Why Leaves Change Color

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.


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.





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.

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