Scenes of stately sugar maples turning shades of orange and red, or red oaks ablaze with crimson leaves accented by a bright blue autumn sky are indelibly etched in the memories of many Iowans. However, have you ever wondered how the leaves of trees and shrubs are able to create such a colorful display? And why is fall color better in some years than in others? There is no simple formula to predict fall color for a given area of the state. Instead, intensity, type and duration of color, and date of peak color are determined by complex environmental factors and the genetic makeup of the plants themselves.
A summer leaf is green because of the presence of a group of pigments known as chlorophylls. Chlorophyll pigments, which help the plant manufacture food, are plentiful in leaf cells during the growing season, and their green color masks the colors of other pigments present in the leaf. But as autumn approaches, shortening daylength causes the supply of chlorophylls to dwindle, and their ability to mask other pigments is greatly diminished. Then other pigments like the carotenoids, which actually have been present in the leaf all summer, begin to show through.
Carotenoids, responsible for the colorations of yellow, brown, orange, and many intermediate hues, give characteristic color to carrots, corn, daffodils and bananas, as well as the fall leaves of deciduous trees like ash, aspen, birch, ginkgo, hickory and honeylocust.
Reds and purples, and their blended combinations that color the autumn leaves of red maple, white ash and red oak come from another group of pigments called anthocyanins. Unlike the carotenoids, anthocyanins are not present in the leaf during the growing season. Instead, they develop in late summer in the sap of leaf cells. Not all trees are capable of producing anthocyanin pigments, and even those that are, depend on the breakdown of sugars in the leaf in the presence of bright light. In fact, when the days of autumn are bright and cool, and the nights are chilly but not freezing, the most brilliant colorations usually develop. Wet, cloudy, warm weather or exceptionally low temperatures in early fall tend to mute the much anticipated autumnal display. Finally, trees growing in dense shade usually do not develop the vibrant colors that trees of the same species produce in full sun.
Each fall, thousands of Iowans and visitors alike enjoy the dazzling leaf display in our state. "Knowing when and where the fall colors are going to be most brilliant, is kind of a roll of the dice," according to John Walkowiak, Urban Forester with the Department of Natural Resources. "But in general, the northern 1/2 of Iowa has prime fall colors during the last week of September to the second week in October, and the southern 1/2 of the state has prime fall colors during the second to the fourth weeks of October," he said. For current leaf conditions across the state, individuals can call (515)233-4110. Callers will receive a recorded message detailing leaf conditions and best color locations. The information is updated weekly and is available from mid- September through late October.
This article originally appeared in the September 12, 1997 issue, p. 137.