Science explains the magic of autumn’s changing colors
Artists and gardeners muse about color. Colors inspire.
Since early October, the Jamestown landscape has gone from brilliant greens to vibrant yellows, reds and oranges…then poof…everything blows off and the trees are bare and the colorful leaves that fell are brown. Then all white with snow a frozen leaves still hanging on some trees. Sure, we know freezing weather changes the colors, but why? It’s science.
Oops, that’s not the best topic to raise these days: Science isn’t. People are seeking other answers to why some things function as they do, especially when science disagrees with how they want their answer to be seen. Natural science is important to know, just as geology and the life sciences. Without losing the special connection we have with autumn’s changing colors, knowing why we have red leaves and yellow helps us understand other sciences as well.
Harvard University’s School of Forestry has a simple description on its website. It shows a diagram of three sets of changes; One shows normal green chlorophyll in summer, while the other two show red and yellow leaf changes. The descriptive narrative is simple and easily understood.
“Leaves change color during the autumn because the amounts of pigments change as the leaves prepare to fall from the trees. All leaves gradually lose chlorophyll during the growing season, and this loss accelerates before leaf fall. Under optimal conditions, this process of chlorophyll loss is very orderly and allows the plants to resorb much of the nitrogen in the structure of the pigment molecule.
“Carotenoid pigments are also lost from the plastids during aging, but some of them are retained in the plastids after the chlorophyll is removed; this produces autumn leaves with yellow colors. In unusual cases, sometimes in winterberry holly, a fair amount of chlorophyll is left in the leaves when they fall. Such leaves are a pale green in color, or perhaps yellow-green from the mixture of chlorophyll and carotenoids.
“Most interesting are leaves that turn red, because this color is the result of the active synthesis of anthocyanin pigments just before the leaves fall from the trees. This is the most common color of autumn leaves; about 70 percent of shrubs and trees at the Harvard Forest produce anthocyanins during the senescence of the leaves. In these leaves, the actual shades of red are the consequences of the amounts of anthocyanin, the retention of carotenoids (or even a little chlorophyll).
“Anthocyanin and chlorophyll produce brownish colors. Anthocyanins and carotenoids produce orange hues. In some plants, the color production is quite uniform, as in hobblebush or blueberry. In other plants, leaves vary between individuals (as sugar maples) or even dramatically within an individual (as red maples), or even within a single leaf (red maples).”
The length of time exposed to cold has a bearing on how much pigment (chemical) is released (revealed) and length of time the leaf remains before being frozen or blown off.
The Harvard illustrations show how a red oak turns from green to red:
“Chlorophylls are broken down, while Anthocyanins increase in concentration; Carotenoids are left in the chloroplasts and the lower cells are still with chloroplasts.” That gives a leaf a visible red color.
A Witch Hazel’s brilliant yellow is described as its “chlorophylls breaking down” but with “no anthocyanin increase.” That pigment change produces yellows.
Understanding the science doesn’t diminish the beauty of the tree as muse. Alfred Joyce Kilmer’s poem “Trees” attests to that.
“I think that I shall never see
A poem lovely as a tree. “
It goes on to describe how trees reach heavenward and become homes for birds. It doesn’t include autumn but does include the one we know so well here;
“Upon whose bosom snow has lain:”
And we can expect snow to weigh pretty heavily on those lovely branches that by May should be filled with chlorophyll colored leaves.
If anyone has an item for this column, please contact Sharon Cox, PO Box 1559, Jamestown, ND 58402-1559.