Nature's Brew Brings Out Fall's Beautiful Palette

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By Joel M. Lerner
Saturday, November 1, 2008

We can look at a calendar and know that it's fall. But how do the trees know?

There are three major factors that alert trees that the season is changing: length of night, pigments in the leaves and weather. Of those three, the amount of light (or photoperiod) is the key factor. Other aspects, such as species and genetic variation, also play a role.

All those factors together make for spectacular color changes. But some of those yellows, reds and oranges actually are there all the time -- we just can't see them.

Trees are far more active than you might realize. They work constantly during the growing season to produce carbohydrates from light, water and carbon dioxide. Light comes from the sun, water from the earth and carbon dioxide from the air.

In the process, called photosynthesis, trees release oxygen and water into the atmosphere. All parts of the tree are dynamically communicating with one another. They are constantly interpreting and reacting to changing conditions in the atmosphere and in the soil. They know how much water they require and how much they're getting, and adjust their fluids and nutrients to the weather. That includes knowing how much light they're getting and making necessary adjustments as it increases or diminishes.

In the spring and summer, leaves are green because they are full of chlorophyll, used in photosynthesis. The diminishing of sunlight triggers a number of changes in the way trees function. One of these is the development of a corky membrane between the branch and the leaf stem, which reduces the amount of nutrients getting into the leaves. Reduced nutrients mean reduced production of chlorophyll. That's why the green color disappears.

As chlorophyll production slows in the fall, other pigments in the leaves begin to show. Among them are carotenoids, which are attached to the cell membranes and give leaves yellow, russet or brown colors.

Reds, purples and bright oranges come from anthocyanins, which some trees produce to help extract all the nutrients from dying leaves. Anthocyanins are produced only in the fall and in response to light and excess sugar in the leaf cells. These pigments are sensitive to the pH levels of the cell sap. Acidic sap produces reds; alkaline sap produces shades that are more purple.

The impact of weather on leaf color is related to temperature and rainfall. If the days are dry and bright and the nights dry and cool, that encourages the production of anthocyanins, making the display more intense.

The mix of pigments in each type of tree governs the colors. Sugar maples become a brilliant yellowish red. Oaks turn red, russet or brown. Dogwoods turn deep red. Hickories are a brilliant yellow; aspens are famously golden, as are birch and yellow poplar. Sweet gums can be deep red or a mixture of yellow and deep red. Chestnuts display a wide range of colors from yellow to red to purple, sometimes on the same leaf.

Deciduous trees don't hold onto their leaves all year because they would lose more water through them than they would gain from the ground. Rainfall might be scarce or come in the form of snow, and the ground might be frozen. The corky membrane shuts off each leaf, and they all fall to the ground.

The spring and early summer this year were most generous with rain, but August and September were dry. This must bode well for a magnificent display because this season has been beautiful.


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© 2008 The Washington Post Company

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