By Robert A. Hedeen, Naturalist
In autumn, as if by magic, Mother Nature, perhaps in consort with Jack Frost, wields her magic paintbrush, and the leaves of deciduous trees begin to change from their greens to a riot of fiery hues.
Because of the large variety of hardwoods in this part of the Midwest, the richness and beauty of the fall foliage surpasses that of many other areas of the country.
Our sumacs are usually the first to display their fall colors, exhibiting magnificent shades of magenta and crimson. A short time later, the brilliant oranges and yellows of the hard maples make a spectacular appearance. The sassafras is not to be outdone, however, as its leaves offer sparkling shades of orange, red and yellow.
Perhaps the most intense of all fall yellows is shown by the ginkgo or maidenhair tree. Unfortunately, the imported ginkgo, a native of China, is rare in the Rock River Valley, and its colorful display lasts for only a few days.
Oaks, with their red, green, deep red, or brown, and the cherries with their deep purplish-red, and the shagbark hickory with its glistening display of yellow, represent a palette any artist would envy.
Contrary to popular belief, cold does not directly affect leaf color change, though a hard frost does serve to deepen leaf colors after they have appeared. The internal physiological workings of a leaf are affected by the gradual onset of cool weather, less light in the atmosphere, and drier climatic conditions that are characteristic omens that fall is advancing.
In preparation for winter, a band of cells called the abscission layer forms at the base of each leaf petiole, or stem, where it is attached to the twig. This layer contracts and restricts the flow of vital elements into and out of the leaf. Another band of cells, similar to scar tissue, forms beneath the abscission layer and serves to cover the wound when the leaf falls from the tree.
These layers of tissue serve to plug up the pipelines, or vascular system, of the tree, preventing moisture from flowing into the leaf. When the leafs water supply is cut off, the fundamental process of photosynthesis ceases until spring, and the changes in the color of the leaf begin. The leaf gradually dies and falls to the ground when photosynthesis stops.
Photosynthesis is an extremely complex biochemical process by which plants trap energy from the sun in the green pigment molecule named chlorophyll. The entrapped solar energy is used to produce carbohydrate and oxygen gas from the raw materials of carbon dioxide and water. The necessary energy for life animals acquire from eating plants, or from animals that have previously eaten plants, is originally derived from the sun and is passed on via the carbohydrate produced by photosynthesis.
During the period of the year when the tree is actively engaged in this vital process, green chlorophyll is abundant in the cells of the leaf and masks the presence of other pigments that may be present: carotenes and xanthophylls responsible for the yellows and gold; anthocyanins, which show up as flaming reds and purples, and some others.
Brown pigments, known as tannins, give the sycamore and some oaks their rich brown color. When photosynthesis ceases in the fall, the green chlorophyll molecules break down, and the other pigments become visible.
The leaves of some trees are not particularly colorful in the fall, falling to the ground without changing color when the brittle abscission layer forms. These less beautiful leaves join the others whose colorful pigmentation gradually fades until the forest floor or the backyard is carpeted with a thick brown layer.
The smell of burning leaves in the fall brings back happy memories of my childhood in north Texas, where everyone disposed of the leaves in their yards by burning. I now know that there are much better and more efficient ways of disposing of dead leaves that are not dangerous to the health of our respiratory systems, but I can remember with nostalgia the distinctive smell that was the harbinger of fall and the winter to come.
When young children ask why green leaves change color in the fall, such things as chlorophyll dissociation, abscission layers, and Xanthophylls are difficult to explain. The fairy tale of Mother Nature instructing Jack Frost to mix his colors and dart about the countryside dabbing his brush here and there will be better received, and does an adequate job of explaining natures changing seasons to inquiring toddlers. The perplexing details of plant physiology can be discussed when a child is better able to understand them.