Fall 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.

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.

Some parts of the San Francisco Peaks seem to peak brighter than other parts, and sunshine is the reason why. The more sunshine a tree receives, 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.

Typically, trees in the highest elevations (such as up on the San Francisco Peaks) begin to change mid-September. Then, like a slow-moving wave, the color descends into town, which is around 7,000 feet. The show usually ends in late October.

Aspen (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.