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Why do leaves change color in the fall?
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Different chemicals cause autumn leaves to change color.
During the fall so many people enjoy looking at the leaves as they change from green to yellow, orange, red and brown. It is such a part of fall that we don’t always stop and think of why it is the way it is. When winter comes the trees need to shed their most parts and the chemicals chlorophyll, cartenoids and anthocyanins all play a role in leaf color.
Leaves are at their most useful to trees during the summer, when the days are longest. When trees take in water and carbon dioxide from their environment they need sunlight to work with the chlorophyll in the leaves. This turns these elements into oxygen and glucose and is called photosynthesis. As the summer ends, the days get shorter and the trees receive less light. This is the trigger that winter is coming on. Thin leaves on deciduous trees are full of sap that can freeze easily. Since the trees have spent the summer making lots of nutrients, they can shut down and live off of stored resources when the days get short. Leaves fall as the veins at the base of the leaves gradually begin to clog as the days shorten. Cells are produced that trap certain chemicals inside the leaves and these chemicals produce colors. Once all of the veins are shut off, the leaf drops.
The main chemicals involved in tree leaf color are chlorophyll, cartenoids and anthocyanins. Chlorophyll is what gives the leaves their green color. Cartenoids (carotene) are responsible for brown, orange and yellow colors in fruits, vegetables and flowers. As the summer process ends, the chlorophyll begins to drain from the leaves, leaving another color like brown or yellow behind. Anthocyanins dissolve in water and bring color to red and purple fruits like strawberries, plums, and red apples. All of these things are usually present in leaves but the cartenoids and anthocyanins only appear when the chlorophyll breaks down. Anthocyanins are not as common during the summer but surge during the fall. The reds, greens, oranges, yellows and browns are always in the leaves, but in the summer the green color from the chlorophyll is stronger.
Colors are usually specific to each type of tree. Maples turn red, beech trees turn pale tan, oaks red or brown, and aspens are yellow. Red is caused when the glucose does not drain from the leaves and becomes stuck inside them. Leaves turn brown as a result of other waste products left behind.
Bright color in tree leaves best develops when there are warm sunny days and cool, but not freezing nights during the fall. The warmth of the days promote sugar growth, but when the sun sets and the air cools the leaves, the veins close and the sugar stays put. Because of the balance of lots of light and lots of sugar the anthocyanins are very productive and lots of color occurs. But, warm days and nights at this time of year will slow down production of these chemicals. Rainy days early in the also make the colors more intense but later in the fall it will dull them. An early frost will also take the zing out of normally bright red leaves. Moisture also affects fall leaves.
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