MadSci Network: Botany

Re: why do pine trees stay green in winter?

Date: Fri Feb 6 15:16:28 1998
Posted By: Justin Remais, Student and Engineer Asst., University of California at Berkeley/Lawrence Berkeley National Laboratory
Area of science: Botany
ID: 886128852.Bt


Cross-Section, Pine Needle During the spring and summer the leaves have served as factories where most of the foods necessary for the tree's growth are manufactured. This food-making process takes place in the leaf in numerous cells containing chlorophyll, which gives the leaf its green color. This extraordinary chemical absorbs from sunlight the energy that is used in transforming carbon dioxide and water to carbohydrates, such as sugars and starch.

Along with the green pigment are yellow to orange pigments, carotenes and xanthophyll pigments which, for example, give the orange color to a carrot. Most of the year these colors are masked by great amounts of green coloring. But in the fall, because of changes in the length of daylight and changes in temperature, the leaves stop their food-making process. The chlorophyll breaks down, the green color disappears, and the yellow to orange colors become visible and give the leaves part of their fall splendor. At the same time other chemical changes may occur, which form additional colors through the development of red anthocyanin pigments. Some mixtures give rise to the reddish and purplish fall colors of trees such as dogwoods and sumacs, while others give the sugar maple its brilliant orange. The autumn foliage of some trees show only yellow colors. Others, like many oaks, display mostly browns. All these colors are due to the mixing of varying amounts of the chlorophyll residue and other pigments in the leaf during the fall season.

As the fall colors appear, other changes are taking place. At the point where the stem of the leaf is attached to the tree, a special layer of cells develops and gradually severs the tissues that support the leaf. At the same time, the tree seals the cut, so that when the leaf is finally blown off by the wind or falls from its own weight, it leaves behind a leaf sear.

Most of the conifers - pines, spruces, firs, hemlocks, cedars, etc. - are evergreens that do not lose all of their leaves in the winter. The needle-or scale-like leaves remain green or greenish the year round, and individual leaves may stay on for two to four or more years.

"Evergreens" keep most of their leaves during the winter. They have special leaves, resistant to cold and moisture loss. Many cut back on the amount of water they lose from their leaves. The dark color around the edge of this cross section of a pine needle is a layer of wax, which reduces dehydration. Others, like holly, have broad leaves with tough, waxy surfaces. On very cold, dry days, these leaves sometimes curl up to reduce their exposed surface. Some, like pine and fir trees, have long thin needles that allow snow to easily fall off the tree. Many evergreens also have chemicals in them which make it harder for them to freeze. Evergreens may continue to photosynthesize during the winter as long as they get enough water, but the reactions occur more slowly at colder temperatures.

Take care Kendra,
justin remais

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