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How & Why Leaves Change Color Each Autumn

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Leaf Color Changes When The Kitchen Closes For The Season
Tree leaves are busy little chemical factories changing sunlight into food for the tree. Think of the leaf as a little restaurant kitchen where the staff is the chlorophyll within the leaf. Chlorophyll is what gives leaves their normal summer green color. All summer the chlorophyll staff is busy mixing sunlight with minerals carried up to the kitchen from the soil by the tree's sap.

The chlorophyll staff does not like cool weather, or a lack of sun. They're a sandy beaches kind of crowd. So when days start getting shorter, and temperatures start to drop in proportion to diminishing sunlight levels, the chlorophyll staff starts closing the kitchen down for the season. Fall color changes in tree leafs begin only when chlorophyll begins to drain out of the leaf. How fast, and how much chlorophyll drains out of each leaf on a tree is a rather complex process driven by several factors.

Falling chlorophyll levels trigger a chemical reaction that forms special cells at the point where the leaf is attached to its twig. These specials cells start to grow in cool weather, and within two weeks or so they completely pinch off the sap flow to the leaf. Once they have completely sealed off the leaf from its twig, the leaf falls to the ground. But prior to this detachment point, the kitchen is a blaze of final activity we humans observe as fall foliage.

Going Out In Style - The Creation Of Great Color
As the green chlorophyll fades away, the underlying yellows and oranges of the carotenoids begin to show through. These colors have always been there but have been covered by chlorophyll. The reds and yellows of the chemicals carotin and xanthophyll appear in many leaves. Tannis creates the deep browns and mahogany seen in the oaks.

It is a common belief that abundant summer rainfall makes for great fall foliage. True, it helps. But it takes more than rain. A great fall foliage display requires sunny fall days and cool nights. It takes nighttime temperatures consistently falling to around 45 degrees Fahrenheit, followed by clear sunny days with temperatures no warmer than 65 degrees Fahrenheit, to produce good color. Why?

Most of the sugars and carbs are made in the leaf during the day in the dwindling fall sunlight. But cool nights prevent movement of sugar through the leaves into the branches. From the sugars trapped in the leaves, red pigment called anthocyanin is formed. But if the weather is cloudy, little sugar is produced because there simply is not enough sunlight. And if the weather stays warm, what little sugar is produced flows out of the leaf at night. The result is no trapped sugars to turn color inside the leaf. Colors end up brown, or muted, not brilliant. Warm, cloudy fall weather wrecks any chance of spectacular displays - no matter how much rain fell over the summer months.

Last Call for the Season
As the weather continues to cool, those special cells in the base of the leaf stem continue to squeeze off the flow of sap to the leaf. Additional chemicals forming within the leaf become trapped, and the foliage display peaks. The chemicals carotin and xanthophyll, along with tannins, continue to create various colors within the leaves. Which chemicals, and how much of each, varies from tree species to tree species, hence the various colors produced by each tree. Reds and browns show in leaves with trapped tannins. Crimson and yellow appear in leaves with trapped carotin and xanthophyll. The same chemicals can create different colors in different tree species.

How to Bust A Great Party
An early hard frost kills leaves before they have had a chance to complete the final fall shut-down. The icy dew we call frost does not, in and of itself, cause color change. It's the cold temperature associated with a frost that starts the process. But a hard frost, or too many nights of frost, kills those special cells squeezing the leaves off the branches. The chemical change inside the leaf cannot be completed. Early hard frosts are total disaster for fall foliage displays. Hard rains will also destroy foliage. The color-bearing chemicals within the leaves are water soluble. Therefore heavy rains not only knock the dying leaves off the tree, but also wash out most of the color. What early frosts, warm cloudy weather, and rain don't destroy, high winds can. Like so many other events in Nature requiring perfect conditions, grand foliage displays are a result of just plain luck!



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