By Meg Hanisch
Minnesota is one of the lucky regions of the world where autumn comes to the landscape in showy bursts of color. Stories tell how Mother Nature picks up her brush and palette of colors and paints the leaves bright yellows, golds, oranges, and reds. Jack Frost is often a character in these stories too -- his crisp temperatures bringing color changes. Although these autumn tales are fun to read or recite, the real story of autumn colors is even more interesting for young naturalists.
Not all trees change color in the fall. Minnesota's trees belong to one of two groups.
The first group is the gymnosperms. Their seeds are in cones, so they are also called conifers (cone bearers). They have narrow, needlelike leaves. Almost all are evergreen, meaning they shed only a portion of their needles each year.
Angiosperms, the second group of trees, have covered seeds and are often called deciduous trees. They drop their leaves each autumn. These aspens, oaks, maples, hickories, ashes, birches, and other trees make our fall forests colorful.
Conifers' narrow, tough needles retain moisture and allow the tree to survive in winter when the air is cold and dry. The broad leaves of deciduous trees evaporate a lot of water and use a lot of sap. So to prepare for winter, deciduous trees shut down and drop their leaves.
Color change is triggered by the shifting rhythm of day and night. As the long, warm days of summer turn into the shorter, cooler days of autumn, a chemical clock inside a deciduous tree urges the tree to begin its trip into dormancy, or winter sleep.
Living leaves contain several kinds of chemicals, called pigments, each with a characteristic color.
During the summer the most visible pigment is chlorophyll. This green pigment converts sunlight, water, and carbon dioxide into sugars (food for the tree), in the process called photosynthesis. In the autumn, the tree starts the dormancy process. As a result, it stops producing food. It also stops making chlorophyll.
As the chlorophyll fades, a group of pigments called carotenoids shows. Like chlorophyll, carotenoids are contained in the plastids, tiny structures in leaf cells. Carotenoids are the yellow, orange, and brown colors we see each fall in deciduous trees such as hickory, ash, maple, aspen, birch, black cherry, and cottonwood.
The reds and purples of some autumn leaves come from another group of pigments called anthocyanins. Unlike carotenoids, they are not always present in the leaf. Instead, they develop in the cells in the late summer, as a result of complex interactions. No longer needed for photosynthesis, phosphate moves out of the leaf and into the twig. This drop in the level of phosphate changes the breakdown of sugar in bright light so that anthocyanins form. The brighter the light, the greater the production of anthocyanins and the more brilliant the color. Anthocyanins color cranberries, red apples, blueberries, cherries, strawberries, and plums. They often combine with carotenoids to give the deeper oranges, fiery reds, and bronzes typical of many deciduous trees.
As fall colors appear, other changes take place. The base of the leafstalk, or petiole, connects the leaf blade to the branch. Tubes in the petiole transport sap in and out of the leaf blade. A layer of cells, called the abscission layer, develops and gradually severs the tissues supporting the leaf. At the same time, nature heals the break, so that after the leaf is finally blown off by the wind or it falls from its own weight, a leaf scar marks the place where it grew on the branch. Above this scar, next spring's leaves are tightly wrapped in buds waiting to unfurl.
Mammals, insects, fungi, and bacteria on the ground help decompose (break down) the fallen leaves. Decomposition releases the leaves' nutrients and adds organic material to the soil. Trees and other plants will take up the nutrients. Nature's cycles of life to death -- spring to winter -- continue.
The brilliance of fall colors depends on the weather. Watch the temperature before and during the time leaf chlorophyll is dwindling.
Are the days warm and sunny? Are the nights cool and crisp but not freezing? These conditions will create the most spectacular color displays. During warm days the leaf produces lots of sugars. During cool nights the leaf veins gradually close, keeping the sugars from moving out. Lots of sugar and lots of sunlight spur production of anthocyanins -- brilliant reds and purples. Because carotenoids are always in leaves, yellows and golds remain fairly constant from year to year.
The amount of moisture in the soil also influences color. Cool, wet summers can cause early displays of color. This is especially true for maples. A mild drought can brighten the display, but severe drought may make colors duller. Sometimes, the leaves die early from a lack of water.
In falls with lots of warm, cloudy, rainy weather, the leaves may have less red. The leaves make smaller amounts of sugar in the reduced sunlight. The sugar moves out of the leaves during warm nights, so the leaves have less sugar to form anthocyanins. It's hard to predict when fall color will be at its best -- its peak. Besides the weather, rich color depends on the eye of the beholder and the health of the tree.
Some people enjoy the first signs of yellow. Others prefer the crazy mix of colors when orange pops out. And some say the deep reds and purples of late fall are their favorite. Trees with lots of leaves naturally make a bigger show of color. Leaves won't show much color if pests, disease, or other environmental problems have damaged them. You can see great color in your own neighborhood. Or, look for forests with a mix of tree species, including maple. Bluffs and hills give you good vistas of the forest. To find out when and where to see great colors, contact the DNRInformation Center, see page 65.
Cut a 2-inch by 6-inch strip from a paper coffee filter. Mince several green leaves into a pulp, put them into a short glass or jar, and add just enough rubbing alcohol to cover. Tape one end of the filter strip to a pencil and place the pencil across the top of the glass so the bottom of the strip just touches the alcohol. After the alcohol has moved about halfway up the strip (at least an hour), remove the strip and let it dry. The various colors are the various pigments found in a fresh leaf.
Will a green leaf change color if you put it into a cold place? Put a green leaf into a plastic bag and into a refrigerator. Watch the color changes for a few days.
Cut a small green branch off a tree and place it in a vase without water. Do not touch or disturb for four weeks, but do observe the leaves. They die, but do they fall?
Meg Hanisch is a Public Affairs Specialist with the DNR Division of Forestry, St. Paul.
A complete copy of the article can be found in the September - October 2000 issue of Minnesota Conservation Volunteer, available at Minnesota public libraries.