What Happens to the Leaves of Most Perennial Trees Each Autumn?

Trees are symbols of endurance and strength as they grow and persevere year after year, even in adverse conditions. In temperate climates, trees have adaptations that allow them to live through severe winter cold, such as losing their leaves. Deciduous trees shed leaves each fall, many showing brilliant color. Leaves, which would die on the tree if exposed to sudden freezing temperatures, are programmed to fall off each year before serious cold occurs, lessening the chance that decaying leaves on trees could spread infection to the parent plant. A number of changes occur in the leaf to allow it to detach from the tree.

Loss of Chlorophyll
- Chlorophyll gives leaves a green color.
  
  During spring and summer, leaves are food-producing factories. Chlorophyll, which reflects green light, is contained in special structures inside leaf cells called chloroplasts. Chlorophyll's job is to take water and atmospheric carbon dioxide and combine them in the presence of sunlight to form sugar, which is taken throughout the plant for food. Chlorophyll doesn't last long, so the plant has to make new chlorophyll all through the warm seasons of the year. When cooler fall temperatures begin, the plant stops making chlorophyll.
Membrane Formation
- Carotenoids are revealed in leaves as chlorophyll disappears.
  
  Cooler temperatures, shortening day length and lessening of sunlight prompt the tree to gradually create a tough membrane between the leaf stem and the tree branch. The membrane, called the abscission layer or separation layer, gradually stops sugars from going from the leaf into the tree and stops water and nutrients from entering the leaf. This helps shut down chlorophyll production, revealing underlying orange pigments called carotenoids, and triggering production of anthocyanins, which are red and purple.
Other Pigments
- Anthocyanins give the red color to fall leaves.
  
  Carotenoids are naturally present in leaf chloroplasts along with chlorophyll, but are masked by chlorophyll's green. When chlorophyll production shuts down, the orange carotenoids are revealed. Other pigments, yellow xanthophylls, are also revealed as chlorophyll disappears. Anthocyanins are usually produced in fall in the leaf's cell sap. Increased sugar buildup in the leaf because of the closing membrane, bright sunlight, dry weather and cool nights produce the most vivid red and purple anthocyanin colors. The acidity of the cell sap influences the colors, with acidic cell sap giving bright red and more alkaline cell sap giving purple.
Falling Leaves
- Fallen leaves furnish food and cover to other organisms.
  
  As the separation layer continues to form, leaf color intensifies and the leaves become drier. Finally the veins that connect the leaf to the twig are completely sealed off, and the leaf is ready to fall to the ground with a passing breeze, a jumping squirrel or a stray raindrop. Without leaves to support, the tree can finish preparing for winter, entering dormancy. Fallen leaves decompose on the ground to return nutrients to the soil and furnish many organisms such as fungi, bacteria, earthworms and insects with food in turn. Layers of leaves furnish insulation to plants, tubers, and small animals that overwinter in the ground.

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What Happens to the Leaves of Most Perennial Trees Each Autumn?

Loss of Chlorophyll

Membrane Formation

Other Pigments

Falling Leaves