Why do leaves change color?
The lush green leaves that provide shade and coolness during the summer start changing color upon the arrival of autumn. The soothing shades of green transforms into vivid shades of reds, browns, oranges and golds before falling off the branches of the tress. The colors of leaves in autumn are beautiful and breathtaking, but there are scientific phenomena at work behind the apparent simple color change.
For decades, scientists have work to comprehend the changes leaves and shrub goes through in autumn. Like most natural phenomenon, we have not been able to uncover all the details of the process, but we do have the basic understanding about this marvelous spectacle. There are three factors that are responsible for autumn leaf colors, the duration of night time, leaf pigment and weather.
The leaves owe their color to pigments. Pigments are natural substances produced by the cells in the leaves. There are three types of pigments involved in autumn colors, namely Chlorophyll, Carotenoids, and Anthocyanins. Chlorophyll is the pigment responsible for giving leaves their basic green color. It is necessary for photosynthesis ‘“ the chemical process that facilitates plants to use sunlight to manufacture their food. Plants and trees in the temperate zones store the food for winters. Carotenoids produce yellow, brown and orange colors in fruits and vegetables such as corn, carrots, bananas, daffodils, etc. Anthocyanins are responsible for color in plants such as red apples, cranberries, blueberries, cherries, grapes, plum and strawberries.
Chlorophyll and carotenoids are present in leaf cells during the growing season while anthocyanins are produced in autumn as a result of bright light and excess sugar in the leaf cells of the plant. During the season of growth, there is continuous production and break down of chlorophyll, hence leaves appear green. The presence of chlorophyll prevents the effects of other pigments from becoming evident. The sunlight regulates the production of chlorophyll. With the arrival of autumn, the length of nights increases which retards the product of chlorophyll but maintains the rate of breakdown, so much so to the point that all the pigment in the leave is destroyed. With the complete destruction of the pigment, the green color fades away.
The carotenoids and anthocyanins, though already present in leaves in dormant state, become dominant and show their colors. The timing of the color changes in leaves depends on the species. Some trees such as sourwood in southern forests become vividly colorful in late summer while others change color once autumn starts. The difference in timing is considered to be genetically inherited, rather than the location and environmental factors.
Pigmentation is not the sole factor in the change of leave color. Weather also affects the color change. The extent and brilliance of the colors in any particular autumn season is influenced by weather conditions. Furthermore, temperature of the region and the moisture content are also factors. Series of warm, sunny days and cool, crisp nights bring about the most vivid and bright colors in leaves. In these conditions, the ample amount of sugar is produced in the leaf, but the cool nights and slowly closing veins connected to the leave keeps the sugar from moving out. The swelling concentration of sugar levels in the cells increases the production of anthocyanin pigments. The excess of sugar and abundant product of anthocyanin pigments (mainly) bring about shades of tint red, crimson and purples. Hence, bright colors are found in sunny autumn days as anthocyanin is dependent on light while overcast weather results in yellow and brown colors.
The carotenoid which is permanently present in the leaves is not controlled by light, hence the shortening of days create no impact on their levels. The sole or major presence of carotenoid in leaves result in yellow color. The presence of both carotenoids and anthocyanins in reasonable amount produce orange color in leaves. In case neither of the pigments is present, leaf color is influenced by plant chemicals. Tannins produce brownish color in certain oak leaves.
The moisture content in the soil also influences the colors in autumn. Soil moisture varies significantly from year to year. Moisture coupled with weather is responsible for ensuring that the colors of autumn in leaves are never exactly the same. The change of color can be delayed by a late spring or severe summer drought. A warm period in fall can reduce the intensity of the color. The most brilliant and vivid colors are produced in a warm wet spring, favorable summer weather and warm sunny days with cool nights.
Everyone enjoys autumn colors as they make up for an incredible spectacle but autumn colors are not predictable. There are factors, as discussed, which affects the onset of color as well its brilliance and intensity. It is highly unlikely that the autumn colors experience one year will return the next year exactly the same, but leaves will likely change color to some extent with the onset of autumn.
USDA Forest Service, 2014, Why Leaves Change Color, Retrieved from: http://www.na.fs.fed.us/fhp/pubs/leaves/leaves.shtm
Anne Marie Helmenstine, 2014, Why do Leaves Change Color in the Fall, Retrieved from: http://chemistry.about.com/od/howthingsworkfaqs/f/fallleafcolor.htm
Carl E. Palm, Jr., 2014, Why Leaves Change Color, Retrieved from: http://www.esf.edu/pubprog/brochure/leaves/leaves.htm