Fall is in full swing and many trees throughout our region have been showing off their festive colors for a number of weeks now. Although children as young as five know why the sky is blue, many adults don’t understand how it is that a deep green leaf just a week ago can turn a brilliant gold seemingly overnight. Leaf pigments, length of night time hours and weather all have an effect on leaf color.
Although not always seen, there are three pigments that are present in leaves during the whole year. Chlorophyll, which gives leaves their basic green color, is the chemical reaction that enables plants to use sunlight to manufacture sugars for their food. Carotenoids produce yellow, orange, and brown colors in such things as corn, carrots, and daffodils, as well as rutabagas, buttercups, and bananas. Anthocyanins give color to such familiar things as cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums. They are water soluble and appear in the watery liquid of leaf cells.
Another chemical in leaves, auxin, controls a special band of cells at the base of each leaf stem, called the abscission layer. During the growing season, auxin prevents this layer from fully developing and blocking the tiny, internal tubes that connect each leaf to the rest of the tree’s circulatory system. In fall, however, cooler and shorter days trigger an end to auxin production, allowing the abscission layer to grow and cut off the circulation of water, nutrients and sugar to the leaves. When this happens, chlorophyll disintegrates rapidly, letting carotene shine through as the yellow in maple, aspen and birch leaves. Anthocyanin, meanwhile, provides the oranges and reds of maples, sumacs and oaks. When there’s less sun, anthocyanin isn’t as chemically active and leaves are more orange or yellow than red.
The 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 spur production of the brilliant anthocyanin pigments, which tint reds, purples, and crimson. Because carotenoids 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 colors.
- Here is a list of a few trees that do well in our area and have great fall color…
- Acer (many varieties)
- Gingko biloba
- Metasequoia glyptostroboides
- Pistacia chinensis
- Quercus (many varieties)
- Zelkova serrata
Marie Stadther’s life in Coachella Valley was void of trees. In 2001, she packed up and headed north. After a drive through the majestic redwoods, she arrived in Redding, where she immersed herself in horticulture as owner of her own landscaping company and as assistant to an arborist. She is now the lead gardener for Turtle Bay’s McConnell Arboretum and Botanical Gardens. Her love of trees is a way of life, and she shares that passion with the community. Send the Tree Goddess your questions at firstname.lastname@example.org.
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