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Why leaves change color

POSTED: October 10, 2012 4:00 a.m.

One of nature's greatest spectacles is the array of colors presented during autumn in the mountains.

Most of us are lucky enough to live within driving distance of a deciduous forest, allowing us to take in the beauty of the color change.

Have you ever wondered what causes leaves to change color and fall off? Have you ever wondered why different trees seem to turn different colors?

According to the U.S. Forest Service, the process of leaf color change begins with the onset of longer nights and decreasing sunlight intensity.

During the summer, trees are constantly producing sugars through a process called photosynthesis. These sugars are sent back down the plant from the leaves.

During fall, the veins that transmit fluids from the tree branches to the leaves close up. A thin layer of cells forms to clog the veins. This action traps different pigments in the leaves, resulting in brilliant fall color displays.

There are three pigments most important in leaf color change.

Chlorophyll is a green pigment that is most dominant in leaves. It captures the sunlight energy and is responsible for photosynthesis.

Anthocyanins are pigment compounds with red and purple hues.

Carotenoids are pigments that exhibit orange, yellow and brown colors.

During the summer, trees are actively producing and breaking down chlorophyll. It masks the other pigments because it is so plentiful.

During autumn, trees begin to shut down in anticipation of the winter.

Chlorophyll eventually gets used up and dissipates from the leaves. The other pigments, the carotenoids and anthocyanins, become trapped in the leaves and begin to show through.

Timing and color variation of leaf change is dependent on species.

Sourwood leaves often change color and fall off while most other species are still green. Oaks tend to be the last to change color. The exact color the tree species will show is dependent on the amount and types of pigment contained in its leaves. Oaks usually turn red or brown, poplars turn golden yellow, dogwoods turn purple-red, and maples have varying colors depending on the type.

Leaf drop is affected by many factors, but the main influence is longer nights. This is a sign to the tree that the cold winter months are coming soon.

Deciduous trees drop their leaves as a protection against cold temperatures. Leaves are the most tender parts of the tree and are susceptible to freeze damage.

Most deciduous leaves are fairly delicate, so ditching them for the winter minimizes cold damage to the trees.

Evergreen trees have found ways of avoiding leaf drop by having a hardened, waxy coating on their leaves for protection.

Environmental conditions can also have an influence on the intensity of leaf color change. Adequate soil moisture seems to have an effect on color change. Drought conditions throughout the summer can delay the onset of fall color and lessen its intensity.

Early frosts tend to decrease the amount of colors displayed. Rainy days and cool nights tend to produce the best fall colors.

Clark MacAllister is the Dawson County extension agent. For more information, call (706) 265-2442.


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