Why Tree Leaves Turn Color in Autumn
Jeffrey O. Dawson, Professor of Tree Physiology
Department of Natural Resources and Environmental Sciences
University of Illinois at Urbana-Champaign
Only 14% of the world’s forests are temperate deciduous forests with a brilliant habit of fall coloration. Each year a landscape patchwork of yellow, orange, pink and red hues burst forth to delight millions of autumn visitors to the broadleaved, deciduous forests of North America. New England is famous for brilliant red leaf displays of sugar maple intermixed with the bright yellows of aspen, beech and birch. Seasonal tourists, referred to as “leaf peepers,” inject millions of dollars into the rural economy of the northeastern U.S. The northern hardwood forests of the Lake States and southern Canada also attract many fall-color tourists. To the south and west broadleaved forests display patches of brilliant color produced by sugar maples blushing red, mixed with the more subtle colors of mixed hardwood forests. These colors include creamy, flat yellow hickory leaves, red leaves of white oaks, yellow brown to russet-red leaves of red oak, the brilliant, early reds of sumacs and Virginia creeper, the luminescent reddish purple of blackgum, the muted, greenish-yellow of silver maple, the dull brown of shingle oak and the green of alders. Mountains and canyons of the far west (think aspens) and the Appalachian highlands have their own regional displays and avid spectators.
The Advantages of Shedding Leaves in the Fall
Temperate deciduous broadleaves, as a whole, avoid having to pay the energy cost of maintenance respiration that would be required if they retained leaves year round. Winter leaf retention would not allow the broadleaved trees, with large masses of leaf tissue, to balance their winter loss of respired carbon (carbon dioxide). Although winter-hardened, dormant leaves of trees in cold temperate climes would have reduced levels of respiration in winter, temperatures are generally too low for an equivalent gain of carbon via photosynthesis. Fall color is associated with the process of leaf senescence leading up to the fall shedding of leaves.
There are popular and scientific myths about the causes of fall color in temperate deciduous forests. Jack Frost is alleged to paint tree leaves with his chilling touch, bringing on color change along with a frosty coating. Another twist on this myth is that Jack Frost brings reds and purples to autumn trees by pinching the leaves with his icy fingers. A less poetic explanation of fall color, favored by scientists for decades, is that the autumnal coloring of leaves was caused by waste products accumulated in the leaves and revealed to us with the fading of green chlorophyll pigments. As it turns out, the waste product theory now seems to be considered a bunch of, well, crud. The fall color pigments are produced, or revealed, only in living leaf cells of deciduous trees during the critical, seasonal process of leaf senescence. In fact, if Jack Frost did his thing too early, or, in other words, if there was an early killing frost, the leaf color display would be dulled, if not stopped altogether.
What triggers these fall changes if not Jack Frost? A specific combination of shortening day length and cooling temperatures in autumn at a given locale is typically “sensed” by plant receptors resulting in the production of plant hormones that initiate leaf senescence. The initiation and timing of the various processes of leaf senescence are genetically controlled for tree populations of a rather narrow climatic zone. How narrow is it? Seedlings originating from a local population in Illinois are usually planted within northern, central and southern seed zones each about 130 miles from south to north. This precise genetic programming, evolving through the impetus of natural selection, allows leaves to escape autumn frost damage in a specific climatic region during senescence. In the western mountains, it is possible to observe the wave of aspen coloration beginning at higher mountain elevations and progressing downwards to milder climates at lower altitudes.
In the living cells of senescing leaves, complex molecules, such as starch and proteins, are broken down into smaller, soluble ones, such as sugars and amino acids, and then exported to storage cells (resorbed). Living storage cells are found in the inner bark of twigs, the outer sapwood of the main stem (in and near wood rays) and in corresponding root tissues. Resorbing and storing these compounds permits the tree to shed its leaves while avoiding loss of the large percentage of their nutrients in leaves. This, in turn, allows the tree to avoid having to compete with other plants and soil microbes for the resorbed nutrients that would otherwise be cycled back into the soil system through leaf litter decomposition. Resorbed nutrients including nitrogen, phosphorus, potassium, sulfur, and carbohydrates. are mobilized from cells and stored within the tree. The following spring the stored nutrients are remobilized and used to support the intense flush of new leaves and spring growth burst in other tissues.
More energy is required for the biochemical breakdown of leaf substances by enzymes, and for loading the soluble products into the leaf-veins for transport out of the leaves, than that which is available as reserves in leaves. Hence it is necessary to protect chlorophyll, at least during the earlier phases of senescence, in order to prolong production of energy rich compounds that initiate the enzymatic reactions necessary for leaf senescence. Additional important biochemical processes supported by photosynthesis in senescing leaves include the production of enzymes and their products that allow leaf cells to better tolerate freezing and drying, that absorb energy from light bursts damaging to the photosynthetic apparatus, that deter leaf predators, that prevent oxidative damage to cell constituents, including membranes, proteins and DNA, caused by free radicals produced during senescence, and that protect and transform the cells of leaf tissue that form the abscission layer at the base of the leaf petiole. The abscission layer allows the leaf to break away cleanly from its branch without forming an opening from which sap could leak and through which disease organisms could enter the tree.
The Functions of Autumnal Leaf Pigments
Carotenoid pigments are found abundantly in such vegetables as carrots and tomatoes. The carotenoids include lycopene and beta-carotene, known to be powerful antioxidants and cancer-fighting substances in humans. Another form of carotenoid found in senescing tree leaves is xanthophyll. Carotenoids are responsible for the yellow and orange colors of autumn leaves. The unmasking of the carotenoids accounts for the yellow fall leaf color of Ohio buckeye, yellow-poplar, sycamore, birches, hickories, ashes, and many other tree species.
Carotenoid pigments and chlorophyll are attached to membranes in intricate structures (organelles) called chloroplasts. Chloroplasts give leaves their green color. Carotenoid pigments assist chlorophyll in the capture of sunlight for photosynthesis. The light energy is converted to a form that charges energy-rich compounds needed to activate enzymatic reactions. These yellowish pigments are always present in leaves, but are not visible for most of the year because they are masked by larger amounts of green chlorophyll. As chlorophyll degrades in the fall, the carotenoid pigments degrade more slowly and persist, revealing their yellowish colors.
In pistachio trees undergoing summer leaf senescence in the Mediterranean, S. Munne-Bosch and J. Penuelas of the Science Faculty of the Autonomous University of Barcelona found that carotenoid substances actually increase during the early stages of senescence. Carotenoids are thought to provide both photo-protection and antioxidative protection to the photosynthetic apparatus. Carotenoids dampen damage, caused by high light intensity, to the susceptible photosynthetic apparatus of senescing leaves.
Tannins cause the brown hues in leaves of some oaks and other trees in the autumn. The golden yellow or copper colors produced in some leaves, such as those of beech, result from the presence of tannins along with the yellow carotenoid pigments. Like the carotenoids, these compounds are always present, but only become visible as chlorophyll and carotenoids both disappear from leaves. They are common products of metabolism in trees, deposited in the cell sap inside the vacuole as well as in cell walls, and often accumulate in dead tissue. Considered waste products, tannins actually act as a defense mechanism in plants against pathogens, herbivores and hostile environmental conditions. Consumed in sufficient quantities, bitter-tasting tannins are toxic to susceptible herbivores. Oaks defoliated by gypsy moths often produce a secondary flush of leaves higher in protective tannins than the first set of leaves. Tannins discourage insect feeding on leaves and bark, and, in the case of the lip-puckering, unripe persimmon fruits, discourage fruit and seed consumption by animals until both the fruit and seed are ripe. Once ripened, fruits are sweet, attracting seed dispersing animals, and seeds have a fully developed seed coat to prevent seed destruction in passage through the digestive tracts of animals.
The leaves of green tea, a woody plant, are a source of beneficial human dietary tannins. Monomeric flavanols, the major components in green tea, are precursors of condensed tannins. Tannins from grapes and oak barrels used for aging play an important role in preventing oxidation in wine. Tannins in fruit juices, such as those found in pomegranate juice, provide antioxidant and other health benefits to humans. Perhaps research will also find evidence for a similar function in senescing leaves.
Anthocyanin pigments are responsible for the pink, red, and purple leaves of sugar and red maple, sassafras, sumac, white and scarlet oak, and many other woody plants. They are formed in sap inside the vacuole, a storage compartment within plant cells, when sugars accumulate and combine with complex compounds called anthocyanidins. Anthocyanidins are a subclass of flavanoids, a group of antioxidant compounds found in plants including fruits and vegetables. The variety of pink to purple colors in leaves is due to many, slightly different compounds that can be formed. Their color is also influenced by cell pH. These pigments usually are red in tree species with acidic sap, and are purplish to blue in alkaline cell solution. Anthocyanins are not commonly present in leaves until they are produced during autumn coloration. A few trees, however, such as 'Crimson King' Norway maple produce reddish leaves throughout the growing season due to anthocyanins. Trees lacking the genes for production of anthocyanin develop yellow and brown shades of autumn color.
With the formation of the abscission layer and with higher viscosity of cell sap under cold conditions, the phloem tissues of a tree’s vascular system, the pathway for conduction of sugars out of leaves, become less efficient and are eventually severed where the leaf petiole joins the tree branch. However, the nonliving xylem vessels that transport water and nutrients from the roots upward, remain intact. This allows them to continue to carry water to the senescing leaves while sugars derived from continued photosynthesis and the conversion of stored starch to soluble sugars are trapped by the impaired phloem of the abscission layer and are available for anthocyanin production. Trees of the same species growing together often differ in color because of differences in amounts of soluble sugars in the leaves for anthocyanin production. These differences are caused by genetic and environmental factors. Leaves exposed to the sun, such as those on the outside branches of the tree crown, may continue photosynthesis and turn red while others in the shade may be yellow. A single tree may even have branches with different colored leaves due to differences in leaf shading. It is common to see sugar maples with reddish leaves only on exposed outer branches of the upper crown.
Fall weather conditions favoring formation of bright red autumn leaf color are warm sunny days followed by cool, but not freezing, nights. Rainy or cloudy days with their reduced sunlight near the time of peak coloration decrease the intensity of reddish autumn colors by limiting photosynthesis and the sugars available for anthocyanin production. There is an old wives' tale that claims rain washes the color out of leaves. It is not true, but the overcast conditions do reduce light intensity, and heavy rains and high winds can sweep the leaves off trees prematurely.
Kevin Gould of the University of Auckland in New Zealand is at the forefront of research on leaf anthocyanins. Senescing leaves seem to need special protection against bright light exposure because the metabolic pathways for the initial capture of energy don’t lose their efficiency as rapidly as the subsequent processes for processing that energy do. Bright light that reaches senescing tree leaves overloads light-gathering chlorophyll and slows it down (photoinhibition). Anthocyanins can offload some of that excess energy, decreasing photoinhibition, sustaining photosynthesis rates necessary to provide energy for nutrient resorption and other critical processes during senescence.
Anthocyanidins, formed from anthocyanins, are flavanoids: antioxidants that are beneficial to human health and possibly able to help prevent such diseases as cancer, Alzheimer’s disease, and cardiovascular disease. While reading an article on the human health benefits of consuming anthocyanin-rich blueberries, Gould decided to investigate the possibility that the antioxidizing powers of the leaf anthocyanins he was investigating also benefited their source plants. In test tube experiments he found anthocyanins purified from tree leaves were four times more effective at soaking up damaging free radicals than vitamins C and E. He and his colleagues devised a method to induce and observe an oxidative burst of hydrogen peroxide by using a needle to pierce the upper layers of a New Zealand shrub that produced red pimples when pierced by aphids. Gould and his coworkers were able to observe bursts of the powerful oxidant hydrogen peroxide one minute after stabbing leaves with a needle. In red leaf tissues the burst faded quickly, while in green tissues the hydrogen peroxide concentrations soared for at least ten minutes. These results suggest that anthocyanins function as protective antioxidants in plant leaves.
Anthocyanins may protect physiological processes in leaves from cold temperatures. Gould notes that a birch species he encountered in Finland held on to its red leaves year round, despite temperatures that plunged to -40 degrees C. William Hoch of the University of Wisconsin-Madison ranked the intensity of red coloration in autumn of species in nine genera of woody plants either from a cold zone in Canada and the northern U.S. or from a milder maritime climate in Europe. The species that produced the most intense red coloration came exclusively from the North American cold zone.
Linda Chalker-Scott of the University of Washington proposes that anthocyanins help leaves retain water. Anthocyanins dissolve in water, whereas chlorophyll and many other cell pigments do not. Water loaded with any dissolved substance has lower osmotic potential: a decreased tendency for water to flow away. Many plants produce soluble anthocyanins that may help leaves retain water when subjected to osmotic stresses from drought, salt buildup on leaf surfaces, and heat. Loading water with solutes also lowers its freezing point, possibly affording added frost protection to senescing leaves.
The evolutionary theorist W.D. Hamilton and Samuel P. Brown of the University of Montpelier speculated in a recent paper that the healthiest trees might put on the flashiest fall displays of (anthocyanin) red leaves. They further speculated that this leaf signal might give fall-feeding insects, such as aphids, a warning to avoid trees that are healthy and have the best defenses. This is an intriguing possibility for yet another role of anthocyanin in tree protection.
Leaf pigments behind the flashy autumn display of color in temperate hardwood forests are much more than cellular trash. Recognizing tree colors not only for their beauty, but also for the complex and vital roles the underlying pigments play in forest function and survival, might just bring new awe and appreciation to the autumnal rite of leaf peeping.