Why Leaves Change Color
by Nina Bassuk
The glorious season that is fall in northeastern North America, is actually the expression of some very necessary physiological processes that prepare deciduous trees for winter. With the advent of shorter days and cooler nights in late summer, a series of responses are triggered which lead to a marvelous array of leaf colors including yellow, orange, red and purple. Every species has its own signature fall color, which in turn varies with site conditions and tree health. Those of us in northeastern North America share this phenomenon with only one other region of the earth, the northern areas of China, Korea and Japan.
Unlike hardy evergreens whose tough leaves are adapted to temperatures below zero, the succulent leaves of deciduous trees are not able to withstand the cold temperatures of winter. All leaves are the factories of trees, photosynthesizing and creating food that allows the tree to grow. Before the leaves fall, much of this food is transported out of the leaves and into the branches, trunk and roots. At the place of leaf attachment, where the petiole (leaf stalk) meets the branch, a series of specialized, thin walled cells form what is called the abscission layer.
The abscission layer is the point where the leaf will eventually detach from the branch, but not before the branch is sealed off to prevent water loss or disease entry. The ‘walling-off’ process requires energy from the tree and generally is completed only after several weeks. Then when only the last vestiges of cells are attached, the action of frost, wind or rain may hasten the leaf’s final departure to the ground.
Between the time when the abscission layer starts to form and the leaves actually drop off, we are treated to a glorious display of leaf colors lasting over a month. While the abscission layer is forming, the flow of fluids into and out of the leaf becomes more and more restricted. The single most important fact at this point is that chlorophyll, the green pigment predominantly responsible for photosynthesis, begins to break down, unmasking the yellow and orange pigments, which were always present but were covered up by chlorophyll. These pigments, the caratenoids (named after the color of carrots) are prominent in the fall foliage of the birches, beech, hickory, honeylocust, and ash, but are also responsible for the colors of the daffodil and sunflower. B-carotene, a pre-cursor for vitamin A, is one type of caratenoid which we are familiar with in food. As a group, the caratenoids also participate in photosynthesis and are thought to protect chlorophyll in the presence of light and oxygen.
However, it is the red and purple pigments of fall foliage when combined with the caratenoids that really cause the eye-catching display we see in this part of the country. Maples, sumac, oaks, poison ivy, sweetgums and Boston ivy are among the species that literally glow with color. Pigments called anthocyanins are responsible for these colors, just as they are in beets, petunias and red roses. Their function is not well understood other than for attracting birds and insects for pollination.
As the abscission layer starts to form, anthocyanins are not present, but form only when cool nights slow the flow of sugars out of the leaves. The presence of sugars in the leaves is necessary for anthocyanin formation. The pH of the soil that trees are grown in can also affect the intensity and color of anthocyanins. The more acid the soil, the redder the color. The more alkaline the soil, the bluer or more yellow the color.
Many people say that you need to have a frost to get good fall color. This is not strictly true, because cool night temperatures above freezing will also trigger anthocyanin formation. The more frequent occurrence of cool night temperatures combined with sunny days favor the production of anthocyanins. It is this combination of sunny days and cool night temperatures during the time when leaves change color that determines whether it will be a good or great year for fall color. Rainy, overcast, warm weather during this time will produce a display rich in yellows but poor in reds.