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Mechanism of Leaf Pigmentation
Leaves display greenish hues because of a green pigment called chlorophyll. The chlorophyll usually masks the effects of the xanthophylls and carotenoids, two other types of pigments also present in leaves. As days grow shorter, leaves stop producing chlorophyll; during this time, xanthophylls and carotenoids introduce yellows and oranges to tree foliage. Some leaves produce anthocyanins, another type of pigment made from sugars, which gives leaves a reddish-purple tinge. These colors do not remain for long. As the pigments break down over time, only the tannins -- brownish pigments also present in tea -- linger, bringing a russet hue to leaves.
Dormancy
Like many other organisms, trees also go through a dormant period during the autumn and winter months. During this time, organisms strive to conserve as much energy as possible because food sources become scarce. Plants, which make their own food, struggle with decreasing amounts of sunlight. Evolution allowed plants to use this time to limit activity to the bare minimum for survival. Plants no longer need features such as leaves, which require too much energy to maintain and provide minimal benefits to the plant during this time. Many plant species have sufficient stores of sugar to last through autumn and winter, although they no longer make food.
Role of Anthocyanins
Professor Kevin S. Gould of the University of Auckland's School of Biological Sciences explored the varied purposes for the production of anthocyanins in autumn leaves. The reddish coloring imparted by anthocyanins deters predators such as leaf-cutting ants and aphids from destroying leaves. Anthocyanins also protect leaves from the damaging effects of the sun's ultraviolet rays. The antioxidant effect of anthocyanins helps the leaves repair themselves by ridding plant cells of harmful free radicals such as peroxide, nitrogen and oxygen.
Coevolution Theory of Autumn Colors
Another explanation of leaf color changes proposed by W. D. Hamilton, a research professor in evolutionary biology at Oxford University, supports the idea that trees evolved brighter colors during the cold months to discourage insects such as aphids from exploiting the plants. Marco Archetti of the Université de Fribourg in Switzerland and Sam P. Brown of the University of Cambridge analyzed several studies that tested and seemed to confirm Hamilton's theory. Insects seemed to prefer green leaves over other colors while trees with the brightest foliage harbored the least number of destructive insects.
Weather Effects
Autumns consisting of warm days and cool evenings produce the most vibrant leaf colors. Trees know when to start changing their foliage colors primarily by detecting shortening day lengths; temperature changes provide a secondary detection mechanism. With the slight increase in global temperatures due to global warming, some trees have delayed their autumn transformations. Warmer temperatures also affect the vivid coloring normally seen. Because global warming brings greater precipitation, the cloud cover would limit the amount of sunlight plants need to produce sugars. The reduction in anthocyanins produced would lead to a decline in the amount of red and purple leaves seen.