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Why Do Leaves Fall

  • Martha Proctor


    Ah - the beauty of autumn!We just returned from a hiking trip to Spain and France where the trees were in full autumn splendor providing a vivid palette of reds, oranges, golds, and purples.In our own garden the lovely reds of the dogwood and blueberry and the yellow of the maple leaves are a small but beautiful reminder that the trees are preparing for the cold of winter.



    Perennial plants, including trees, must protect themselves to be able to survive through any harsh winter weather.Evergreen trees - pines, spruces, cedars, firs - are able to survive the winter because their needle-like foliage is covered with a heavy wax coating and the fluid inside their foliage contains substances that resist freezing.Stems, twigs, and buds have a protective coating so are able to survive extreme cold.The thin, tender leaf tissues in broad-leaved trees would freeze in winter so plants must either toughen up and protect their leaves or dispose of them.Thus, leaf fall precedes each winter in temperate climate zones.


    Through a process called photosynthesis, leaves are nature’s food factories. Sugars are produced from carbon dioxide and water in the presence of chlorophyll, using energy from the sun. Glucose is the sugar plants use as food for energy and as a building block for growth.Chlorophyll also gives leaves their basic green color.As days shorten and temperatures become crisp, protective processes begin to take place within the plant during which the green palette of spring is transformed into the vivid colors of autumn.


    The most critical factor which influences autumn leaf color pigmentation is photoperiodism—the length of day and night.As days grow shorter, and nights grow longer and cooler, biochemical processes in the leaf begin to change the landscape to its autumn palette.The processes induced by photoperiodism are called “senescence,” a term for the collective process that leads to the aging and death of a plant or plant part, e.g., a leaf.Senescence is a part of the larger process by which a plant goes into dormancy.


    In early autumn, in response to the shortening days and declining intensity of sunlight, leaves begin the process leading up to their fall from the trees.Throughout the growing season, both chlorophyll and carotenoids are present in the chloroplasts, the organelles in leaf cells active during photosynthesis.As the photoperiod decreases, the plants ability to synthesize chlorophyll decreases and the yellow and orange carotenoids, always present in the leaf, begin to show.Carotenoids are responsible for the yellow and orange colors in such foods as corn, carrots, rutabagas, and in flowers, e.g., daffodils and buttercups.Additionally, in response to bright light and excess plant sugars, senescent cells produce other chemicals, particularly anthocyanins, responsible for the red and purple colors evident in cranberries, blueberries, strawberries, and plums.Some species, particularly oaks, contain high quantities of tannins which give rise to brown colors.


    The plant efficiently converts nutrients from the leaves, i.e., water, sugar, amino acids, into a form that serves as an antifreeze which it draws away from the leaves into the stems and roots.The veins that carry fluids into and out of the leaf gradually close off as a layer of cells forms at the base of each leaf.Eventually the veins of the leaf are all that hold the leaf to the tree.At the conclusion of the process, the only tissues left in the leaves are cell walls and nutrient-depleted protoplasm. The veins break or are torn off by the wind and the leaf falls, leaving what is called a bundle scar and a bud for next year’s growth.


    The amount and brilliance of the colors that develop in any autumn season are related to the weather conditions that occurred before and during the time the chlorophyll in the leaves is dwindling.Temperature and moisture are the main influences.A series of warm, sunny days and cool, crisp but not freezing nights is thought to bring about the most spectacular color display.If conducive conditions exist, lots of sugar is produced in the leaf but the cool nights and gradual closing of veins prevent these sugars from moving out.These factors unite to promote production of the anthocyanin pigments, which induce the leaves to turn red, purple and crimson.Because carotenoids are always present in leaves, the yellow and gold colors remain fairly constant from year to year.


    In addition to protecting plants from the cold of winter, autumn brings a unique beauty and purpose to the landscape.As the leaves decompose, the resulting compost provides nutrients to the soil which, ultimately, are recycled to the plants and trees living in the soil.Now is the time to enjoy observing this process of senescence and rebirth in your own garden and in the wider vistas within Marin.