September 22, 2019

Understanding Autumn Foliage Colorations: The Science Behind Leaf Color Change

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We are soon approaching the time of year with shorter days, longer nights and cooler temperatures; when these changes occur, the vibrant green leaves on trees begin to slowly change to reds, yellows, oranges, browns and purples before they “fall” before winter.

Leaf color change is caused by a variety of factors. In the summer, chlorophyll, a plant’s pigment, is rapidly replaced in the leaves due to the longer, sunnier and hotter summer days.

“Leaves are green because they keep a constant supply of chlorophyll, the photosynthetic pigment that captures energy from sunlight,” said Clarice Guan, a graduate student researching plant biology.

In warmer, humid weather, leaves consistently photosynthesize to provide sugars for the rest of the plant or tree.

“For photosynthesis to work, specialized structures in leaf cells called chloroplasts have to capture the energy from light, and to do so, they use a variety of pigments,” said Joseph Cammarata grad.

According to Michael Miller grad, both chlorophyll and carotenoid pigments function to harvest energy from light for photosynthesis. The carotenoids, which are usually hues of orange, red and yellow, are far less abundant, but still critical for photosynthesis.

“Every fall as the days get shorter and the nights start to get colder, deciduous trees respond by going into a temporary state of dormancy,” said David Wickell grad.

According to Wickell, this dormancy creates drastic changes in plants’ leaf compositions, which can explain why we see these beautiful color transformations. This process is referred to as senescence.

Cammarata explained that leaves undergo senescence in the fall. During senescence, plants store nutrients and distribute them to seeds and buds that will become next year’s leaves or flowers.

Miller said that the senescence process resulted in the degradation of the chlorophyll pigments, removing the color from the leaf. This reveals the “underlying” oranges, yellows and reds of the carotenoids.

“Often, these plants will also produce anthocyanins, which add red or purple colors to the mix,”  Guan said.

Prof. Karl Nikas, plant biology, said that a distinctive combination of rainfall and temperature provides the most prominent fall colors each season.

“A wide array of factors such as temperature, precipitation and genetics all affect the rate of change and intensity of color,” Wickell.

Thus, fall colors can be prominent during one year and barely visible the following.

Last year, Prof. Taryn Baurle, plant science, accurately predicted that the fall colors of 2018 would not be vibrant because of the combination of warm weather and excessive precipitation.

According to Wickell, color change differ among species of trees as well.

“The exact biochemical makeup of pigments, and their amounts can vary by species,” Guan said.

Red maples, for example, have notable color changes and become one of the most colorful trees in the fall with their red-purple anthocyanin hues.

“Trees in the bean (Fabaceae) family also produce beautiful, but extremely ephemeral, yellow leaves,” Miller said.

“My favorite fall foliage has to be that of smooth sumac (Rhus glabra) which hosts a broad range of colors from bright orange to a deep purple color,” Wickell said.

Regardless of the species, watching trees change color in autumn is a magical thing to witness.

“When we look a little closer at the trees around us, we see that even a “simple” process like reducing chlorophyll content can produce a wonderful, seasonal diversity in plants,” Guan said.