Autumn leaf color

Autumn leaf color is a phenomenon that affects the normally green leaves of many deciduous trees and shrubs by which they take on. During a few weeks in the autumn months, one or many colors that range from red to yellow. The phenomenon is commonly called fall colors and autumn colors, while the expression fall foliage usually connotes the viewing of a tree or forest whose leaves have undergone the change. In some areas in the United States (notably New England), and in much of Canada "leaf peeping" tourism between the beginning of color changes and the onset of leaf fall, or scheduled in hope of coinciding with that period, is a major contribution to economic activity.

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Chlorophyll and the green color

A green leaf is green because of the presence of a pigment known as chlorophyll. When they are abundant in the leaf's cells, as they are during the growing season, the chlorophylls' green color dominates and masks out the colors of any other pigments that may be present in the leaf. Thus the leaves of summer are characteristically green.

Chlorophyll has a vital function: that of capturing solar rays and utilizing the resulting energy in the manufacture of the plant's food — simple sugars which are produced from water and carbon dioxide. These sugars are the basis of the plant's nourishment — the sole source of the carbohydrates needed for growth and development. In their food-manufacturing process, the chlorophylls themselves break down and thus are being continually "used up." During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.

In late summer, the veins that carry fluids into and out of the leaf are gradually closed off as a layer of special cork cells forms at the base of each leaf. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. It is during this time that the chlorophyll begins to decrease.

Often the veins will still be green after the tissues between them have almost completely changed color.

Pigments that contribute to other colors

Carotenoids

  As autumn approaches, certain influences both inside and outside the plant cause the chlorophylls to be replaced at a slower rate than they are being used up. During this period, with the total supply of chlorophylls gradually dwindling, the "masking" effect slowly fades away. Then other pigments that have been present (along with the chlorophylls) in the cells all during the leaf's life begin to show through. These are carotenoids and they provide colorations of yellow, brown, orange, and the many hues in between.

The carotenoids occur, along with the chlorophyll pigments, in tiny structures called plastids within the cells of leaves. Sometimes they are in such abundance in the leaf that they give a plant a yellow-green color, even during the summer. Usually, however, they become prominent for the first time in autumn, when the leaves begin to lose their chlorophyll.

Carotenoids are common in many living things, giving characteristic color to carrots, corn, canaries, and daffodils, as well as egg yolks, rutabagas, buttercups, and bananas.

Their brilliant yellows and oranges tint the leaves of such hardwood species as hickories, ash, maple, yellow poplar, aspen, birch, black cherry, sycamore, cottonwood, sassafras, and alder.

Anthocyanins

The reds, the purples, and their blended combinations that decorate autumn foliage come from another group of pigments in the cells called anthocyanins. These pigments are not present in the leaf throughout the growing season as are the carotenoids. They develop in late summer in the sap of the cells of the leaf, and this development is the result of complex interactions of many influences - both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of phosphate in the leaf is reduced.

During the summer growing season, phosphate is at a high level. It has a vital role in the breakdown of the sugars manufactured by chlorophyll. But in the fall, phosphate, along with the other chemicals and nutrients, moves out of the leaf into the stem of the plant. When this happens, the sugar-breakdown process changes, leading to the production of anthocyanin pigments. The brighter the light during this period, the greater the production of anthocyanins and the more brilliant the resulting color display. When the days of autumn are bright and cool, and the nights are chilly but not freezing, the brightest colorations usually develop.

Anthocyanins temporarily color the edges of some of the very young leaves as they unfold from the buds in early spring. They also give the familiar color to such common fruits as cranberries, red apples, blueberries, cherries, strawberries, and plums.

In autumn forests they appear vivid in the maples, oaks, sourwood, sweetgum, dogwood, tupelo, black gum and persimmon. These same pigments often combine with the carotenoids' colors to create the deeper orange, fiery reds, and bronzes typical of many hardwood species.

Function of autumn colors

Deciduous plants are believed to shed their leaves in autumn primarily because the high costs involved in their maintenance would outweigh the benefits from photosynthesis during the winter period of low light availability and cold temperatures.^[1] However, there is no reason why leaf fall should necessarily be preceded by the production of vivid autumn colors, and the function of the color change is still uncertain. Autumn colors (especially red) are not just due to the breakdown of chlorophyll; in fact anthocyanins (red-purple) are actively produced in autumn. What use is the production of pigments in leaves that are about to fall? A number of hypotheses have been proposed, including photoprotection, coevolution and allelopathy:

Photoprotection

According to the photoprotection theory, anthocyanins protects the leaf against the harmful effects of light at low temperatures^[2][3]. It is true that the leaves are about to fall and therefore it is not of extreme importance for the tree to protect them. Photo-oxidation and photo-inhibition, however, especially at low temperatures, make the process of reabsorbing nutrients less efficient. By shielding the leaf with anthocyanins, according to the photoprotection theory, the tree manages to reabsorb nutrients (especially nitrogen) more efficiently.

Coevolution

According to the coevolution theory ^[4], the colors are warning signals towards insects that use the trees as a host for the winter, for example aphids. If the colors are linked to the amount of chemical defenses against insects, then the insects will avoid red leaves and increase their fitness; at the same time trees with red leaves will have an advantage because they reduce their parasite load. The coevolution theory of autumn colors was born as a branch of evolutionary signalling theory. It is a general feature of biological signals that, when a signal is costly to produce, it is usually honest - that is it reveals the true quality of the signaller, because it does not pay for a low quality individual to cheat. Autumn colors might be a signal if they are costly to produce, or they could be an index, which is maintained because it is impossible to fake (because the autumn pigments are produced by the same biochemical pathway that produces the chemical defenses against the insects). Although it is not certain that aphids have red receptors, there is some evidence that they preferentially avoid trees with red leaves. This is what the coevolution theory predicts at the intraspecific level (more insects on dull leaves). It is also known that tree species with bright leaves have more specialist aphid pests than do trees lacking bright leaves^[5], which is the interspecific prediction of the theory (autumn colors are useful only in those species coevolving with insect pests in autumn). The coevolution hypothesis has been subjected to criticism.^[6]

The change of leaf colors prior to fall have also been suggested as adaptations that may help to undermine the camouflage of herbivores.^[7]

See also: Deciduous and Plant defense against herbivory

Many plants with berries attract birds with especially visible berry and/or leaf color, particularly bright red. The birds get a meal while the shrub, vine or typically small tree gets undigested seeds carried off and deposited with the birds' manure. Poison Ivy is particularly notable for having bright red foliage drawing birds to its off-white seeds (which are edible for birds, but not most mammals).

Allelopathy

Researchers at New York's Colgate University have found evidence that the brilliant red colors of maple leaves is created by a separate processes then those in chlorophyll breakdown. At the very time when the tree is struggling to cope with the energy demands of a changing and challenging season maple trees are involved in an additional metabolic expenditure to create anthocyanins. These anthocyanins, which create the visual red hues, have been found to aid in interspecific competition by stunting the growth of nearby saplings in what is known as allelopathy. (Frey & Eldridge, 2005)

Tourism

See also: Leaf peeping

 Although some autumn coloration occurs wherever deciduous trees are found, the most brightly colored autumn foliage is found in four or five regions of the world: most of mainland Canada; most of the United States; Scandinavia and Northern Europe, Russia and Eastern Asia, including much of China, Korea, and Japan.^[8][9] Eastern Canada and the New England region of the United States are famous around the world^[8] for the brilliance of their "fall foliage," and a seasonal tourist industry has grown up around the few weeks in autumn when the leaves are at their peak. Thick forest cover and distinct seasonal changes make this part of the world an ideal setting for the types of deciduous trees that produce wonderful fall foliage. Fall colors are typically at their peaks in Early to mid October for much of the northern and interior parts of the area, late October for areas further south, and early November for the warmer subtropical areas of the region. Some television and web-based weather forecasts even report on the status of the fall foliage throughout the season as a service to tourists, the most well-known of which is The Weather Channel. Fall foliage tourists are often referred to as "leaf peepers". Fall foliage tours to the Rocky Mountain states, the northwestern United States and far western Canada are becoming more popular as well. The Japanese momijigari tradition is similar, though more closely related to hanami.^[10]

References

Further reading

• Guy., R. (2003). "Autumn Colours – Nature's Canvas is a Silk Parasol". Davidsonia 14 (4): 111-120. Retrieved on 2007-05-18.