Why do leaves change color in the fall?

 

Photo showing many different colors of fall foliage

Changing leaves herald the coming of autumn to the Coconino National Forest as early as mid-September. That's when forest roads and trails begin to hum with the crowds of nature lovers who come to enjoy the display. The Coconino encompasses such a broad range of habitats that it's possible to stretch this most colorful of seasons into more than a month of celebration within its boundaries. The gold rush begins on the higher slopes of the Forest's volcanic highlands as the aspen leaves change to amber while summer is still in the air. From there, the transformation gradually migrates to the crimson sumacs and fuchsia maples of the desert canyons as brisk nights confirm the full onset of autumn. The climax of this parade of color generally occurs around the second week of October, but remnants of reds, oranges and golds can linger in the canyons as late as mid-November.

What Makes Leaves Turn Colors?

The leaves on deciduous trees do not really “turn” colors. They just lose their green. Leaves actually begin to prepare for autumn in the spring. At the base of each leaf is a layer of cells called the “abscission” or separation layer. All summer, small tubes pass through the abscission to carry water into the leaf. The leaf uses this water with carbon dioxide, sunlight and chlorophyll to produce food. This process is called photosynthesis.

The word photosynthesis means “putting together with light.” The food, or “sugar,” is carried back out through the tubes in the abscission into the tree. 

In the fall, the cells of this abscission layer begin to swell and form a cork-like material, reducing and finally cutting off flow between the leaf (leaves) and the rest of the tree. Also, because of cool nights, the sugar still produced in the bright fall sun is not readily transported from the leaves to the stems and roots. The less sunlight (shorter days at the end of summer, beginning of autumn), the less green chlorophyll is produced, and is, in fact, broken down.

This is when you begin to see the yellow xanthophyll and orange carotene that are present in the leaf all year around but are covered by the green chlorophyll. No one is sure what purpose these other elements serve, but scientists believe they also have something to do with photosynthesis. Certain species of deciduous trees will begin, at this time of warm days and cool nights, to produce anthocyanins. Anthocyanins are the reds and purples and are produced only in autumn; they are not present in the leaves all year.

During this dormant period, the trees use the reserves they have stored up over the summer to stay alive. They need this time to take a rest from producing.

What do autumn leaves and bananas have in common?

The green color in unripe bananas comes from chlorophyll, the same pigment that gives green leaves their color. As bananas ripen, the chlorophyll breaks down and disappears, revealing the yellow color, which has been there all along. The yellows and oranges of autumn leaves are also revealed as their chlorophyll breaks down.

Why Is It That Some Years We Get Great Color And Relatively Bland Results In Other Years?

Good reds are produced when the days are warm and sunny and the nights are cool (45°F or less, but not a frost), coming one after another. In this setting, the leaves produce lots of sugar using the abundant sunlight, but the cool nights prevent the sugar from flowing through the leaf veins. At this point, anthocyanins (the reds and purples) are produced. Yellow and orange are fairly constant, because they are already in the leaf and do not require specific weather situations.

A warm, wet period in the fall will cause the changing to be not so brilliant because sunny days producing the sugar and cool nights halting it’s spread produce the best color. A severe frost will kill the leaves, ending the show immediately.


Why Is It That Some Portions Of The Mountain Or Flagstaff Get Better Color Results From The Changing Leaves?

Sunshine is the key. The more sunshine, the more sugar is produced. While there will not be enough sunshine to produce chlorophyll, some sugar and anthocyanins (the red/purple/blue colors) will be produced as a last ditch effort to bring nutrition to the leaves. In trees where anthocyanins are not produced, we will see only the yellows and oranges.

The sunnier spots of the mountain and less-shaded trees in town will become reddish/purplish, if they are predisposed to do so. In addition, a single tree may be red on one side, the side more exposed to the sun, and yellow and orange on the other.

When Do The Leaves Typically Change Color In Flagstaff? Closer To The Peaks?

Typically, trees in the highest elevations begin to change mid-September. Then, like a slow-moving wave, the color descends into town. The show usually ends in late October.

What Are The Types Of Trees In Northern Arizona That Turn Colors?

Fall Colors on the Mogollon Rim thumbnailAspen (yellow), Maple (yellow, orange, red), Oak (yellow, orange, brown), Cottonwood (yellow), Various fruit trees (red, orange, yellow), Elm (yellow), ash (yellow), poplar (yellow), willow (yellow), and even weeds and bushes like sumacs (Orange, red) and poison ivy will change colors and decorate the roadsides.

What Makes Evergreens…Well, Evergreen?

Evergreen trees (pines, spruces, cedars and firs) don’t lose their leaves in the fall. They are covered with a heavy wax coating and the fluids inside the cells contain substances that resist freezing. Evergreen leaves can last for several years before they fall and are replaced by new growth. They may, however, become brownish if the winter is particularly cold.

 

Visit Fall Colors on the Coconino National Forest for current conditions and recommendations for enjoying the season.

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https://www.fs.usda.gov/detail/coconino/learning/nature-science/?cid=FSEPRD559441