What makes those bright fall leaf colors?
Why do tree leaves change color in the fall?
The simple answer is that, during summer, chlorophyll in the leaves is so densely green that it hides the yellow and orange colors of other chemical compounds that are also there. But when chlorophyll begins to disappear with the oncoming cold season, the oranges and yellows are exposed. At the same time, in autumn, red chemical compounds are produced in large quantity by some trees, and these are added to the orange and yellow. The brown color is that of tannin, a waste product of the tree's life processes.
Why do some years have autumns more colorful than others?
The intensity of autumn leaf coloration each year is different. This is because the amount and exposure of colored chemicals in leaves depend on the weather. Green chlorophyll disappears fastest during an autumn with bright sunlight and cold days. This disappearance quickly exposes the leaves' orange and yellow pigments to view. Dry weather concentrates sugar in the cell sap, which speeds up manufacture of red compounds. So the most colorful autumns depend on the weather. Dry, sunny days of an Indian summer that are followed by cool, dry nights will create the best season for the fall leaf show. You can predict good viewing if you analyze the weather conditions.
Where and when is it best to view the fall folliage?
People in some parts of the world never see the wonderful variety of fall leaf colors that we have here in Western New York and the adjacent Niagara Peninsula. Their climate may be too cold or too hot for trees that drop their leaves in fall, or they don't even have seasons, such as in the tropics. In cool parts of the world, such as areas of northern Europe, they have no Maple trees and, except for the occasional Oak tree, most of their fall foliage is yellow! Maples, especially Sugar Maple and Red Maple, are especially endowed with red chemical compounds produced in the fall and are found throughout the northeastern USA Best times for viewing change every year, depending on the weather conditions, but mid-October is best for our area. There is an interesting reversal of seasonal change in our region because higher elevations are in the south, towards Allegany State Park. You must go earlier, at the beginning of October, to catch the display at higher elevations, such as south in Allegany State Park, or to the east in the Adirondacks, or north to the of our area in Canada. The foliage display peaks latest near Buffalo. The northeastern United States is the best place in the world to view the fall foliage show! Because of the differences in elevation you can plan a long auto trip to see the full exhibition, driving from mountainous areas in the south or far east back towards Buffalo. The fall foliage business is booming in the fall. In some states tourists seeking the fall color display spend over a billion dollars.
How are trees different in fall coloration?
Evergreens do not change color with the cold season, but their leaves have thick, weather-resistant waxy skins and are green all winter long. The leaves do not fall each annually but instead last from two to four or more years. Evergreens include cone-bearing trees like Pine, Spruce, and Fir, and also seed-bearing trees, like Holly. Only deciduous trees, trees that drop their leaves every fall, have colorful foliage at the end of the growing season. Trees with leaves that are usually yellow in autumn are Birch, White Ash, Poplar, Basswood, Beech, Witch Hazel, Aspen, and Hickory. The best red colors are shown in leaves of Red Maple, Sugar Maple, Red Oak, Dogwood, Sumac, Mountain Ash, and Sour Gum. Trees are not the only plants that have brightly colored leaves in fall, Goldenrods, for instance, are famous for their bright red foliage, Wild Grape has dark purple berries, and vines, like Virginia Creeper and Poison Ivy have yellow and red variegated leaves.
Exactly what takes place in the leaf to product fall colors?
- During summer, green chlorophyll is produced. Chlorophyll is a pigment that is found only in green plants, and is the plant's food factory. It allows plants to chemically combine carbon dioxide in the air with water from the ground to make glucose, a kind of sugar, a basic building block and energy storage substance for life on Earth. Sunlight provides the energy for this manufacture. Cellulose, which is the primary ingredient in wood, is made from long chains of different kinds of sugars. Chlorophyll is green, so it absorbs all colors except green, which it reflects. Chlorophyll has a metal atom, magnesium, in the middle of its molecule. This molecule is oddly similar in chemical structure to another important life pigment: hemoglobin in warm-blooded animal, which differs largely in having an iron atom in its center.
- Two other pigments in plants, carotenes (which are orange) and xanthophylls (which are yellow), absorb the wasted green light, and are thought to help chlorophyll by capturing and saving energy from light that chlorophyll reflects away. Carotene is well known as an important antioxidant in human diet. These two additional pigments are held in the leaf in fat layers, and are also found in carrots and daffodils.
- Plants not only manufacture sugar with chlorophyll, they also use oxygen, just like animals, to carry on normal processes of life. Two waste products of metabolism are carbon dioxide, which can be re-used to make sugars by chlorophyll using the energy of sunlight, and tannins, which remain as brown pigments in the leaf. During summer, the orange, yellow and brown pigments are all camouflaged by the intense green color of chlorophyll.
- The shortening of day length in September is the major signal to begin arrangements for over-wintering, but cold air is also important. Hormones in trees with deciduous leaves start a chemical reaction that forces nitrogen compounds from the leaves back down into the roots for storage during the winter season. After this happens, a membrane, called the abscission layer, forms at the base of the leaf-stalk. This cuts across the tiny nutrient-carrying tubes that connect the leaf with other living parts of the tree: the green inner bark and the roots. These tubes carry water and minerals from the roots to the leaves, and also return sugar manufactured in the leaves to the rest of the tree. Deprived of water by the strangle of the abscission layer, the leaf begins to die. Sugar and waste products begin to accumulate in the leaf. Before it falls, however, the leaf will give us all the colors of autumn.
- Chlorophyll is not a very stable chemical compound and is broken down by bright sunlight. As the leaf begins to die in the fall, the plant can no longer replace its chlorophyll. Carotenes and xanthophylls, however, are more resistant to sunlight than chlorophyll, and they remain bright in the leaf, providing its yellow and orange fall colors. The yellow and orange colors were always there alongside the green color, but are only seen when the deep green fades. You see this happen when new bananas lose their green color to reveal the yellow that was always there.
- Lots of sunlight in fall makes what remaining chlorophyll there is produce lots of sugar, but cool air temperature limits life processes in the leaf, so sugars that are produced are not used up. The concentration of sugar in the leaf cell sap is increased if the weather is also dry. Various members of a group of red pigments called anthocyanins are produced in the fall when the amount of sugar increases. Like sugar, anthocyanin (a general term for this group) is soluble in the leaf cells' vacuole (a bubble of water in each cell). Anthocyanin absorbs all colors except red, which it reflects and gives the leaf you see its red color. Sunny days also make life processes go faster in the leaf, and more carbon dioxide is produced as a result. This carbon dioxide is not used up because the amount of chlorophyll is getting less and less. The carbon dioxide dissolves in the cell sap and makes it more acid. The more acid the cell sap is, the more anthocyanin is produced. So sunny, cold but not freezing, dry weather makes leaves more red in fall. You can see the influence of sunlight on anthocyanin production by red color found on only one side of some apples. The purple color of grapes is one kind of anthocyanin. Some anthocyanin pigments may act like litmus paper, producing red if the cell sap is acidic (as in Apples) and blue (as in Grapes) if the sap is alkaline. This is the reason you sometimes find blue colors on some autumn leaves.
- In spring, life-giving nitrogen is returned, from where it had been stored all winter in the roots, back to the buds through the rising sap. New green leaves grow from the buds to begin the cycle of the seasons all over again.
Analyzing the Fall Coloration Pigments at Home
You can separate the different chemicals using chromatography right at home. You will need isopropyl alcohol (the same as rubbing alcohol), hot water, a coffee cup, a soup dish, a pencil, scissors, plastic wrap, and some filter paper used for making coffee. Take two or three green tree leaves and cut them into small pieces with scissors. Then crush or crumble the leaves into the coffee cup. Heat some water (hot water from the tap is good enough) and fill the soup dish half full. Add isopropyl alcohol to the coffee cup just enough to cover the leaves in the bottom. Let the leaves soak in the warm isopropyl alcohol for an hour or more. Put plastic wrap over the mouth of the cup to slow evaporation of the alcohol. Pour off the water in the soup bowl if it gets cold and replace it with hot water. Wait until the liquid in the cup gets dark, showing that pigments are dissolved in it.
Using scissors, cut the filter paper (if you don't have filter paper, use paper toweling) into one or two strips about 4 inches long by 1 or 2 inches wide. Put the pencil down across the mouth of the cup and drape the filter paper across the pencil so that one bottom end touches the bottom of the cup through the liquid. Let this stand and watch what happens. The alcohol will rise up the filter paper and slowly drag the pigments from the leaf with it. It works best if you seal the coffee cup with plastic wrap so that the alcohol does not evaporate but saturates the air in the coffee cup instead. (If it doesn't work well, try soaking leaves in isopropyl alcohol in a bottle overnight.)
The pigments will move at different rates through the paper, and if you wait for 30 or 45 minutes, you will see them separate. Can you name them? Try this with green leaves that you know will make different fall colors, yellow or red or some combination. See if you can force the hidden colors from hiding! You can dry the strips of paper, write down the names of the leaves you analyzed, and keep the strips in a notebook to record your experiments. Try this with other substances. For instance you can make a mark with a black ink ball point pen on the filter paper, just above the level of the alcohol, and see what different colors are used to make the ink black—you will be surprised! Chromatography is an important scientific tool.
Pressing and Keeping Autumn Leaves
The time-honored way to preserve colorful autumn leaves is to put them between two sheets of wax paper, cover them with a cloth, then press the fabric with an iron warm enough to seal the wax paper together with the leaves in between. You can cut the leaves out with scissors, allowing a margin of sealed wax paper around each leaf edge. Another way is to soak the leaves in a mixture of one part glycerin to two parts water for several days. Let them dry flat, pressed between newspaper that will absorb any excess liquid. The leaves will be soft and leathery and should last a long time.
This is an educational production of the Herbarium, Division of Botany,
Buffalo Museum of Science, 1020 Humboldt Parkway, Buffalo, NY 14211.