Autumn Color

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The Science Behind Beautiful Fall Foliage

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.

Check out NASA images from space

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 colors?

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.Table of Contents

• How Do Leaves Get Their Color?
• Why Do Leaves Change Their Colors?

The Blue Ridge Parkway is one of the most famous drives in the US. Last fall, I had the good fortune of driving through it at the perfect time. It was undoubtedly the most beautiful ride of my life. I had no destination and I never wanted the drive to end. Surrounded by a wonderland of yellow and orange leaves, it felt like I was driving through a garden, not on a public road. The sight of all that fall foliage was simply breathtaking.

When autumn arrives, life suddenly becomes more enjoyable. When you see all those beautiful yellow and orange leaves on the road, all your worries about traffic and work stress just vanish. People around you seem happier, eager to soak up the natural beauty of the world around them.

However, have you ever wondered how all those leaves magically change their green color to yellow, orange and red when autumn finally arrives?

How Do Leaves Get Their Color?

During summer vacations, when you are playing outside, the trees are working hard to provide you with oxygen for all of your activities. Then, when you get tired and are sweating from the summer sun, these trees provide you with shade beneath their green leaves.

Photosynthesis

Trees take water from the ground through their roots and carbon dioxide from the air. They use sunlight to turn water and carbon dioxide into oxygen and glucose. Glucose is a type of sugar that trees use as food. This process is known as photosynthesis, and a vital chemical called  chlorophyll helps in photosynthesis.

Chlorophyll is a green color pigment that is abundantly found in leaves; therefore, it is chlorophyll that gives leaves their green color.

Why Do Leaves Change Their Colors?

Yellow and Orange

Other than chlorophyll, leaves contain two other pigments: carotene (orange color) and xanthophyll (yellow color).

During the summer, chlorophyll is the busiest pigment, as it is required for photosynthesis. The abundance of chlorophyll masks the orange and yellow color pigment that lie beneath.

Trees work hard in the summer season to produce food, which they can store for the autumn and winter season. Autumn is the time for the trees to finally take a rest after all that hard summer work.

In some parts of the world, during autumn and winter, days become shorter and nights become longer. Therefore, the amount of sunlight decreases significantly, and as a result, plants are not able to perform photosynthesis. As the amount of sunlight reduces, the amount of chlorophyll also reduces and becomes less dominant. Due to this, the carotene (orange) and xanthophyll (yellow) pigments that were already present in the leaves begin to show.

Red Color

Leaves look even more beautiful when they are red in color. This red color is the result of the anthocyanin (red color) pigment. Anthocyanins, unlike carotene and xanthophyll, are not present in leaves all the time. The tree synthesizes anthocyanins only after the chlorophyll content reduces significantly. Anthocyanins are produced as a form of protection for the leaves; they allow the tree to recover nutrients in the leaves before they fall off.

In short, leaves do not actually change their color. They simply lose their green color during the autumn.

We should be very thankful for trees , as they let us enjoy the beauty of nature in every season. Autumn is like a second spring, when every leaf is a flower.

References:

1. Environmental Science and Forestry
2. US Forest Service
3. Why do we celebrate Halloween, go trick or treating and carve pumpkins? (goodto.com)
4. Impossible – Nat King Cole