What Is Behind Those Dazzling Autumn Colors?
THE North American Indians tried to explain the mysterious beauty of autumn leaves. Their legend claimed that every autumn hunters in the sky killed the Great Bear. His spilling blood, they said, splashed many leaves with red, while fat spattering from the hunter’s kettle tinted others yellow.
This explanation probably does little to satisfy your curiosity about autumn’s color extravaganza. What really goes on in those colorful leaves? Why do some turn red, others yellow or orange or purple, and still others just brown? What makes the same tree turn several different colors? And why do just a few areas of the earth enjoy spectacular autumn displays?
You may be surprised to know that much of the color is right there in the leaves all summer long! You just cannot see it. The superabundant supply of green chlorophyll in the leaves during summer masks over the other colors. But during autumn, something happens to the chlorophyll in broad-leafed trees. Understanding this is vital to knowing why the leaves change color as they do.
These deciduous trees, in contrast with the evergreens, lose their leaves every year. The annual blazing forth of autumn colors is merely a reflection of physical and chemical changes that occur during this process. The colors signal to observers that a marvelous sealing off of the leaves is taking place. How so?
Well, in a still little-understood chemical reaction, the chlorophyll in leaves uses sunlight all summer to make food (sugars) from water and carbon dioxide in the air. To do this, the leaves have been drawing up water from the soil, and much of it evaporates into the atmosphere.
But during winter very little water is available. Often it is frozen in the soil. Hence, loss of vital water through the leaves must be stopped. Trunks and branches also need sealing off from winter’s chill. For the tree’s own good, its leaves must go. So, as the sunlight wanes in the period before winter, most broad-leafed trees begin to shut down their food-production shop.
A Sealing That Reveals
As the days grow shorter, a layer of specialized cells begins to form between the leaf stem and the twig from which it grows. This corky abscission layer gradually cuts off the supply of water from below, as well as stopping the flow of sugars from the leaves into the tree. When the sealing is finally completed, the weight of the leaf and its twisting in the breeze are enough to send it floating to the ground. But, in the meantime, remarkable. things are happening.
Losing its abundant supply of water and summer sunlight, the busy chemical laboratory is deprived of raw materials. So the unstable chlorophyll in the leaves begins to break down and fade away, revealing the colorful pigments already there. These are mainly the carotenoids, pigments responsible for colors from pale yellow (xanthophylls) to carrot color (carotenes). Carotenoids are far more stable than chlorophyll, and so remain in the leaves of poplars, birches, aspens and others to brighten the landscape with shades of gold.
But what about the vivid reds, purples and even blues that make the autumn display so spectacular in certain parts of the world? These are the anthocyanins. They also give apples their red color, cabbages their purple, violets their blue, and so on. Anthocyanins are so dominant in some trees, such as the Japanese maple (red) and the purple-leafed plum, that they are seen all summer. But in most plants this pigment is formed only during autumn.
Anthocyanins are much more sensitive to outside influences than are the other leaf pigments. If leaf fluids are acidic, they appear red; if the fluids are neutral, violet; and if alkaline, blue. So any fluctuation in the chemical composition of the anthocyanins or of leaf acidity can make for a great range of colors.
Since these pigments are made from sugars, and bright, sunny days make for good sugar production, variations in weather during autumn can also affect the brilliance of the leaf display. If bright days are followed by cool, crisp nights, the chill slows the movement of sugars from the leaves into the trees during the night. Sugar concentrations build up, making for greater color production. But if during autumn the weather is cloudy or the nights warm, the colors are much more subdued.
Anthocyanin production in some plants is so sensitive to light that if one leaf shades another, an image of the upper leaf will appear in green or yellow on the lower one, outlined in red where the sun strikes! This also explains why parts of a tree with more exposure to the sun may be brightly colored, while other parts of the same tree have little red coloration.
Finally, there are the browns that often combine with yellows to make the beautiful yellow-golds and gold-browns that further enhance the autumn display. The browns generally appear in aging cells, in a process similar to what happens when a cut apple turns brown when exposed to the air. In the case of the beech tree and some oaks, this is a vibrant brown because the leaf cells are still very much alive, though aging, when the brown is formed.
On the other hand, some leaves turn brown only when they are almost dead or are already on the ground. This brown and some yellows are the only autumn colors seen in most parts of the world where there are some few deciduous trees. Why is it that so few places have brilliant autumn displays, with their wide range of colors?
A Show for a Few
Only limited regions of the earth are blessed with the conditions that can produce these dazzling displays. First, there must be large numbers as well as a variety of deciduous trees. They must have the genetic ability to produce the pigments that make the leaves so colorful. Many varieties simply do not produce anthocyanins. Another important factor is the bright, crisp autumn weather. Few parts of the world meet these requirements, and they are mostly in the northern hemisphere.
The British Isles and west central Europe have large deciduous forests, as do eastern China, Korea and parts of Japan. And these put on some very beautiful autumn coloration. But many believe that it is in the eastern United States and southeastern Canada that the most spectacular displays of color occur. The greater diversity of tree varieties with red-coloration potential and ideal fall-weather conditions seem to work together for truly breathtaking results.
The autumn leaf displays in some states draw people from distant places as a yearly tourist attraction. Bulletins are even issued describing the stage of coloration in different areas. Traffic clogs the back roads of the normally sleepy countryside with those the local people often call “leaf freaks.”
But these beauty lovers are rightly in awe of what they see. It is a Chemist without equal, the Creator himself, who can make such magnificence a part of what is otherwise a routine exercise in survival for trees.