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The Science Of Fall Color

We all marvel at the fall colors in our region. That’s because we’ve been blessed with the right climatic conditions. However, there are a lot of explanations for this phenomenon, many of them wrong.

Back in 2012, the International Society of Arboriculture (ISA) posted a release on its treesaregood.org website explaining the science of autumn beauty. It’s all about the temperature swings and diminishing hours of daylight. The release says, “Leaf pigments play a crucial role in the colors we see. Chlorophylls, carotenoids, and anthocyanins present in a leaf help determine what color the leaf will display.

“The pigment that gives leaves their green color is chlorophyll. Chlorophyll is used in photosynthesis, which is the process that uses sunlight to transform carbon dioxide and water into carbohydrates (sugars) that fuel tree growth. During the spring and summer, with more hours of sunlight and warmer temperatures, this photosynthetic process is most active, thus leaves are green. 

“When daylight hours become less and temperatures are cooler, photosynthesis slows down, and there is less chlorophyll. This decline reveals a yellow or orange pigment, carotenoid. Carotenoids, the same pigment found in carrots and corn, are usually masked by the chlorophyll.

Unlike chlorophyll and carotenoids, which are present in leaf cells throughout the growing season, anthocyanins are produced in autumn. Anthocyanins give color to familiar fruits, such as cranberries, red apples, cherries, and plums. These complex, water soluble compounds in leaf cells react with excess stored plant sugars and exposure to sunlight, creating vivid pink, red and purple leaves. A mixture of red anthocyanin pigment and yellow carotene often results in the bright orange color seen in some leaves.

“Weather conditions that occur before and during the decline of chlorophyll production can affect the color that leaves may display. Carotenoids are always present, so the yellow and gold colors are the least affected by weather. 

“Colors most affected by weather are the red tones created by anthocyanin. On warm sunny days lots of sugar is produced in the leaves. Trees exposed to brighter sunlight generate the reaction between the anthocyanins and the excess sugar creating the bright red hue. Cooler temperatures cause the veins in the leaves to gradually close preventing the sugars from moving out which preserves the red tones. Thus, a succession of warm sunny days and cool crisp nights can paint the most spectacular display of color.

“The level of moisture in the soil can also affect autumn color. A severe summer drought can delay the onset of color change by weeks. Ideal conditions for producing the most brilliant colors are a warm wet spring, favorable summer weather, and sunny fall days with the cooler temperatures at night.”Now that you know the science between the color change, enjoy your fall leaf peeping.

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