The why of leaf color

Oct 03, 2012
As a former middle school teacher, the onset of fall would bring me joy and trepidation. This year, instead of focusing on lesson plans and back-to-school night, I am looking forward to taking my time to quietly and peacefully watch autumn unfold.


As a former middle school teacher, the onset of fall would bring me joy and trepidation. I love fall, but sadly have had little time in the past 26 years to enjoy it. This year, instead of focusing on lesson plans and back-to-school night, I am looking forward to taking my time to quietly and peacefully watch autumn unfold.

One thing I always thought I understood about the natural world was that elevation and temperature are what change the color of the leaves. But that just isn’t the case! It turns out leaves change color because of length of night and the pigments inherent in them. Sure, environmental factors like temperature, rainfall, food supply are part of the equation, but these factors are highly variable year after year and only play a part in the story. Length of night is the constant. As days grow shorter and nights grow longer and cooler, biochemical processes in the leaf begin the transformation.

There are three types of pigments in leaves: chlorophyll, carotenoids and anthocyanins. Chlorophyll gives leaves their basic green color. Chlorophyll is not a very stable compound. It requires sunlight and warm temperatures. In fall, the chlorophyll gets sluggish as nights grow cooler and days are shorter. Once chlorophyll slows down, special layers in the leaf prohibit any water from reaching it. When this happens trees break down chlorophyll and the leaves’ true colors are exposed.

The carotenoids and anthocyanins that are present in the leaf are unmasked once chlorophyll production ceases. Carotenoids produce the yellow, orange and brown colors found in pumpkin, corn, carrots, and daffodils. Red and purple colors come from anthocyanins which are found in cranberries, red apples, plums and blueberries. Once the chlorophyll is gone, we see what is really there.

Leaves change color at different rates and times. This is genetically inherited. Even elevation does not affect this timing. I used to think the higher the elevation, the colder the air is, thus the earlier the change in the leaves. But this isn’t so. It’s not the temperature that signals change, but temperatures and weather do play a part. The amount and brilliance of colors are related to weather conditions during the time the chlorophyll in the leaves is dwindling.

If the summer has ample moisture followed by a dry, cool and sunny autumn, with warm days, cool nights and no frost, leaves will be their most brilliant. Lack of wind and rain in the fall prolongs the colors. Wind and heavy rain cause leaves to drop before they reach their full potential. If we have a late spring and drought, there will be a delay of color. If the fall is too warm, there is less color intensity.

October is here. Sweaters are becoming important again. There are candles at the dinner table once more. We are eating squash and baked apples instead of strawberries and peas. A new season is here. Autumn has arrived.  And as this budding naturalist observes the new season, the real truth about why leaves change color emerges. I am glad I finally understand.

Sources:
USDA Forest Service: Why Leaves Change Color
PBS.org: Why Do Leaves Change Color.