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The Chemistry of Fall Colours
Autumn is a second spring when every leaf is a flower ~ Albert Camus
Nature’s most spectacular colors decorate the trees as the days shorten and the warmth of summer fades. This transformation is created by a surprisingly limited range of chemicals that produce a myriad of fall hues.
Gold and yellow were not created; instead, they are revealed when the leaves begin to close for the winter. The lush green chlorophylls that color our world are unstable compounds. An active plant needs to constantly replace chlorophyll, because, paradoxically, sunlight destroys it. Chlorophyll is aided in its quest to harvest light by a group of carotenoid auxiliary compounds, compounds such as carotene (orange) and xanthophyll (yellow). Both chlorophyll and carotenoids are physically bound to plant cell membranes and cannot move freely throughout the plant. Carotenoids give color to things like: corn, carrots, daffodils, buttercups and bananas.
The beginning of autumn signals the plant to stop creating new chlorophyll and conserve its energy. As chlorophyll fades, carotenoids remain present, resulting in their chemical resistance, and the green mask disappears. Consequently, the yellow and orange colors are created simply by removing the green pigment from the leaves.
What causes autumn to begin?
The main sign of autumn is shorter days, but cold nights also trigger changes in the tree. However, summer drought can delay fall as the trees try to produce a little more sugar before winter. To protect against potential damage caused by frozen leaves, to conserve water and minerals, to expel waste products and to avoid disease, the tree has a corky membrane between the branch and the leaf stem. Once the day length is reduced enough, the tree gradually begins to reduce the production of chlorophyll and the veins in the leaves slowly close. When the separation is complete and the connective tissues are sealed, the leaf is ready to fall. The sealing membrane not only stops the inward flow of nutrients, but also blocks the outward flow of sugar, trapping it in the leaf. The blocked sugar in the leaves of some trees, such as maples, reacts to produce reddish pigments, called anthocyanins. Anthocyanin pigments give color to things like: cranberries, red apples, grapes, blueberries, cherries, strawberries, and plums, among others. The color produced by anthocyanins is sensitive to the pH of the cell sap. If the juice is quite acid, the pigments give a bright red color; if the juice is less acidic, its color is more purple.
It is not known exactly why these trees would waste the sugars in their leaves like this, since nature generally abhors waste. Some scientists believe this helps the trees keep their leaves longer because anthocyanins lower the freezing point of the leaves. Others think it’s because anthocyanins, once absorbed into the soil from composted fallen leaves, help prevent some other plant species from growing where the leaves have fallen, reducing competition for nutrients in the soil around the tree.
As we know, different trees can take on different colors in autumn. Oak trees tend to turn brown or red or a mixture of the two. Hickory becomes bronze; aspen, golden; dogwood is a mixture of purple and red. Maples vary quite a bit depending on the species. Red maple will turn crimson. Sugar maple will turn orange/red. Black maple will turn bright yellow. On the other hand, elm leaves tend to just fall off without really changing color much before they fall.
The eventual brown color of dead leaves comes from waste left in the leaves when the veins closed, especially from oxidized tannin. Tannin is a bitter plant compound that binds to proteins and other organic compounds.
What determines how colorful autumn will be?
The most vivid colors on tree leaves can usually be seen after a series of very warm sunny days in autumn, which in turn give way to cold but not freezing nights. This is because a lot of sugar will be created in the leaves, but will be trapped because the veins are closed. This in turn leads to increased anthocyanin production. Anthocyanins develop best where the soil is acidic; there are few nitrates, and there is an abundance of light. Thus, the brightly bathed tops of the maple leaves and the sunny sides of the apples are the reddest. Leaves that primarily contain anthocyanins will appear red. Leaves with good amounts of anthocyanins and carotenoids will appear orange. Leaves with carotenoids but little or no anthocyanin will appear yellow. Some leaves contain oxidizing tannins, which are responsible for the brownish color of some oak leaves.
Autumn is supposed to come in waves. First wave, yellow dominant. Willow, poplar, birch and some maple. Second wave, orange. Silver maples and white oaks. Finally, a third wave of red from trees like maple. The best fall colors are between the second and third wave
Autumn wins you over best by this, with its silent appeal to sympathy for its decay.~Robert Browning Hamilton
Why do we have autumn colors?
The evolutionary function of autumn colors is less clear. As described above, some fall colors are not intentionally produced but are revealed as the green fades, others are actively produced by the tree during this period. There are two current theories that help explain why a tree can benefit from fall colors: photoprotection and coevolution. Photoprotection can be a method by which autumn color chemicals help the tree to make the most of the nutrients that can be reabsorbed from the leaves before they fall. Research has shown that anthocyanins ‘protect’ the leaf from the harmful effects of light at low temperatures, which would otherwise prevent the reabsorption of nutrients such as nitrogen from the leaves into the branches. Coevolution suggests that the colors act as a warning to insects that cling to trees or their eggs during the winter. The colors visibly signal the amount of chemical defense against insects. Branches with the brightest or most intense colors deter most insects. Insects profit by finding the least defended trees on which to lay their eggs. Trees with the reddest leaves reduce the number of parasites they carry. The theory is based on a broader branch of evolutionary ‘signaling theory’, which says that trees must defend themselves in this way, otherwise they wouldn’t put so much effort and expense into it. There is some evidence that aphids avoid trees with red leaves.
The most useful photography tool in autumn is a polarizing filter. It works in two important ways; firstly enhancing the blueness of the sky and enhancing the contrast with autumn colors and secondly reducing the reflection of light from the leaves making the colors more vivid. Additionally, keep an eye on the woods in the autumn mists, keeping an eye on the exposure range as the meters will be fooled by the mist. The effect of the fog is enhanced by the sunlight through the misty woods showing beautiful ‘crupuscular’ rays.
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