• Shorter days, longer nights
• Pigments in the leaves
• Weather

Shorter days, longer nights
The most constant influence is the changing length of the days and nights. Longer nights trigger biochemical changes in the trees. The long days of intense summer sunlight are disappearing and trees begin to prepare for the short cold days of winter. A layer of cells, called an abscission layer, is produced between each leaf stem and the woody part of the tree. It cuts off the flow of nutrients and water between the leaves and the rest of the tree, changing the chemical processes that take place within the leaves. These changes cause the color changes that follow.

Pigments
The beautiful autumn colors can be found in the flowers, fruit and stems of plants, not only in their leaves. Ripe red berries, golden yellow sunflowers, purple eggplants and red cabbage owe their color to the same pigments that appear in trees in the fall.

Green

Chlorophyll gives leaves their green color. It is the substance that is most responsible for the manufacture of food (by photosynthesis) in the leaves. During the summer, bright sunlight breaks down the chlorophyll molecules and new chlorophyll is constantly being created within the leaves. When the abscission layer forms, chlorophyll becomes trapped. Without water or nutrients flowing into the leaf, the chlorophyll that is trapped breaks down, and new chlorophyll cannot be produced. The green color of chlorophyll is so strong, that it hides any other pigments in the leaves. It isn’t until fall, when the chlorophyll breaks down, that we can see the other colors that had been hidden.

Yellow

The yellow color comes from carotene, a pigment found in the cell membrane of many leaves. Carotene helps the leaves absorb light, bringing in more energy for the chlorophyll to use in photosynthesis. It is a much more stable compound than chlorophyll. When the chlorophyll breaks down, carotene remains, giving the leaves a yellow color.

Red

Red is the color of anthocyanin, a pigment that is present in sap, not in the cell membranes of the leaves like chlorophyll and carotene. Production of anthocyanins is dependent on the acidity of the sap, the concentration of sugar in the leaves and sunlight. Trees with higher acidity, and a higher concentration of sugar trapped in their leaves in autumn will be redder. Since sunlight is also necessary to produce anthocyanins, you will often see trees with red leaves at the top of the tree and more gold or yellow ones below. This explains why some apples are red on one side and yellow on the other. As the apples ripen, they will only turn red on the side in the sunlight.

Orange

A combination of carotene and anthocyanin will produce gold and orange colors.

Brown

Tannin is the brown pigment of waste products that collect in the leaves. It is the slowest to break down. There are some trees with a high concentration of tannin that turn right from green to brown in the fall.

Weather
Cool temperatures at night will break down chlorophyll, revealing the other pigments. Bright sunny days and dry weather, which cause a buildup of sugars in the leaves, are the best conditions for developing the red anthocyanins,. But freezing temperatures will inhibit or stop their production. A severe drought during the year may delay the color change for several weeks; wind and rain may force the leaves off the trees early.

The best colors appear in years with warm, wet springs, bright sunny fall days and cool autumn nights.
Here in the Midwest, we are fortunate to be able to watch the green of summer turn magically into the beautiful warm colors of autumn. Walk along the trails at Ryerson Woods, under its dense deciduous canopy, and enjoy the palette of the changing seasons!

(leaf photographs by Sue Auerbach)

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Pustelnik Designs and Friends of Ryerson Woods