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  1. shinichi Post author

    TAG Forests, Botany, Autumn, Leaves

    Fifty shades of autumn leaves: The why (and how) of changes in foliage color

    by Jim Algar


    Chemistry behind the brilliant colors of U.S. forests in fall explained. Why leaves go from green to red, orange and yellow.

    (Photo : ForestGladesiWander, Flickr, Creative Commons)

    Have you ever wondered about the science behind the colorful changes in America’s forests as summer turns to fall? Well, wonder no more, just prepare for a little chemistry lesson.

    The chemistry of photosynthesis, to be exact – the process whereby trees take sunlight falling on their leaves and convert that into energy to break down carbon dioxide into sugars for food and oxygen that is in turn necessary for our own survival.

    The chemical pigment in the leaves primarily responsible for making the process happen — chlorophyll — allows trees to take energy from the sun by absorbing light in the wavelengths of red and blue while reflecting green wavelengths, which is why we see the leaves as green.

    However, as days grow shorter and colder with the arrival of fall, many varieties of trees no longer depend on leaves for their energy needs, so the chlorophyll becomes unneeded.

    That’s the point at which other pigments in the leaves, some of which have always been there but were overpowered in their effect by the chlorophyll, now come to the fore to provide the brilliant warm hues so emblematic of fall.

    Pigments known as xanthophylls and carotenoids reflect yellow and orange, respectively, and break down more slowly than chlorophyll with the coming of fall, which is why the colors become visible in the season.

    “The amount of carotenoids is pretty much constant, unlike chlorophyll,” says Robert Moreau, a plant biochemist with the U.S. Department of Agriculture.

    The red that appears is from pigments known as anthocyanins, which the leaves don’t make during the summer and only appears well into fall.

    Anthocyanins are also found in distinctively red fruits such as strawberries, cherries and cranberries.

    The onset of fall coloring varies from year to year, with 2014 expected to provide a bounty of seasonal hues.

    An excellent growing season in the U.S. Northeast has set the stage for a colorful display, University of Maine ecologist Michael Day told Accuweather.

    “In addition, adequate precipitation and lack of wind disturbances has resulted in trees with an exceptional amount of foliage still attached,” he said.

    Weather over the next 4 to 6 weeks will strongly determine how vivid a display the region’s forest put on, he said; cold, dry weather through September and October is usually ideal for producing the best colors.

    A cold snap over that period would bring out vibrant colors and offer an “exceptional” year for fall foliage in the Northeast, he said.


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