The rich colors of autumnAutumn is the season of change with shorter days, cooler evenings and redder sunsets. This can be depressing to some people but to many others it is a time to enjoy the colors.
By: Vern Quam, Jamestown City Forester, The Jamestown Sun
Autumn is the season of change with shorter days, cooler evenings and redder sunsets. This can be depressing to some people but to many others it is a time to enjoy the colors.
There is a sense of wonder in autumn to search, how does this all happen? The progression of fall coloration is actually the process of degradation.
It begins earlier than most would guess, after about mid-summer, a specialized layer of cells form within the leaf’s petiole or stem.
These cells form as a wall that will separate leaf from the twig of the tree and is called an abscission layer. Shorter day length may be the trigger for this to grow in preparation for the coming winter.
Dry conditions can also cause the abscission layer to shed excess leaves. This layer will regulate the movement of water and nutrients into the leaf and carbohydrates (food) produced within the leaf out to the rest of the plant.
Tree leaves are green during the growing season due to the content of chlorophyll which masks the colors of the other substances.
Chlorophyll is a group of compounds present within the leaf cells where sunlight plus carbon dioxide plus water plus nutrients from the soil are brought together to produce carbohydrates which equals oxygen.
This process is known as photosynthesis, a miracle that only plants can produce their own food and food for man and animals.
It is with lower temperatures and even a hard frost that the abscission layer is stimulated to grow further and gradually limits the flow of water and nutrients into the leaf.
The amount of chlorophyll starts to decrease and other colors are unmasked in a gradual process. Carotenoids and xanthophyll are little specks in the plant cells called plastids and give a bright yellow to orange color to the leaf.
Carotenoids give carrots, corn, canaries, bananas, egg yolks, daffodils and buttercups their yellow to orange pigments. These brilliant yellows and orange colors can be seen in ash, maple, aspen, birch, black cherry and cottonwood leaves.
In other trees, certain sugars accumulate and a red pigment is displayed. The pigment that gives maples their autumn crimson is anthocyanin. Different from carotin or chlorophyll, anthocyanin resembles a stain such as iodine.
Anthrocyanin gives red, blue and purple hues to fruits such as cherries, cranberries, red apples, blueberries, strawberries and plums. These combine with the carotenoids to give the deeper orange, fiery reds and bronzes.
Environmental factors such as temperature, light and water can all affect the intensity of fall coloration. Low temperatures above freezing will favor the production of anthocyanin of red colors. Early frosts, contrary to certain beliefs, produce less brilliant red colors.
Tree leaves that are highly exposed to sunlight tend to produce more red coloration than shaded leaves.
Droughty conditions tend to favor more red coloration. If the fall weather is rainy, there will be a decrease in the intensity of fall coloration.
Cool to warm, sunny and dry conditions favor more brilliant fall colors. In North Dakota our high soil pH limits the general growth and production of red fall colors and so yellows and rusty yellow colors are predominate.
Some tree species will produce only green to yellow fall colors due to their plant chemistry. Some trees don’t form the abscission layer until after a hard frost. In these leaves the chlorophyll is actively photosynthesizing and drops green.
Whatever the autumn colors bring enjoy them now that you know their secret.