Why Leaves Change Color in the Fall

Even if you live in the most tropical of climates, where summer is your only season, you still can’t help but marvel at the splendor of a brightly colored autumn vista. So, what’s behind this annual event? Why do leaves change color every autumn and why are some years more vivid in color than others?

To understand what’s involved each fall in the transformation from a sea of cool green to a kaleidoscope of red, orange, yellow and every shade in between, it is helpful to know two important points. The timing of leaf color change is primarily affected by the calendar and the intensity is a product of three main factors: color pigments, length of night and weather.

First are the pigments. There are three most responsible for leaf color. Most of us are familiar with the first, chlorophyll. You know it as having something to do with providing the basic green color found in leaves and grass. It’s required for photosynthesis, the chemical process that allows plants of all sizes to use sunlight to produce food.

During the warmer months when plants are actively growing, the chlorophyll pigment dominates the color we see in leaves. However, another pigment is also present at this time, carotenoids. They produce the yellows, oranges and browns. But because chlorophyll is so dominate as a pigment, it is not until fall, when the photosynthesis process shuts down that carotenoid pigments begin to become apparent. Eventually the photosynthesis process ceases and all chlorophyll is depleted, eliminating the green color completely from certain leaves and allowing the carotenoid pigments to take center stage.

A third pigment, anthocyanins are not present in leaves until autumn. Warm bright days of fall produce lots of sugars in the leaf. But as the days shorten and the nights cool, these excess sugars are trapped in the leaf as veins leading into and out of the leaves gradually close. The combination of bright light and trapped sugars stimulate the production of the anthocyanin pigments. They produce the vivid shades of reds and purple and the many hues in between.

The next factor contributing to fall leaf color is the longer nights. Days become shorter, reducing the amount of sunlight available for plants to photosynthesize. This is nature’s way of signaling plants that winter is on the way. Energy begins to shift from food and energy production into storage and reserves. As the photosynthesis process slows down in response to shorter days, so does the production of chlorophyll. In the absence of the dominant green pigment, carotenoid and anthocyanin pigments light up the fall landscape.

The third factor and the one most responsible for the intensity of autumn color is the weather. The most brilliant fall displays are the result of a warm wet spring, a mild summer, bright sunny autumn days and cool but above freezing nights. When this combination comes together, the result is the most vivid color exhibition.

So, as you think back on this season’s display of color, hopefully you were rewarded with exceptional views. And next spring, when it’s warm and wet, be happy. It may be the makings of a spectacular fall!

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