By Sonny Macaluso - February 28, 2019
As the days get cooler and frost is in the air, deciduous trees and shrubs put on an autumn show in all shades of red, yellow, purple, and brown. The splendor of crisp fall days and vibrant leaves brings to mind memories for nearly everyone who lives in an area where deciduous forests are the dominant vegetation. In many parts of the country, autumn leaves are an important factor in tourism.
Many think that cool weather or frost cause the leaves to change color. While temperature may dictate the color and its intensity, it is only one of many environmental factors that play a part in painting deciduous woodlands in glorious fall colors.
To understand the whole process, it is important to understand the growth cycle of deciduous trees and shrubs. Most have a relatively short period of annual growth. New stems begin to grow from overwintering buds when the days become long enough and the weather is warm enough to support growth. For most trees, growth is usually completed by late June in the Northern Hemisphere. Next year’s leaf buds are set at this time and will not open until they experience the chill and short days of winter followed by the warmth and longer days of spring. Once the leaves are fully expanded and the buds are set, the work of manufacturing and storing carbohydrates to support next year’s growth goes full speed ahead. These carbohydrates are stored in the branches, roots, and buds throughout the growing season to support next year’s growth.
The process that starts the cascade of events that result in fall color is actually a growth process. In late summer or early autumn, the days begin to get shorter, and nights are longer. Like most plants, deciduous trees and shrubs are rather sensitive to length of the dark period each day. When nights reach a threshold value and are long enough, the cells near the juncture of the leaf and the stem divide rapidly, but they do not expand. This abscission layer is a corky layer of cells that slowly begins to block transport of materials such as carbohydrates from the leaf to the branch. It also blocks the flow of minerals from the roots into the leaves. Because the starting time of the whole process is dependent on night length, fall colors appear at about the same time each year in a given location, whether temperatures are cooler or warmer than normal.
During the growing season, chlorophyll is replaced constantly in the leaves. Chlorophyll breaks down with exposure to light in the same way that colored paper fades in sunlight. The leaves must manufacture new chlorophyll to replace chlorophyll that is lost in this way. In autumn, when the connection between the leaf and the rest of the plant begins to be blocked off, the production of chlorophyll slows and then stops. In a relatively short time period, the chlorophyll disappears completely.
This is when autumn colors are revealed. Chlorophyll normally masks the yellow pigments known as xanthophylls and the orange pigments called carotenoids — both then become visible when the green chlorophyll is gone. These colors are present in the leaf throughout the growing season. Red and purple pigments come from anthocyanins. In the fall anthocyanins are manufactured from the sugars that are trapped in the leaf. In most plants anthocyanins are typically not present during the growing season.
As autumn progresses, the cells in the abscission layer become more dry and corky. The connections between cells become weakened, and the leaves break off with time. Many trees and shrubs lose their leaves when they are still very colorful. Some plants retain a great deal of their foliage through much of the winter, but the leaves do not retain their color for long. Like chlorophyll, the other pigments eventually break down in light or when they are frozen. The only pigments that remain are tannins, which are brown.
Temperature, sunlight, and soil moisture greatly influence the quality of the fall foliage display. Abundant sunlight and low temperatures after the time the abscission layer forms cause the chlorophyll to be destroyed more rapidly. Cool temperatures, particularly at night, combined with abundant sunlight, promote the formation of more anthocyanins. Freezing conditions destroy the machinery responsible for manufacturing anthocyanins, so early frost means an early end to colorful foliage. Drought stress during the growing season can sometimes trigger the early formation of the abscission layer, and leaves may drop before they have a chance to develop fall coloration. A growing season with ample moisture that is followed by a rather dry, cool, sunny autumn that is marked by warm days and cool but frostless nights provides the best weather conditions for development of the brightest fall colors. Lack of wind and rain in the autumn prolongs the display; wind or heavy rain may cause the leaves to be lost before they develop their full color potential.
The character of autumn color is different in different parts of the world. In New England and the northeast sections of Asia, a few species dominate the deciduous forests. The display there tends to be short but intense because the change is rapid and rather uniform. In the southern Appalachians, the change is often gradual and the fall foliage season may last for more than a month because of the greater diversity of plant species found in the forest there. Mixed forests that have both evergreen conifers such as spruce and deciduous trees such as aspen or larch are found in the far north or at high elevations. Here, the dominant color is yellow and the change is rapid, with trees often going from green through brilliant yellow to bare over a period of two weeks. Tropical forests often have many deciduous trees that lose their leaves in response to drought; typically the leaves do not change color before they drop. In areas that are often cloudy for much of the autumn, with rather warm temperatures, fall colors are dull at best. This is often the case in much of Europe.
While the whole process of fall color is fairly well understood, the reason for it is less clear. Scientists have long known that xanthophylls and carotenoids play an important part in photosynthesis by helping to capture light energy, but the benefit of anthocyanins is not well understood. It might seem more logical for plants to remove all the carbohydrates they possibly can from the leaf before making it fall off. If this were the case, we wouldn’t have the red and purple pigments that we see in sugar maple, black gum, burning bush, or sweet gum. Carbohydrates are needed to manufacture these pigments. Some entomologists believe that the evolutionary reason that plants expend energy to produce fall color is to warn pests. A plant that is healthy is able to produce lots of carbohydrates, and therefore more anthocyanin. This may cause certain insect pests laying eggs in the fall to seek another host plant for their offspring that is weaker and drab by comparison. Some scientists believe that anthocyanins may act as a sunscreen to inhibit the destruction of the chlorophyll, help to prevent frost injury to leaf tissues, or limit water loss during dry spells in autumn. As far as the fall foliage watcher is concerned, their purpose is simple—they signal a last hurrah for the growing season and delight the optic nerve.