Ever wondered why leaves change color in the fall? The dazzling switch to oranges, yellows and reds that trees and plants make are the result of complex chemical changes. As trees switch from creating food and energy to conserving it, they cast off their leaves in the brilliant display of arboreal beauty that enables them to survive the winter.
Yellows and Oranges are Always There
The yellows and oranges that makes fall colors so spectacular are the result of carotenoid and xanthophyll pigments; chemical compounds found in foliage that are also responsible for the color of carrots. These carotenoids are actually always in the leaves, but we can’t see them because they are masked by the greens of chlorophyll.
Chlorophyll is another chemical that can be found in leaves. During the summer months, trees produce chlorophyll to enable photosynthesis–the process whereby they convert sunlight to food. The leaves absorb carbon dioxide and water and use sunlight to convert it to glucose and oxygen.
The green chlorophyll covers up the carotenoids so you see the leaves as green. As the temperatures begin to fall and the days get shorter, trees slow (and eventually stop) the production of chlorophyll in the leaves, and it’s color fades. Now we are able to see the underlying oranges and yellows.
Reds Play a Special Role
The red, scarlet and crimson colors are harder to explain. Unlike the carotenoids, the red pigments are not in the leaves over the summer months. The trees and plants that turn red actually produce this pigment called anthocyanin in the fall. Anthocyanin is what gives cranberries, cherries and plums their signature colors.
This anthocyanin pigment is actually a kind of “sunscreen” used to protect the leaves from the fall and winter sunshine. It is also thought to protect the leaves from freezing. Too much light can interfere with the transportation of nutrients from the leaves to the stems during fall–a process vital to conserve the tree through the cold winter months. Another theory is that the bright red leaves help to ward off insects and other pests in the same way red frogs and insects ward off predators.
Some plant and tree varieties add depth to the display with a range of browns. These result when chlorophyll in the leaves breaks down, leaving tannins which show through as brown.
What Makes For a Colorful Fall?
August should be sunny with steadily cooling nights which allow the trees to produce sugars. This sugar production triggers anthocyanin production in leaves. A sunny August must also contain a little rain because drought makes for water-stressed trees which will not make for good fall colors. Early frosts will put an end to fall colors as leaves halt all chemical production.
Leaf peepers can find the best places to view fall colors in Canada here and in the USA here.
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