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Fall Leaf Color Science

Fall Leaf Color ???

Dr. Wayne Clatterbuck, Professor of Forest Management and Silviculture, UT Department of Forestry, Wildlife and Fisheries, provides some insight into this question in the following exerpt from his article that appeared in the Departmentís Newsletter:

"We wish we could be more definitive with leaf color predictions because people want to make plans to view the landscape when leaf color is at its peak. However, leaf color really depends on the weather in the next few weeks. Can we confidently predict the weather two or three weeks in advance? Leaf color will depend on cool nights with temperatures in the high 30's and in the 40's and the amount of moisture we have in the next few weeks.

"The color of leaves changes first at the higher elevations where it is cooler and progresses to the valleys at the lower elevations. Color generally begins in the mountains during the second week of October and advances to the valleys by the end of October and even lasting into the first two weeks of November. The changing leaf color is triggered by the shorter days of sunlight, and then influenced by temperature and moisture."

The following information about the science of foliage color can help you enjoy the colorful fall season. In addition, our handy Fall Leaf Color Guide can help identify trees by the color and shape of leaves.

The Geometry of Fall Leaf Color

Leaf Geometry Leaves begin to change color along their outer margins first. These outer edges are the last to get nutrients and water, and as the nutrient supply is decreased, these cells are the first to lose their chlorophyll. The midrib stays green the longest since it gets nutrients first and easiest, and here the chlorophyll can remain active longer.


The Weather and Fall Leaf Color

The following description appeared in the UT Agricultural Extension Service publication number SP 529 and was written by Dr. Wayne Clatterbuck, UT Professor of Forestry, Wildlife & Fisheries.

Rain, Sun, Temperature The amount, duration and brilliance of autumn color depend on weather conditions that occur before and during the time chlorophyll in the leaves is declining. Temperature, light and water supply are the primary factors that influence the synthesis of carbohydrates (sugars) that favors anthocyanin formation and bright fall color. Cool, but not freezing, temperatures favor anthocyanin production. Early frost is more likely to kill leaves, making them turn brown and fall sooner from the trees. Bright light favors red colors, so red color often develops on exposed leaves. Water supply also affects anthocyanin production, with mild drought favoring bright reds. Rainy days occurring near peak coloration will decrease color intensity. Late summer droughts can delay the onset of fall color by a few weeks. Temperature, sunlight and moisture are highly variable each year, assuring that no two autumns are alike.

The Chemistry of Fall Color

During the summer, it is chlorophyll that gives leaves their green color. This complex chemical, which is essential in the photosynthetic production of food (sugars), is continually being manufactured and broken down at approximately equal rates.

As fall approaches, the steadily decreasing length of day and cooler temperatures interact to biologically trigger the formation of a corky layer of cells called the abcission layer across the base of the leaf petiole. This formation gradually decreases the supply of water and minerals to the leaf; reduces the manufacture of chlorophyll; and traps sugars in the leaf.

When chlorophyll is reduced, a pigment known as anthocyanin becomes prominent. This pigment is responsible for the red and purple coloration of autumn leaves and is built from the sugars trapped in the leaf. Bright, sunny, fall days increase the photosynthesis of sugars, therefore, increasing the production of anthocyanins. This condition gives the most brilliant red colorations in the fall.

If no anthocyanins are produced, the leaf will turn yellow or orange due to the presence of the pigments known as carotene and xanthophyll. These pigments were present in the leaf all summer, butr had been masked by chlorophyll until now. Since carotene and xanthophyll are not built from sugars, the yellow fall coloration is not as dependent on weather conditions, but rather on the absence of other pigments.

An additional substance which affects foliage color is tannin. Most prevalent in oaks, walnuts, and hickories, tannin turns leaves brown or dark red.

Leaves begin to change color along their outer margins first. These outer edges are the last to get nutrients and water, and as the nutrient supply is decreased, these cells are the first to lose their chlorophyll. The midrib stays green the longest since it gets nutrients first and easiest, and here the chlorophyll can remain active longer.

Leaf coloration may vary within a species due to genetic differences in pigment production. Environmental conditions, such as soil acidity, can also cause variation in leaf color. Leaves of Red Maples growing in very acidic soils turn brilliant red, while trees in neutral or alkaline soil may have yellow leaves.

The best fall foliage occurs when there is a dry, late summer to start formation of the abcission layer to trap sugar in the leaf. Then, to prevent leaves from falling too soon, rain is needed in early fall. An alternation of heavy rain and bright sunshine along with the gradual dropping of temperatures gives the most brilliant colors.

The presence of a large number of plants in North America having brilliant fall foliage colors is more unusual when it is noted that the only other places in the world with a similar abundance of foliage colorations are northern China, Korea, and Japan.

The Physics of Fall Leaf Color

The following description appeared in the UT Agricultural Extension Service publication number SP 529 and was written by Dr. Wayne Clatterbuck, UT Professor of Forestry, Wildlife & Fisheries.

Several pigments in leaves are responsible for color: chlorophyll, carotene, xanthophyll and anthocyanins.

Chlorophyll is the pigment in chloroplasts of plants that reflects green light. Plants use the energy absorbed by chlorophyll in photosynthesis to produce food for plant growth and development. Chlorophyll is continually broken down during photosynthesis and being replenished by the plant.

Carotene and xanthophyll are pigments that reflect orange and yellow light respectively. Both are present in the chloroplasts, with chlorophyll enabling the plant to absorb a wider range of wavelenghts of light and thus capture more energy. These pigments are in such small quantities that they are masked by the more dominant chlorophyll during the growing season.

With the passing of summer, days become shorter. The phytochrome, the light-sensing mechanisms in leaves, recognizes the shorter day lengths. The shorter days and lower temperatures arrest chlorophyll production. Chlorophyll breaks down faster than it is replaced, allowing the yellow and orange pigments to be unmasked.

The molecules reflecting red wavelengths, anthocyanins, are water-soluble pigments that occur in the cell sap creating the red, pink, and purple hues. Not all trees produce anthocyanins. These pigments are not present during the summer, but their formation is encouraged during a succession of cool nights and sunny days. During these days when photosynthesis and chlorophyll production are decreasing, an abundance of sugars accumulates in the leaf. The cool nights promote a separation layer of cells in the petiole, where the leaf attaches to the twig, that prevents sugar from flowing out of the leaf, and also arrests the flow of nutrients into the leaf. The formation of anthocyanin requires bright light, a diminishing water supply and the accumulation of sugars trapped in the leaf.

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