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The awe in Autumn: What makes leaves change their colors




Eastern Kentucky is home to one of our nation’s most beautiful regions with the Cumberland Mountains as the backdrop for one of Mother Nature’s most vivid displays. With the approaching season, sometimes breathtaking in its beauty, you might wonder how the trees can put on such a magical change. Keep in mind that although this magnificent change is a product of chemistry within the trees, it is also the product of weather.

Every autumn, magic transformations begin as trees begin to prepare for winter. In certain regions, such as our own, the shedding of leaves is preceded by a sometimes spectacular color show. Formerly green leaves will turn to brilliant shades of yellow, orange, and red. These color changes are the result of transformations in leaf pigments.

All during the spring and summer, leaves serve as factories where most of the foods necessary for the trees’ growth are manufactured. This food-making process takes place inside the leaf in numerous cells that contain the pigment chlorophyll, which actually masks the true color of the leaf by absorbing all other light colors and reflecting only the green light spectrum – thus the leaves appear to our eyes as green.

The energy of the light absorbed by chlorophyll is converted into chemical energy stored in carbohydrates (sugars and starches). By the plant absorbing water and nutrients through its roots, and absorbing carbon dioxide from the air, the chemical process of photosynthesis takes place. This chemical change drives the biochemical reactions that cause plants to grow, flower, and produce seed.

Chlorophyll is not a very stable compound; bright sunlight causes it to decompose. To maintain the amount of chlorophyll in their leaves, plants continuously synthesize it. The synthesis of chlorophyll in plants requires sunlight and warm temperatures. During the summer months, chlorophyll is continuously broken down and regenerated in the leaves of the trees.

The shortening of the days and the cooler temperatures at night trigger several changes within the tree. One of these changes is the growth of a corky membrane between the branch and the leaf stem. This membrane interferes with the flow of nutrients into the leaf. As this nutrient flow is interrupted, the production of chlorophyll in the leaf declines, and the green color begins to fade.

The different chemical components that are always present inside the structure of the individual leaf will determine its fleeting brilliant color in the fall. But weather also plays a factor in determining color. The degree of color may vary from tree to tree. For example, leaves directly exposed to the sun may turn red, while those on the shady side of the same tree may appear yellow or orange. Also, the intensity of colors on the same tree may vary from year to year, depending upon the combination of weather conditions.

According to the tree experts at several universities, the U.S. Department of Agriculture, and the U.S. Forestry Service, the most vivid colors appear after a warm, dry summer combined with early autumn rains which will help prevent early leaf fall. Long periods of wet weather in late fall will produce a rather drab coloration. Wet springs and early summers, followed by late summer droughts favor the chemical formation of anthocyanin, which is the chemical most responsible for the brilliant reds and purples. Drought conditions, especially in late summer, also favor red pigment formation due to the reduction of nitrate absorption through the tree’s roots.

However, a prolonged period of extreme drought as has been experienced this summer may produce a different outcome. One survival mechanism for trees in extreme drought is an early leaf drop. If there is no leaf exposure, demand for water decreases. The tree may go dormant earlier in the season in order to survive. As one looks at the woods across east Kentucky, there is an indication that this might be the case this year as many oaks, poplar, and sycamore trees have already turned yellow, then brown, and began shedding their leaves by the second week in September. Early leaf drop does not mean that the tree is dying, according to forestry officials. It is just an indication of extreme stress.

Just as rainfall, or lack of rainfall, plays a significant role in the brilliance of the autumn colors, temperature also plays a part. The best colors seem to be displayed when warm and sunny days are followed by cold nights through early October. Light frost will enhance the colors, but a hard, killing frost will hasten the actual fall of the leaves from the trees. The right combination of temperature, rainfall, and sunshine can prolong the autumn foliage brilliance by as much as two weeks.


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