This article was published originally on 9/16/2009
As the days grow shorter, I am reminded why fall is such a wonderful time. Not only is the weather cool enough to open the windows ─ but still warm enough for long walks outdoors without multiple layers of clothes – fall also offers up a kaleidoscope of colors on trees and shrubs. The brilliant display in my neighborhood and on campus tempers my thoughts of a long winter ahead.
While Iowa may not be especially noted for fall color and "leaf peeping" like the New England states, it really is quite spectacular. Growing up in the deep South, where fall color is more subdued, I have come to appreciate the varied colors of an Iowa fall.
How does fall color actually happen inside the leaf of those deciduous trees and shrubs each year? The science behind fall color revolves around 3 pigments in the leaf and the weather (of course!). The three pigments are chlorophyll, carotenoids, and antohcyanins. Chlorophyll is the green pigment vital to the ultimate production of carbohydrates via photosynthesis in plants. Chlorophyll makes most plants green. Carotenoids are yellow or orange pigments that make carrots, corn, banana skins, daffodil flowers, and egg yolks colorful. Anthocyanins are the red or purple pigments found in red skinned apples, grapes, plums, and other fruit like strawberries, blueberries, and cranberries.
While plants are actively growing, chlorophyll is often the dominant pigment in each leaf. This is not surprising since it is needed for photosynthesis. Because it is so important, chlorophyll is constantly being produced as long as the weather is warm and the days are long. Carotenoids are also present in the leaves throughout the growing season; they are simply masked by chlorophyll until fall. So what happens in fall to displace the ever-present chlorophyll? As the summer progresses into fall, the length of the day shortens. Or more appropriately from the plant's perspective, the length of the night gets longer. Long nights are a signal to the plant to prepare for winter and, so the veins that connect the leaves to the stems start to close. This limits the amount of water and other necessary resources that can travel to the leaf…and prevents the continual production of chlorophyll. As chlorophyll declines, the green pigment of leaves goes away – exposing the other pigments present in the leaf.
While yellow and orange carotenoids are present in the leaves throughout the growing season, the red and purple anthocyanins are produced in the leaf in late summer and early fall. As the leaf veins restricted the transport of resources into the leaf, it also restricted the exit of carbohydrates out of the leaves. Many tree and shrub species will use these carbohydrates to create anthocyanins. The amount of anthocyanin produced and the resulting intensity of color are affected by environmental conditions. Warm, dry summer days followed by cool, dry nights in fall usually produce the most brilliant fall color.
Not every tree or shrub species produces carotenoids or anthocyanins in quantities that lead to a dramatic color display. And because the weather is different each year, the amount of these pigments in species that do normally produce them will vary – causing the fall color to be better or worse than the year before.
So take a hike outdoors and enjoy the autumnal display in your neighborhood or nearby park. You too will be impressed at the brilliant picture a couple of pigments can paint in the landscape.
Redbud Leaves, Fall Color. Photo by Cindy Haynes
Amur Maple Leaves, Fall Color. Photo by Cindy Haynes