WHAT MAKES IT UNIQUE AND SPECIAL
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, various shades of red, yellow, purple, black, orange, pink, magenta, blue and brown. The phenomenon is commonly called autumn colours or autumn foliage in British English and fall colors, fall foliage, or simply foliage in American English.
In some areas of Canada and the United States, “leaf peeping” tourism is a major contribution to economic activity. This tourist activity occurs between the beginning of color changes and the onset of leaf fall, usually around October in the Northern Hemisphere and April to May in the Southern Hemisphere.
A green leaf is green because of the presence of a pigment known as chlorophyll, which is inside an organelle called a chloroplast. 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. During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.
In late summer, as daylight hours shorten and temperatures cool, 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, lowly 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.
Chlorophylls degrade into colorless tetrapyrroles known as non-fluorescent chlorophyll catabolites (NCCs). As the chlorophylls degrade, the hidden pigments of yellow xanthophylls and orange beta-carotene are revealed. These pigments are present throughout the year, but the red pigments, the anthocyanins, are synthesized de novo once roughly half of chlorophyll has been degraded. The amino acids released from degradation of light harvesting complexes are stored all winter in the tree’s roots, branches, stems, and trunk until next spring when they are recycled to re‑leaf the tree. Carotenoids are present in leaves the whole year round, but their orange-yellow colors are usually masked by green chlorophyll. 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. Carotenoids are the dominant pigment in coloration of about 15-30% of tree species.
The reds, the purples, and their blended combinations that decorate autumn foliage come from another group of pigments in the cells called anthocyanins. 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. Anthocyanins are present in about 10% of tree species in temperate regions, although in certain areas — most famously New England — up to 70% of tree species may produce the pigment. In autumn forests they appear vivid in the maples, oaks, sourwood, sweet gums, dogwoods, tupelos, cherry trees and persimmons. These same pigments often combine with the carotenoids’ colors to create the deeper orange, fiery reds, and bronzes typical of many hardwood species.
The brown color of leaves is not the result of a pigment, but rather cell walls, which may be evident when no coloring pigment is visible. 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.
In the matter of apple trees, 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 exists between fruit size, leaf color and aphids resistance as varieties with red leaves have smaller fruits suggesting a cost to the production of red 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). Autumn colors would be a signal if they are costly to produce, or be impossible to fake (for example if autumn pigments were produced by the same biochemical pathway that produces the chemical defenses against the insects). The change of leaf colors prior to fall have also been suggested as adaptations that may help to undermine the camouflage of herbivores.
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).
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 southern mainland Canada; most of the eastern part of the United States as well as smaller areas of forest further west; Scandinavia; Northern, and Western Europe north of the Alps; the Caucasus region near the Black Sea, Russia and Eastern Asia, including much of northern and eastern China, as well as Argentina, Chile, southern Brazil, Korea, Japan and New Zealand’s South Island. 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. 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. In Finland the time of the year is called ruska (russeting). In Latvia during russeting, leaf peeping is promoted both internationally and locally. Other lands may also promote this, but mushrooming may be more culturally significant, for example in Lithuania provides many more arbor species (more than 800 species and about 70 oaks, compared to 51 and three respectively 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 were protected in more southern regions along north–south ranging mountains, which was not the case in Europe.
Global warming and rising carbon dioxide levels in the atmosphere delay the usual autumn spectacle of changing colors and falling leaves in northern hardwood forests, and increase forest productivity. Experiments with poplar trees showed that they stayed greener longer with higher CO2 levels, independent of temperature changes. However, the experiments over two years were too brief to indicate how mature forests may be impacted over time. Also, other factors, such as increasing ozone levels close to the ground (tropospheric ozone pollution); can negate the beneficial effects of elevated carbon dioxide.