Fall Foliage: The Science Behind The Beauty
From the Northeast to the Midwest, people annually flock to see a beautiful transformation take place- fall foliage. Every autumn, leaves transcend into vibrant shades of red, orange, yellow, and gold. The fall foliage display begins in early September, and ends in early November. As the earth rotates around the sun, varying regions receive decreased amounts of sunlight.
When this occurs, the days become shorter and the nights grow progressively longer. The photoperiod (length of day) triggers the tree to begin the process of leaf senescence to prepare for winter. Trees respond to the decreased amounts of sunlight by producing less chlorophyll (the green pigment in trees). Pigments are natural substances produced by leaf cells to harvest energy from sunlight. Trees also contain other pigments, but they are overshadowed by an excessive amount of chlorophyll in the summer. Eventually, the production of chlorophyll ceases, which unmasks other color pigments allowing them to shine through.
Chlorophyll and carotenoids are two light absorbing chemical pigments. Carotenoids are responsible for the yellow, orange, red and brown colors seen in leaves. Unlike chlorophyll and anthocyanins, carotenoids are always present in leaves regardless of the weather. Corn, carrots and bananas are just some of the vegetables that contain carotenoids.
Anthocyanins are responsible for the red, purple, and crimson colors in leaves. An interesting fact about anthocyanins is that not all trees can produce them. They are usually produced only in autumn in response to lower temperatures, bright sunlight and increased sugar concentrations in leaves. Anthocyanins also add red to cranberries, strawberries, cherries and apples.
Autumn leaf colors are usually specific to certain tree species but can vary even within trees of the same species. Trees that produce red foliage are mountain maple, red maple, dogwood, sweetgum, black gum, white oak, red oak, scarlet oak, sumac and sassafras.
Trees that produce yellow foliage are hickories, silver maple, Norway maple, white ash, black oak, striped maple, black maple, American elm, beech, yellow poplar, willow, paper birch, yellow birch, ginkgo and witch hazel. Sugar maples may produce bright orange, red and yellow leaves on the same tree.
Color variation and intensity of fall foliage is often dependent on the weather and soil conditions. Weather dictates the timing of the appearance of foliage color, the intensity and colors seen, and the duration of time that leaves remain on trees. The best autumn colors can be seen when there has been a warm and wet spring, a summer that is not excessively arid, and mild sunny autumn days with cool evenings (above 32 f). Less then ideal weather conditions include hard frost, wet weather and wind. Hard frost causes leaves to quickly wither and drop to the ground, and wet weather can lead to a drab coloration. Along with wind, rain can knock leaves off the trees before full fall coloration is realized.
Healthy, well fertilized trees growing in good soils often produce the best fall colors. Trees that are under stress usually begin to show color earlier then healthy trees but tend to drop these leaves rather quickly. The best way to enhance fall foliage is to maintain species diversity on your property and make sure that your feature trees and shrubs receive professional care including fertilization, insect and disease control, and pruning by a certified arborist.
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