A tree's roots, branches and twigs can endure freezing temperatures, but most leaves are not so tough. On a broadleaftree -- say a maple or a birch -- the tender thin leaves, made up of cells filled with water sap, will freeze in winter. Any plant tissue unable to live through the winter must be sealed off and shed to ensure the tree's survival.
As sunlight decreases in autumn, the veins that carry sap into and out of a leaf gradually close. A layer of cells, called the separation layer, forms at the base of the leaf stem. When this layer is complete, the leaf is separated from the tissue that connected it to the branch, and it falls. Oak leaves are the exception. The separation layer never fully detaches the dead oak leaves, and they remain on the tree through winter.
Evergreen trees -- pines, spruces, cedars and firs -- don't lose their leaves, or needles, in winter. The needles are covered with a heavy wax coating and the fluids inside the cells contain substances that resist freezing. Evergreen leaves can live for several years before they fall and are replaced by new growth.
On the ground, fallen leaves are broken down by bacteria, fungi, earthworms and other organisms. The decomposed leaves restock the soil with nutrients, and become part of the spongy humus layer on the forest floor that absorbs and holds rainfall. In nature, nothing goes to waste!
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