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 season, 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 and 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.
Chlorophyll and the green color: A green leaf is green because of the presence of a group of pigments known as chlorophylls. 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 which contribute to other colors:
Carotenoids: Cross section of a leaf showing color changes; click to enlarge.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 our autumn forests, they show up vividly 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.
Fall foliage peak times in the United StatesCompared to western Europe, North America provides many more arbor species (more than 800 species, about 70 oaks compared to 51 and three in Western Europe) which adds many more different colors to the spectacle. The main reason was the different effect of the ice ages—while in North America, species could evade to warmer regions along north–south ranging mountains, this was not feasible in Europe.
According to a study in the journal Global Change Biology, climate change delays the autumn spectacle of multi-colored leaves but increases forest productivity. The study suggests that rising carbon dioxide levels in the atmosphere act directly to delay the usual autumn spectacle of changing colors and falling leaves in northern hardwood forests. The researchers found that the forests in North America and Europe stayed greener longer as CO2 levels rose, independent of temperature changes. However, the experiments were too brief to indicate how mature forests may be impacted over time. Also, the research suggests that other factors, such as increasing ozone levels in the part of the atmosphere closest to the ground, can negate the beneficial effects of elevated carbon dioxide.
Purpose of leaf loss:
The conventional reasoning behind leaf fall is that leaves may be unable to survive winter, or that they may run short of water and minerals, although plants leaves can and do survive to winter, and even aquatic plant shed leaves. Evergreens are misnamed, as also shed their leaves albeit not in autumn. Brian J. Ford has proposed that the shedding of the leaves is a primary means of plant excretion.
Deciduous plants, including trees, shrubs and herbaceous perennials, are those that lose all of their leaves for part of the year. This process is called abscission. In some cases leaf loss coincides with winter - namely in temperate or polar climates. While in other areas of the world, including tropical, subtropical and arid regions of the world, plants lose their leaves during the dry season or during other seasons depending on variations in rainfall.
Although some autumn coloration occurs wherever deciduous trees are found, the most brightly colored autumn foliage is found in three regions of the world: most of Canada and the United States; and Eastern Asia, including China, Korea, and Japan.
Function of autumn colors: Deciduous plants were traditionally 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. In many cases this turned out to be over-simplistic - other factors involved include insect predation, water loss, and damage from high winds or snowfall. Anthocyanins, responsible for red-purple coloration, are actively produced in autumn, but not involved in leaf-drop. A number of hypotheses on the role of pigment production in leaf-drop have been proposed, and generally fall into two categories: interaction with animals, and protection from non-biological factors.
Photoprotection: According to the photoprotection theory, anthocyanins protects the leaf against the harmful effects of light at low temperatures. 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.
According to the coevolution theory, 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. This has been shown in the case of apple trees where some but not all domesticated apple varieties unlike wild ones lack red leaves in autumn. A greater proportion of aphids that avoid apple trees with red leafs manage to grow and develop compared to those that do not. A trade off moreover exists between fruit size, leaf color and aphids resistance as varieties with red leaves have smaller fruits suggesting a cost to the production of read leafs linked to a greater need for reduced aphid infestation. Consistent with red leaved tree providing reduced survival for aphids, tree species with bright leaves tend to select for more specialist aphid pests than do trees lacking bright leaves (autumn colors are useful only in those species coevolving with insect pests in autumn).
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 than 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)
Eastern Canada and the New England region of the United States are famous around the world 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. Some television and web-based weather forecasts even report on the status of the fall foliage throughout the season as a service to tourists. 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 (the Japanese tradition of going to visit scenic areas where leaves have turned red in the autumn) tradition is similar, though more closely related to hanami (the Japanese traditional custom of enjoying the beauty of flowers).
Leaf peeping is an autumn activity in areas where foliage changes colors. Leaf peepers are those who participate in photographing and viewing the fall foliage.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)