This time of year, you hopefully admire the changing colors. Deep greens are turning into vibrant reds, flaming oranges, golden yellows and dusky browns. (See Figure 1) Personally, it’s one of my favorite seasons. But, have you ever wondered what causes leaves to change or why some trees lose their leaves and others don’t?
First off, there are two types of trees, coniferous (evergreen) and deciduous. Conifers produce cones, and the ones we are familiar with in North America have needle/scale-like leaves instead of the big, broad, flat leaves like deciduous trees. An exception to this rule is the Gingko, but it’s not native anyway.
Deciduous trees not only have flat leaves, but they produce flowers and fruits like nuts, apples and those little helicopters that twirl down from maples trees. A big difference is that conifers typically do not lose their leaves in the fall when deciduous trees do, but in southern areas where winters are mild, deciduous trees will keep their leaves too.
The conifers’ leaves have thick, waxy layers in them. This and special compounds in their leaves help them keep from freezing and allow them to be used for several years. Deciduous leaves are flat and thin. They freeze and are useless to the tree after the winter, so the tree sheds them. Some trees will keep their dead leaves throughout the winter and loose then when they are ready for new ones in the spring. So why the change in color? It starts with the sun and our planet’s relationship with it.
Our little planet rotates on an angle (See Figure 2). In the summer, North America is tilted towards the sun so we get more daylight and more intense sunlight. In the winter, we tilt away from the sun, so we get shorter days and less intense sunlight. Plants sense this change and prepare for winter.
Throughout the spring and summer, chlorophyll (green leaf pigment) and carotenoids (yellow, brown and orange pigments) are produced in the leaves. When there is a lot of chlorophyll, the green color masks the other colors. As light intensity decreases, the leaves stop producing chlorophyll, and the yellows, oranges and browns of the carotenoids are revealed. (See Figure 3) Shortened days also cause the veins that lead to the leaf to start to close and trap sugars in the leaf. This buildup of sugar triggers the production of other pigments called anthocyanin. These give leaves the purples and reds we love. (See Figure 4) Eventually the veins close completely and the leaf is ready to fall.
A leaf’s journey does not stop there. Once they have fallen, they provide food for many small animals and recycle nutrients back into the soil to maintain a healthy forest ecosystem. They might also fall into streams where they act as part of the base of the stream’s food web, feeding bacteria that many insect larvae eat. The larvae then feed other organisms and so on and so forth.
People talk about the beauty of the trees on the East Coast. They are lovely, but our native trees here in the Midwest have their own splendor. Oaks will take on reddish browns and russet colors. Hickories will turn a golden bronze. Trees like tulip poplars and persimmon trees will be brilliant golds. (See Figure 5) Sweet gum trees and bushes like sumac will be a deep red or purple. (See Figure 4)
When is the best time to see the colors? The Farmer’s Almanac website predicts that northern Illinois, northern Missouri, and Iowa’s peak will be Oct. 5-21 and southern Illinois and Missouri will be October 12-28. See the full list of states and times at the Farmer’s Almanac.
You can also use an interactive fall leaf prediction map for this year (See Figure 6) at www.smokymountains.com/fall-foliage-map. It also has some coloring sheets you can download for your kids to color on rainy days. While the sun is shining and colorful leaves are falling, be sure to take some time to experience it all. Collect and dry the leaves, use them in craft projects, or just sit on the blanket or park bench and take it all in.