Nature up close: Autumn colors

By “Sunday Morning” contributing videographer Judy Lehmberg.

I’m a biologist so I enjoy understanding biological processes. However, I have a place in my heart, rather than in my brain, for color changes in nature. When I watch a chameleon change color I don’t think of what its skin cells are doing; I just think, “Isn’t that wonderful!” When I learned what really causes them to change color, I was disappointed. I wanted it to be magic. I feel the same way when I see leaves changing color in the fall. It is so beautiful, it must be magic, right?

I remember a comic strip from maybe thirty years ago. A kid asks his mother why leaves change color in the fall. She then goes into a lengthy discussion of the role of chlorophyll and other plant pigments. Then the kid says something like “I thought it was magic.” Well, even though I thought it was magic, too. before I understood the process, I still think it is magical.

There are many aspects of biology that appear strange and magical, whether we understand them or not.

Almost all life on Earth, with the exception of life around the deep ocean vents, is dependent on photosynthesis to make food. Photosynthesis in turn is dependent on chlorophyll to absorb light and initiate the process that turns light energy into chemical energy. Chlorophyll is continually produced during the growing season and give leaves their green color. So why do we also see yellows, oranges and reds in leaves, especially in the fall?

Chlorophyll does a great job of absorbing the reds and blues from sunlight but it does not absorb green; it reflects it back to our eyes and out into the atmosphere. If a plant leaf contains only chlorophyll it can’t utilize any of the green wavelengths from sunlight. Other pigments, primarily yellow and orange carotenoids, have evolved to absorb green light which can then be used to make glucose during photosynthesis. 

It basically amounts to this: the more types of pigments in a leaf, the more food it can make. During the spring and summer plant leaves contain several types of chlorophyll, yellow xanthophylls, and orange carotenoids -- pigments which absorb many different colors of light and produce glucose. Some of this glucose undergoes chemical changes to make fats, proteins, and other nutrients the tree needs.

In many areas on Earth it is too cold for photosynthesis to occur during the winter, so leaves in winter are a waste. They also have a lot of surface area where the plant can lose over 90% of the water it absorbs each day. A tree with leaves on it in the winter risks loosing a lot of water. It is better to shed those leaves, reduce the tree’s surface area, and thus be able to retain more water. That is why most trees lose their leaves in the fall.

The rate at which tree leaves die determines whether they turn the brilliant yellows, oranges and reds we enjoy seeing, or whether they just quietly fade to brown before they fall. This change is triggered primarily by shorter day lengths. In more northern areas of North America the fall weather can change from warm to cool relatively quickly. When that happens the leaves begin to die. A corky group of cells form at the base of each leaf, limiting water movement. Chlorophyll production declines. The red and purple anthocyanin pigments are generally produced from glucose after the leaves begin to die, and form more readily during cool nights and bright days. The role of anthocyanin is not completely understood but may protect the leaves from excess sunlight. Once the chlorophyll breaks down, it no longer masks the carotenoids and other pigments present. If it becomes windy or rains a lot, the leaf breaks where the corky cells formed.

In warmer climates, such as the southern parts of the U.S., the temperature change from hot summer to cool winter can be very gradual, so most leaves lose their pigments at about the same time, and we don’t see the pretty yellows, oranges, and reds. If there is a cold snap in the fall, we may have pretty leaves in the South, but usually there isn’t much of a show.

If you would like to go see some pretty fall foliage you had better hurry as it has already peaked in some places. There are several “foliage forecast” maps online, including: smokymountains.com, weather.com, newengland.com, and the U.S. Forest Service.  And there are state-by-state fall foliage phone hotlines.

Judy Lehmberg is a former college biology teacher who now shoots nature videos.

For more info:

• Judy Lehmberg (Official site)
• Judy Lehmberg’s YouTube Channel

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