SciSun: The last one to Fall

This time of year, you’ll see leaves start to turn from green to yellow, red, purple and brown, and eventually fall off trees and other plants.  The changing colors bring brilliant shades into our neighborhoods and communities for a few weeks, but what’s really happening?   Why are the leaves changing like that, and does it hurt the tree when the leaves fall off?

Actually, leaves contain the red and yellow colors all year; these colors are just masked by green.  Yellow is created by chemicals called Carotenoids, and reds and purples come from Anthocyanins.  These are the same chemical pigments that make bananas yellow, or blueberries blue, or strawberries red.   In the Spring and Summer, another chemical, Chlorophyll, is made and used by the trees and plants in such large amounts that the green Chlorophyll hides the yellows and reds of the other chemicals.

Chlorophyll, along with water, light, and carbon dioxide (another chemical), is what plants use to make their own food!  That’s one reason you’ll never see a plant ordering at a restaurant, they can make much of the food they need.  Also, they usually don’t have any money.

For much of the year, plants make and save their food (called Glucose – a type of sugar).  But when the days become shorter and the nights longer, the chemicals begin to change.   Chlorophyll becomes less and less, and as it goes, the yellows and reds start to become more and obvious, or even grow more vibrant.   The change in colors has more to do with the changes in daylight than with cold weather, or rainfall, or the number of tourists that drive into the forest to see the pretty Fall colors only to find they are about a week early.

All Summer the plants have stored Glucose sugars in preparation for the coming winter.   The tough branches and trunk of the tree can survive the cold and snow, but leaves are too tender and they would die in the winter, so each Autumn the leaves of trees and other plants fall off and help the tree prepare for the winter.   Next Spring, new leaves will grow Thanks to Charles Willgren and Wikimediaand the cycle will begin again.

But even when they fall, the leaves’ work is not finished.   When they fall and remain on the ground, they can be food for small animals, help stabilize the soil and hold rainfall near the trees, and eventually they will decompose and become part of the earth, again contributing nutrients to the same tree where they once helped to create food.

Hold on, you say, not all trees lose their leaves – there are plenty of trees that are green all winter.   Very true, wise one!   But that’s something that will have to wait until another SciSun!

Dig Deeper!

There are lots of great places to learn more about changing Autumn colors.   Here are a few to get you started:


And always check out your local library!

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