Why Do Tree Leaves Change Color?

Sarah’s post last week got me thinking about fall foliage. Everyone loves the changing colors, but until recently I didn’t know why it happened. There are three primary factors that affect fall color: the length of the day/night, weather, and leaf pigments. 

As we near the end of the year and fall approaches, the days get shorter and the nights become longer. They also become cooler. These two changes induce a biochemical process in the leaf cells that alters their expression of pigments.

Warm, sunny days followed by cool — not freezing! — nights often induce the most brilliant colors. Moisture in the soil also influences the timing and expression of fall colors. For example, summer droughts can delay the change in the colors by several weeks.

There are several pigments that play a role in creating beautiful fall foliage. Chlorophyll is responsible for the green color of most leaves and is critical for photosythesis. Cartenoids are pigments that produce yellows, oranges and browns. Anthocyanins are blue, violet, and red pigments. All three are found in plants.

Chlorophyll and cartenoids are already present in leaves throughout the year, but anthocyanins are produced in the autumn as the days get shorter. At the same time, chlorophyll production is slowing down (and eventually stops). This allows the colors from the cartenoids and anthocyanins to become more visible. Different species may have different characteristic leaf colors in autumn. Some species have leaves that just dry up and fall off.

It’s important for deciduous trees to drop their leaves because they are full of fluid that would freeze during winter months, damaging them beyond use. In other words, leaf drop is a survival mechanism for non-evergreen trees to successfully overwinter.

For more information, check out the USDA Forest Service.

Images: Christopher S. Penn, KimberlyKV and NHN_2009


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