Autumn's Palette
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
Many people, including myself, believe that Nantucket is loveliest in the fall; the weather is cool and often sunny, the water is still relatively warm, and our landscape begins to show its autumn colors. Driving down Polpis Road in late September, you can see the large palmate leaves of the Virginia creeper (Parthenocissus quinquefolia) turning red and draped like haphazard Christmas garland on the Eastern red cedar (Juniperus virginiana) lining the road. Even poison ivy has the decency to reveal itself in red splotches creeping up trees. Both of these vines change colors early in fall as their chlorophyll begins to shut down for the winter and their nascent red colors appear. Fall foliage is created as orange, gold, brown, and red colors always present in leaves begin to out-compete the green color supplied by chlorophyll dies off when plants prepare for winter dormancy. The salt marsh plants begin to turn brown and red, even the pickle weed found in the salt marsh (Salicornia) turns a bright red early in fall. Our moors and heaths also gradate into soft browns, gold, and reds. The showiest members of the plant kingdom that draw visitors from around the country to New England are deciduous trees such as oaks, maples, birches, beeches, and elms. As we’ll see below, there are many factors that determine whether fall is a spectacular show or a more modest production.
The primary factors that influence autumn leaf color are the changes in the amount of darkness or light (length of day) called photoperiodism and environmental processes including rainfall, temperature, and food supply. The timing of color change and leaf fall is primarily regulated by the increasing length of night although the temperature and number of sunny days also play important roles. None of the other environmental influences such as temperature, rainfall or food supply 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. This colorful progression signals the death of these leaves (senescence) which becomes apparent through color change and the falling of leaves, leading the tree into its winter dormancy. The colors present in leaves are provided by three pigmented organic chemical complexes of which chlorophyll, which gives leaves their basic green color, is the most well known. Chlorophyll is necessary for photosynthesis, the chemical reaction that enables plants to use sunlight as energy to combine with carbon dioxide to manufacture sugars for their food. Trees in the temperate zones store these sugars for their winter dormant period. Chlorophyll is green because it absorbs all colors except green, which it reflects. Chlorophyll has a metal atom, magnesium, in the middle of its molecule. This molecule is oddly similar in chemical structure to another important natural pigment: hemoglobin in warm-blooded animals, which differs largely in having an iron atom in its center.
Another natural fat-soluble pigment found in plants that contribute to the glory of fall is the carotenoids, which produce yellow, orange, and brown colors in such things as corn, carrots, and daffodils, as well as rutabagas, buttercups, and bananas. Carotenoids include several phytonutrients such as lycopene, beta-carotene, lutein, and zeaxanthin. Medical and food nutrient research has indicated that these phytonutrients act as antioxidants in our bodies and may inhibit the growth of human cancer cells by reducing free radicals in our bloodstream. Another essential plant pigment group is the anthocyanins, which give color to such familiar things as cranberries, red apples, concord grapes, blueberries, cherries, strawberries, and plums. Anthocyanins are water soluble and appear in the watery liquid of leaf cells. Both chlorophyll and carotenoids are present in the chloroplasts of leaf cells throughout the growing season. Most anthocyanins are produced in the autumn, in response to bright light and excess plant sugars within leaf cells.
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 carotenoids and anthocyanins that are present in the leaf are then unmasked and show their colors. Water and nutrients are drawn into the stems and away from the leaves. As they shut down, senescing cells also produce additional anthocyanins, which are responsible for red and purple colors. Some species, particularly the oaks, contain high quantities of tannins in the leaves which are responsible for brown colors. When you gaze upon an orange or brown stream or pond, you are looking at stained water colored by tannins, not unlike the color imparted by tea leaves.
In preparation for winter and to prevent or minimize damage from cold, plant cells switch from production of chlorophyll for growth, to production of sugars and amino acids, which act as antifreeze for the plant. As the daylight shortens and the nights get cooler, hormones in trees with deciduous leaves start a chemical reaction that forces nitrogen compounds from the leaves back down into the roots for storage during the winter season. After this happens, a membrane, called the abscission layer, forms at the base of the leaf-stalk. This membrane cuts across the tiny nutrient-carrying tubes that connect the leaf with other living parts of the tree such as the green inner bark and the roots. These tubes carry water and minerals from the roots to the leaves, and also return sugar manufactured in the leaves to the rest of the tree. The leaf starts to die as water is restricted by the presence of the abscission layer. Sugar and waste products begin to accumulate in the leaf. Before it falls, however, the leaf will give us all the colors of autumn. Eventually the wind will break this tenuous hold and the leaf will fall to the ground. The nitrogen in the fallen leaves is broken down by soil bacteria and microbes and returns to the ground to be reabsorbed by plants the following spring. When we remove leaves from our lawns, we are short-circuiting this natural regeneration.
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 leaves in 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. This past summer’s drought may delay the colors we would normally see on Nantucket, but the concentration of sugar in the leaf cell sap may be increased if the weather is dry, resulting in a more magnificent fall.
Not all species or even individuals within a stand of trees decides to change colors in the same day. Variables like amount of shade and depth to groundwater or wetness of the soil affects when a plant starts its winter journey. For instance, several species of oaks tend to turn last in New England, lending golds and browns to the graying landscape.
Winter is a certainty that all vegetation in the temperate zones must face each year. Perennial plants, including trees, must have some sort of protection to survive freezing temperatures and other harsh wintertime influences. Stems, twigs, and buds are equipped to survive extreme cold so that they can reawaken when spring heralds the start of another growing season. Tender leaf tissues, however, would freeze in winter, so plants must either toughen up and protect their leaves or dispose of them.
Nantucket’s evergreens such as pines, spruces, hollies, and cedars are able to survive winter because they have shored up their defenses to the cold. Their needle-like or scale-like foliage is covered with a heavy wax coating and the fluid inside their cells contains substances that resist freezing. Thus the foliage of evergreens can safely withstand all but the severest winter conditions. Evergreen needles survive for some years but eventually fall because of old age. On Nantucket, the salt content of the surrounding ocean can also lend some protection against freezing temperatures by raising the temperature of freezing water.
The leaves of broadleaved plants, on the other hand, are tender and vulnerable to damage. These leaves are typically broad and thin and are not protected by any thick coverings. The fluid in cells of these leaves is usually a thin, watery sap that freezes readily. This means that the cells could not survive winter where temperatures fall below freezing. Tissues unable to overwinter must be sealed off and shed to ensure the plant's continued survival.
All this scientific talk about pigments and senescence does not diminish the joy of a cool crisp sunny day framed by the red pigments of sassafras trees in Squam Swamp or the diffuse ribbons of color seen in the Middle Moors or Folgers Marsh. You are looking at plants enjoying their moments of leisure as they relax in restfulness, unimpeded by the urgent desire to bud and grow. As the hectic activity of summer fades, make sure to take some time to enjoy nature’s fall fashion.
