Welcome to You Ask Andy

David Milner, age 11, of Toronto, Ontario, Canada, for his question:

Why do the fall leaves change color?

Come fall, the maples of Canada display their patriotism in a blaze of scarlet and the silver stemmed birches hang their boughs with leaves like golden coins. After these days of glory, the withered leaves litter the ground and the dazzling display is a dying memory. Or so it seems. Actually, the color change is but one step in a fantastic recycling program that has nothing at all to do with dying.

Trees that shed in the fall have broad papery leaves that need lots of moisture. Frost turns their sap into tiny daggers of ice that shatter their fragile cell walls    and frost freezes the moisture in the hard ground. This is the basic reason why these trees shed their leaves before winter comes. As usual the main part of the project goes on behind the scenes. And in this case it happens to be an intricate recycling program    in two parts.

The total program involves a multitude of plant chemicals and chemical processes. We don't notice, but it starts in summer, when the trees are greenest.

The shorter hours of daylight cause changes in the DNA, the miracle biochemical that directs all operations in every living cell. Orders are given to reduce certain chemicals and produce others. A system of seeping liquids is set up to transport reusable ingredients from the leaves to winter storage in the living cells of wood, bark and roots.

The colorful pigments are just a few of thousands of chemicals involved in operations that go on in the leaves. During the summer, the reds and yellows are masked by overpowering quantities of green. When the fall recycling program begins, the green chlorophyll is reduced and the more vivid pigments show their colors. In some cases, more red pigments are manufactured in the fall, especially in cool weather.

Meantime, as dazzling fall colors light up the landscape, the serious programs of recycling goes on behind the  cenes. Thousands of  proteins are broken apart and reassembled to transport nitrogen and phosphorus, potassium and sulphur, manganese and magnesium to winter storage. Next spring, the rising sap will transport these vital growth ingredients back to work. They will nourish the bursting buds until their young green leaves can manufacture the next year's plant foods.

The other half of the program begins when the withered leaves fall to the ground. Rains and decay bacteria break up their reusable plant foods, which seep down to nourish the roots.

The intricate program is highly complex and nothing is wasted. The greens change to vivid fall colors as the summer period of maximum food production changes to the preparation for winter storage. Even the tough leafy skeletons are useful. They cover the ground with a porous blanket of mulch that helps to keep the soil warm and moist.

 

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