A fiery fall display; How weather impacts the show

Posted 8/21/12

NORTHEAST U.S. — In some years, the displays of autumn leaves are brighter than others. The following explanation as to why comes from the Forest Service of the United States Department of …

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A fiery fall display; How weather impacts the show

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NORTHEAST U.S. — In some years, the displays of autumn leaves are brighter than others. The following explanation as to why comes from the Forest Service of the United States Department of Agriculture.

“The timing of color change and leaf fall are primarily regulated by the calendar, that is, the increasing length of night. None of the other environmental influences—temperature, rainfall, food supply and so on—are as unvarying as the steadily increasing length of night during autumn. As days grow shorter, and nights grow longer and cooler, biochemical processes in the leaf begin to paint the landscape with nature’s autumn palette.

“During the growing season, chlorophyll is continually being produced and broken down and leaves appear green. As night length increases in the autumn, chlorophyll production slows down and then stops, and eventually all the chlorophyll is destroyed. The carotenoid pigments and anthocyanin pigments that are present in the leaf are then unmasked and show their colors.

“The amount and brilliance of the colors that develop in any particular autumn season are related to weather conditions that occur before and during the time the chlorophyll in the leaves is dwindling. Temperature and moisture are the main influences.

“A succession of warm, sunny days and cool, crisp but not freezing nights seems to bring about the most spectacular color displays. During these days, lots of sugars are produced in the leaf but the cool nights and the gradual closing of veins going into the leaf prevent these sugars from moving out. These conditions—lots of sugar and lots of light—spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoid pigments are always present in leaves, the yellow and gold colors remain fairly constant from year to year.

“The amount of moisture in the soil also affects autumn colors. Like the weather, soil moisture varies greatly from year to year. The countless combinations of these two highly variable factors assure that no two autumns can be exactly alike. A late spring, or a severe summer drought, can delay the onset of fall color by a few weeks. A warm period during fall will also lower the intensity of autumn colors. A warm wet spring, favorable summer weather, and warm sunny fall days with cool nights should produce the most brilliant autumn.”

Some climate scientists say that because of global climate change, the turning of the leaves each fall could be pushed back by a few weeks by the end of the century. Writing in the Journal of Global Ecology and Biology, researchers from Princeton University say that this possible seasonal shift, which may push back the change by a few weeks in some areas, may not be such a bad thing. When leaves are green, it means they are performing photosynthesis and in the process pulling carbon out of the atmosphere, and the current thinking suggests that the longer the trees are green, the more carbon, which is the main greenhouse gas, is pulled out of the atmosphere.

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