Fall Colors: Part I – Yellow, Gold and Orange

Here in PA it’s the peak of the fall colors. Deciduous trees are transforming, leaving the green of spring and summer behind and turning into the bright yellows, reds and oranges of autumn. Seeing this change all around me, it got me thinking about what causes this change. So I did some research and found some fascination information about the fall colors.

The entire process begins with the tree knowing that winter is coming and that the growing season is about to end. I had always thought that the onset of the changes in leaf colors was due to the cooler days of autumn. But trees have evolved a much more reliable way of knowing when winter is on its way. Temperatures can vary from year to year but the one constant is the increasing length of night. As the days get shorter, the trees know it’s time to start shutting down and going into dormancy.

During the growing season there are two main pigments present in leaves of trees – chlorophyll and carotenoids. Chlorophyll is the green pigment that traps the sun’s energy and, through the process of photosynthesis, allows the leaves to produce sugars to feed the tree. Carotenoids are yellow/orange pigments that give carrots, bananas and corn their color. In the cells of leaves, both of these pigments are located in organelles called chloroplasts. The interaction between these two pigments is interesting. Chlorophyll is constantly breaking down and being replaced by the leaves. If this green pigment is exposed to the full energy of the sun with no protection, it’ll break down faster than the leaf can produce it. What protect the chlorophyll are the various carotenoids that are also present in the chloroplasts. These yellow pigments act as a “sun block,” absorbing some of the sun’s energy to protect the chlorophyll while still allowing enough energy to fuel photosynthesis. A leaf in the summer has a perfect balance of greens and yellows to allow it to feed the tree.

When the days become shorter, the leaves produce less chlorophyll until there is none left. When this happens, the carotenoids, which have been masked by the green pigments, are revealed and we see the yellow, gold and orange colors of autumn leaves.

Carotenoids Revealed

Since these carotenoids are always present in leaves, the display of yellow colors in the fall are fairly consistent from year to year. Environmental factors have little effect on them. The red, crimson and purple colors are a different story and I’ll explore them in my next post.

When I drive by a tree that’s exploded in bright yellows and golds, I’m seeing the carotenoids that have been in the leaves all year. But with the chlorophyll gone, these yellow pigments get their chance to shine. After spending all summer protecting the chlorophyll, I think it’s only appropriate that they have their moment of glory, short-lived as it might be.

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