The wonder of fall color in the natural world can be scientifically explained, but sometimes a story is the best way to describe and remember this annual phenomenon.
The story told to my daughters, who are now 24 and 27, goes something like this:
“Daddy,” she says, “tell me a story.”
“Let’s see, I know a story about why the leaves turn color in the fall and it might have a baby bear in it, too.”
“Tell me,” she says.
Way back in the spring of the year when Mother Nature was hanging leaves on the trees she decided to paint them with bright colors of red and yellow. After all the painting was done, Mother Nature felt that all these brightly colored leaves were a little too much and that they could clash with her spring wild flowers. So, she covered them up with green peanut butter. This green peanut butter was spread over all the leaves hiding the bright reds and yellows that lay underneath.
Meanwhile, up in the trees lived these little tiny creatures that spent their days eating the green peanut butter that was spread on the leaves. While the little creatures feasted on the peanut butter, Mother Nature was busy making more and spreading it on the leaves.
This went on all summer long and right up into the early fall. The little creatures kept on eating the peanut butter and Mother Nature kept on spreading it, keeping the leaves green.
Well around October, it finally happened, Mother Nature ran out of peanut butter.
And the little creatures never missed a beat. They kept right on eating that green peanut butter off those leaves. It didn’t take long before some of the bright colors started to shine through.
Soon the little creatures had eaten all of the green peanut butter off the leaves and the mountainsides where covered in beautiful red and yellow leaves, and all the baby bears were very happy.
The science story goes something like this:
In a plant that is deciduous (shedding its leaves annually), chlorophyll and carotenoids are present and are continually being made in the chloroplast of the leaf cells throughout the growing season. In autumn, anthocyanins are produced due to longer nights, bright light and excess plant sugars within these same leaf cells. (Chlorophyll creates the basic green color and is necessary for the chemical reaction of photosynthesis (6CO2 + 6H2O —> C6H12O6 + 6O2) taking place within leaves and enabling sunlight to manufacture sugars for the plant to store during winter dormancy.) Carotenoids are yellow, orange and brown producing substances in plants like carrots and daffodils, bananas and buttercups. Anthocyanins are best remembered for the color they produce in cranberries, concord grapes, blueberries and cherries. They appear in the watery liquid cells of leaves and are water-soluble sugars. Carbohydrates are required for the manufacture of these colorful anthocyanin pigments. Anthocyanins are not completely understood and are theorized to act as a protectant to plants in either warding off insects or acting as a sunscreen of sorts to inhibit the destruction of chlorophyll (remember the bedtime story?) or in the prevention of frost injury or extreme water loss during drought.
As nights lengthen and chlorophyll production slows, the remaining chlorophyll is used or destroyed revealing the carotenoids and anthocyanin colors.
Yes, the lengthening of night actually triggers the color change. Other determinants of fall color change include temperature, moisture and climate events.
Let’s dissect these effects.
1.) Shorter days and longer nights initiate leaf color change.
2.) Temperature and moisture affect color brilliance. When sunny warm days are followed by cool nights, the most spectacular anthocyanin colors are revealed. Carotenoid colors are always present so that there will always be good gold and yellow fall color year after year. As each year’s weather combination is different, there are different effects produced in the fall color forecast.
3.) Early frost means that fall color production inside leaves comes to an end.
Why do leaves fall? At the onset of longer nights, veins carrying fluids into and out of the leaf begin to close, and a small layer near the base of the leaf or leaf petiole begins to form. This layer is called the abscission layer,and once it becomes corky and dry, the leaf will fall from the tree or shrub branch. The scar left on the branch is called a leaf scar and is useful in plant identification. Drought stress can cause early formation of the abscission layer, cutting short the time for fall color to occur. Wind and rain may cause early leaf drop due to the agitation of leaves.
Why are certain fall colors exhibited in certain plants? Fall colors are characteristic by species and are thought to be driven by plant genetics. The variations in color are not affected by altitude, as the same species along a similar latitude will show color similarly, whether located at mountaintops or lowland valleys.
Dominant tree species around the world affect the character of autumn color. Short seasons and intense colors are exhibited in the northeast U.S. and in Asia where maples species are the dominant forest trees. In the northern and western U.S. forests, evergreen tree species grow alongside larch, birch or aspen that typically exhibit yellow coloration for fall colors lasting for as little as two weeks. In the southeastern U.S. where there is more plant diversity, the season may be longer and have a greater diversity of color. In cloudy regions of the world like Europe and some upper Midwest states, fall color is typically dull.
According the U.S. Forest Service you can drive over 3,000 miles in 31 states to see autumn colors changing from late August through early November. The USFS Fall Color Hotline is 800-354-4595.