Why Leaves Change Color

Why do leaves turn beautiful colors? Why do some turn yellow and others red? Why is fall color better some years than others? In summer, leaves are green because of a pigment known as chlorophyll, which is vital to the tree's food-making process (photosynthesis). Leaves manufacture simple sugars from water and carbon dioxide, using energy captured from the sun by chlorophyll. These sugars are the sole source of the carbohydrates needed for the tree's growth and development.

As fall approaches, chlorophyll is replaced at a slower rate. Shorter days cause a layer of cork cells to form at the base of each leaf, gradually closing off the flow of water and nutrients into the leaf. As the supply of chlorophyll dwindles, other pigments are slowly unmasked and begin to show through.

Unmasked pigments result in brilliant yellows and oranges. Another group of pigments is responsible for the reds, purples, and blended combinations of these colors. These pigments develop in late summer in the sap of the leaf cells. Their formation depends on the breakdown of sugars in the presence of bright light. The brighter the light; the more brilliant the color.

There is no formula to predict fall color. The intensity and peak time of color are determined by complex environmental factors and the genetic makeup of the plants themselves. For example, trees and shrubs of European origin evolved where the growing season is longer and cooler, so they stay green into the fall.

The "best" fall color for an area occurs during the shortening days of autumn when days are bright, sunny and cool, when nights are cool but not below freezing, and when there has been ideal rainfall.

Generally, plants growing in shade do not produce the brilliant colors that plants of the same species produce in full sunlight. Trees along the edge of woodlands and tall trees that make up the forest canopy usually change color first. Leaves on smaller saplings and more shaded trees stay green until the leaves of the taller, more exposed trees have changed color and fallen. However, plants that are in poor health or are stressed, usually change color earlier than their healthy, unstressed neighbors.

We don't yet fully understand all of the complicated interactions—involving pigments, sunlight, moisture, chemicals, hormones, temperatures, length of daylight, site, genetic traits, and so on—that make for a perfect autumn color display. As research probes deeper into the basics of plant life, we will understand more about the processes that color the autumn landscape.