 0.5 kg of shot at a time.

However, trees are not only yellow and red in autumn: they are brown, golden bronze, golden yellow, purplish red, light brown, crimson and orange red. Different trees have different proportions of these pigments. The amount of chlorophyll remaining and the proportions of other pigments determine the color of the leaf. The combination of anthocyanins and chlorophyll gives brown color, while anthocyanins and carotenoids create orange leaves.

Low temperatures, which are just above freezing, promote the formation of anthocyanin, which gives a bright red color. Early frost weakens the colors because anthocyanin production stops. But, for example, drought can lead to falling leaves without changing color.

Trees have a special mechanism to shed their leaves. At the point of attachment of the leaf, a special layer of cells is formed, they eventually cut the tissue that attaches the leaf to the tree. Subsequently, the sheet can fall freely with a gust of wind, gravity or rain. When the leaves begin to shrivel, their chloroplasts are completely destroyed, changing their color to a dirty brown.

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Why do leaves change color, turning yellow or red in autumn?

In spring, when all plants come to life after winter dormancy, leaves begin to appear on them and they are usually green. They are painted in such a shade due to a special pigment - chlorophyll, which is part of plant cells. But when autumn comes and everything living and blooming prepares for cold and frost, the foliage on the bushes and trees changes its usual color to a brighter one. Why do leaves change color?

Pigments: their purpose

Biopigments are specific substances found in plant cells that help color plants in different shades.

These pigments perform various functions. They determine the color of plants, for example, the leaves of trees, which is very important for their adaptation to the external environment. The bright flowers attract the special attention of insects, which help pollinate them. Brightly colored berries and fruits attract the attention of animals and birds, and they, in turn, help spread the seeds.

And there are those pigments that perform a protective function - they reliably protect from scorching sunlight or warn with their bright color that the plant is poisonous and should never be touched.

Chlorophyll is considered to be the most important pigment. It colors the leaves green. But he is responsible not only for coloring, but also participates in an important process for all plants on the planet - photosynthesis. An important function is assigned to this process - the conversion of the energy of sunlight into the energy necessary for the connection of organic matter. In a more understandable language for a person ignorant of biology, photosynthesis helps plants grow actively and produce a good harvest.

Chlorophyll is not the only component, there are other pigments responsible for certain functions:

  • Carotene, lutein, lycopene and others are a separate group of pigments called carotenoids. This class is the most common of all. It is they who paint the foliage in all a variety of yellow and reddish hues. They are considered additional elements that take an active part in photosynthesis. Beta-carotene is the precursor to retinol, the most powerful antioxidant.
  • Aurones, kakhetins and others are a group of flavonoids required by plants for metabolic processes and participation in photosynthesis. It is these pigments that color the foliage and flowers in blue, burgundy, pink and purple hues. Most of the components of this group are the strongest antioxidants.
  • Porphyrins are nitrogen-containing pigments that take an active part in metabolic and photochemical processes. This group includes chlorophyll, as well as pheophytin, which is similar to the first in structure. If you imagine visually, when vegetables from a saturated green color are cooked in an acidic environment, they become dark olive, and pheophytin is formed. In the summer, almost all the pigments described above predominate in the leaves, except for anthocyanins in different proportions. But with the onset of autumn, everything changes dramatically. Why do leaves change color in autumn?

Autumn transformation

Throughout the spring and summer, the cells are dominated by the pigment - chlorophyll, it is thanks to him that you can admire the rich green foliage. But in the fall, everything changes dramatically, and what's the matter? Why do leaves change their color in autumn? Where does chlorophyll go?

In autumn, everything living, and this applies not only to animals, but also to plants, prepares for the cold. Daylight hours become shorter, the temperature gradually decreases, the water in the soil freezes. As a result, almost all plants have to free their trunks from foliage. If the trees did not shed their leaves, then the process of active evaporation would not stop, and there would be nowhere to absorb moisture, because the water in the ground froze. As a result, the plants simply withered. This reason leads to the fact that the foliage falls even in the summer, when there is heat and drought outside for a long time.

But before the trees are completely free of leaves, you can notice a change in the shade of the leaves on the trees. What is the reason for such a striking transformation?

Chlorophyll is characterized by increased sensitivity to light and oxygen and is rapidly destroyed due to lack of it. If in summer it is easily restored under the influence of bright sunlight, then in autumn there are no conditions for this. And water and nutrition from the soil are already getting worse into the foliage, because a protective layer is formed at the base of the trunk, helping to protect the plant in the cold. Initially, in early autumn, this process is slow, but with the approach of winter, it accelerates significantly.

Chlorophyll is slowly destroyed, and its place is taken by carotenes and xanthophylls, which are responsible for the bright and saturated color of foliage. In parallel, the formation of anthocyanins responsible for lilac and violet tones occurs in the foliage, but why plants need this process has not yet been clarified. Many scientists are studying this issue, but they have not come to a consensus. It is believed that this happens for photoprotection against the background of the fact that plants are actively losing magnesium.

Chlorophyll stays on the leaves almost until cold weather, gathering in veins. All processes that occur with plants depend on air temperature and daylight hours.

When can the most intense colors be observed?

The brightest and most saturated foliage can be observed on plants when the weather is cold, but dry and sunny for a long time, and the temperature is kept within 0-+7 degrees.

It is noted that you can see the most beautiful colors of plants in the state of Vermont. But in England, where heavy rains pour most of the autumn and cloudy weather is more common, the foliage is dominated by a dull yellowish or brown hue. Autumn passes quickly, and more often it comes earlier than its allotted time. Foliage loses its bright color. The leaves are attached to the branches with the help of special cuttings, but with the advent of cold weather, this connection becomes weak. They are held only thanks to thin vessels, through which food and moisture go to them, but a strong breath of the breeze and that's it, even this connection is lost, and the leaf falls off.

Every plant - flower, bush or tree - stores up nutrients to easily survive the harsh winter weather.


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