A Common Life - Letter #2 on Leaf Color
Do you know why leaves change colors in the fall?
Wendell, pictured above, found some neat leaves walking home from school. Naturally, he started a new collection. He showed me his new collectables with pride, posed for my picture, and then trotted off to show his treasure to anyone else who crossed his path.
I was surprised to see the leaves had already started to change. I shouldn't be. Here in Huntsville, AL we have had a bout of cooler nights and warmer days with bright sunshine. The perfect recipe for red, orange, purple, and yellow leaves. Which leads me to my question: Do you know why leaves change color? Let me try and explain.
The reason leaves are green is because of a pigment in the leaves known as chlorophyll. In spring and summer, as days are increasingly getting longer, chlorophyll is produced in abundance. As summer days begin to shorten and trees start receiving less and less sunlight each day, a signal goes off inside the tree. This signal indicates that it is time to start degrading chlorophyll faster than it is produced. Over time, this leads to less and less chlorophyll being present in the leaf. The absence of chlorophyll allows another, ever present pigment in the leaf to reveal itself - the carotenoids. This group of pigments gives us our yellow fall color. Shorter days > Chlorophyll degrades> Yellow pigment revealed!
Another pigment, anthocyanin, gives us our red fall color. When it is mixed with the yellows, it gives us our purples, oranges, and browns. Unlike yellow pigment, the red pigment is not always present in the leaf. It's also not produced in all trees either. It's understood that this pigment is designed to protect vulnerable leaves from intense sunlight. With this in mind, it makes sense that it is produced early in the spring to protect new, young leaves and late in the summer when the sun is its brightest and leaves are post-mature. As nights begin to cool, degradation of this red pigment slows. Warmer, bright days encourage its production. When the days begin to shorten, chlorophyll begins to degrade. When nights get cool and days stay bright and warm, this red pigment gets left behind in the leaf to mingle with the remaining yellow pigments and gives us the beautiful fall colors we all love and enjoy.