The color of falling fall leaves is one of the most dramatic acts of nature. Sugar maples are spectacular, turning reddish-orange and complementing the monochromatic vibrancy of the aptly named red maple, which was Thoreau’s favorite tree. In his essay “Autumnal tints” he remarks that ” By the twenty-fifth of September, the red maples generally are beginning to be ripe…. conspicuous with all the virtue and beauty of a maple – Acer rubrum. We may now read its title, or rubric, clear. Its virtues, not its sins, are as scarlet…. The whole tree thus ripening in advance of its fellows attains a singular preëminence, and sometimes maintains it for a week or two. I am thrilled at the sight of it, bearing aloft its scarlet standard for the regiment of green-clad foresters around, and I go half a mile out of my way to examine it. A single tree becomes thus the crowning beauty of some meadowy vale, and the expression of the whole surrounding forest is at once more spirited for it” and, with perhaps a touch of sarcasm “I do not see what the Puritans did at this season when the maples blaze out in scarlet. They certainly could not have worshipped in groves then. Perhaps that is why they built meeting-houses and fenced them round with horse-sheds for.”  It is hard to be dour in the kaleidoscope of autumn leaves.
The dark red oaks and crimson tupelos also stand out against the prevalent yellows of the hickories and tulip poplars that turn golden as if touched by Midas. Had there been maple trees in the Levant, the biblical rainbow covenant against another flood may well have been the painted forest. This would follow the anthropocentric view that prevailed through most of recorded history – that the reds and yellows were created to alert mankind to the onset of winter with the promise of spring’s return. But that is surely not the case. All things in nature have a reason. So why do leaves change their colors in the fall? And, specifically, why red? The fundamental mechanisms attributed in lore to the palette of Jack Frost are established botanical principles. Leaves change color in the fall because the plant senses the colder temperatures and shuts down the production of chlorophyll. When greenness abates, other colors of the leaf are revealed depending on what pigments are present for that particular plant. The yellow and orange colors come from carotenoid compounds (carotene and xanthophyll) and the red color from a flavonoid pigment called anthocyanin. Ultimately, they all turn brown due to tannin, and most of them fall off as leaf litter; some trees like white oaks and beeches retain their withered leaves all winter, a phenomenon called marcesence. 
The importance of photosynthesis that occurs within plant cells in bodies called chloroplasts cannot be overstated, as almost all living things depend on it directly or indirectly. The reaction of carbon dioxide and water that produces sugars and oxygen using the photon energy of the sun is the essential elixir of life. In chemical terms:
6CO2 + 6 H2O + 672 kilocalories => C6H12O6 (glucose) + 6O2
The chlorophyll molecules (C55H70O5N4Mg) in the chloroplasts absorb the energy of light extending from the longer wavelength infrared through the visible spectrum to the shorter wavelength ultraviolet and execute the reaction in a complex series of steps in two separate operations called photosynthesis I and II. The process is not very efficient, converting only about 3 percent of the absorbed energy into chemical energy, but that is enough for rain forests and buffalo herds. An interesting and revealing feature of the photosynthetic processes is that the atmosphere’s supply of oxygen for animal respiration comes from the water that chlorophyll electrolyzes to use the energetic electrons of hydrogen and not from the carbon dioxide that it consumes in equal measure. Another interesting point about chlorophyll is that magnesium and four nitrogen atoms framework molecular structure to which all of the other elements bond ― and why these two elements are so critical to plants. Chlorophyll absorbs light primarily at the red and violet/blue ends of the spectrum and not in the middle green wavelengths which is the reflected color we observe. Chlorophyll makes up about 20 percent of the volume of a leaf cell.
The other relevant components of the plant cell are the chromoplasts, which contain some of the yellow and orange carotenoids, and the vacuoles, which contain anthocyanin. The function of the carotenoids is not well established … they are not directly involved in photosynthesis. However, they are there for a reason, which is thought to involve protecting chlorophyll from excessively bright sunlight and indirectly supporting photosynthesis. One irrefutable fact is that they look yellow because they absorb the other wavelengths of the visible spectrum. Vacuoles are essentially cell cisterns. Plant cells start off completely filled with protoplasm containing the nucleus, chloroplasts and other organelles. As plant cells mature, vacuole chambers form that are essentially repositories for any substances created by the cell not necessary or desirable in the cytoplasm ― they can also function as support or growth expansion reservoirs. They are similarly used by fungi and animals to a lesser extent than plants. The generic name for the material occupying plant vacuoles is cell sap. The PH of the sap determines whether the anthocyanin molecules that they contain are red or blue according to relative acidity. 
A more scientific explanation of autumnal leaf senescence is a bit more complicated. Deciduous trees (those that lose leaves … evergreens are ever green) have a layer of cells at the base of each leaf called the abscission. Seasonal temperature fluctuations eventually reach a sensory limit based on tree type and habitat signaling the abscission cells grow a cork-like membrane to interrupt the flow of nutrients to the leaf. The seasonal variations of environmental influence on biological function is called phenology, the scientific field that governs the degree and timing of fall colors. The leaf, now bereft of any nutrition, begins to die. The first thing to go is the chlorophyll, as it requires a robust nutrient flow to maintain the photosynthetic factory, which is officially closed for the season. So much for the verdant hues of summer. The yellow carotenes and xanthrophylls are large molecules sharing space in the chloroplasts with the now defunct chlorophyll, and also populating the separate chromoplasts. They are more stable than chlorophyll since they are not directly involved in photosynthesis so they persist, resulting in the crown of yellow leaves that invite the sun’s brilliance to the dark of the woods. The red of anthocyanin is another matter. It is not a permanent leaf chemical constituent but must be manufactured by the plant, a matter of some complexity and energy expenditure. The classic explanation for anthocyanin is that it is produced by plants that have high sugar content. When the abscission layer forms in the fall, the sugar is trapped in the leaf and is converted to anthocyanin. Thus, when you have a dry, low H2O summer, little sugar is produced, and the fall colors are subdued. In point of fact, however, quite the opposite is true, as a hot, parched summer is likely to yield more color. Research into the phenology of fall foliage over the last several decades has upended the traditional rationale. Anthocyanin production by different plant species is a complicated phenomenon and not just a matter of sugar. 
Anthocyanin has been studied by scientists for several centuries. Originally called ‘colored cell sap,’ it is formed by the reaction between the sugar produced by the plant and proteins in the sap. It was named by the German botanist Ludwig Marquart in 1835, the Greek anthos meaning flower combined with kyanos meaning blue, it can also be red as is the case with most tree leaves (there are a few trees with bluish leaves or needles – blue spruce for example). Early research focused on the red and blue anthocyanin coloration of fruits and flowers, as the color was important in attracting seed dispersing and pollinating animals and insects to economically important agricultural products. More recently, the fundamental question as to why (some) leaves turn red, or, more broadly, why some leaves produce anthocyanin became a matter of serious investigation. There are several theories. One involves a phenomenon known as photoinhibition. Under bright light conditions, damage to photosynthetic plant tissues occurs when one part of the two-part photosynthesis (recall chlorophyll and photosynthesis I and II) process is blocked or inhibited. Anthocyanin has the property that it absorbs damaging light wavelengths of photoinhibition which are outside the wavelength range of other leaf chemicals. Anthocyanin is thus one of several strategies that an individual plant may evolve to limit the damaging effects of photoinhibition and maintain the tree’s sugar production capacity under adverse light conditions.
Anthocyanin is also an antioxidant. Intense sunlight results in the production of reactive oxygen species and free radicals (molecules with a negative charge due to having one or more free, unpaired, electrons), which react strongly with cell membranes, proteins, and DNA, the destruction of which can lead to the death of the cell. This problem is experienced by all living things whose survival is a matter of organic chemistry. Vitamin C or ascorbic acid and vitamin E are noted antioxidants, recommended as dietary supplements to reduce their deleterious effects; anthocyanin has four times the antioxidant capacity of these vitamins. This is the source of the general precept that a glass of red wine (containing the anthocyanin of the grape skin) a day is good for you, the hyperbole of the market economy driven by artificial media-driven consumer demand. Anything with colored cell sap would do just as well, like apples and plums (or apple jack and plum brandy). 
Even with the demonstrated protective capacity of anthocyanin to reduce photoinhibition damage and to neutralize free radicals, it is not clear why a tree would produce this rather large molecule (with constituents that might better be invested in food storage for the winter) just before it sheds its leaves. There are a number of other theories that have been advanced as alternative. One is that the anthocyanin is a catalyst that allows the plant to reabsorb nutrients such as nitrogen from the leaf before it falls, reinforcing the plant for its eventual emergence from the somnolence of winter in the sap rising spring. A second thesis concerns biological evolution – that the red color either acts to protect the leaf from being eaten by other animals or that it attracts selected animals to eat the leaf for propagation purposes. There is some evidence that there is a correlation between trees that are weakened and leaf color suggesting that anthocyanin may be a remedy against parasites, notably aphids. Or even that aphids recognize a weakened tree by its color and look elsewhere for promising egg-laying sites. Reds and oranges are not infrequently employed by animals as a signal of toxicity (known as aposematism) to ward off predators … red efts and monarch butterflies are good examples. There is also evidence that some tropical trees have red tips to ward off predators until they mature, at which time the leaves turn green to maximize production. Conversely, chimpanzees and monkeys in Uganda use the red coloration of leaf tips to locate the tenderest leaves. Berries are red to attract birds.
So, why do leaves turn red? They turn red because that they contain anthocyanin. Why do leaves produce anthocyanin? Not yet altogether certain on that account. Empirical evidence favors the so-called sunscreen effect, as brighter colors will always follow a late summer period of intense solar heating. There are some theories about the nature of anthocyanin production, but, if it is so beneficial to a plant, why do only some plants have it? And why aren’t more leaves red all the time? The answer is that chance in the form of random mutation propels evolutionary change. The plants that make anthocyanin survived more frequently and had more offspring in the environment where this proved to be a winning stratagem. Others did not. The climate change of the current Anthropocene Epoch is one such environmental forcing function. Increased levels of carbon dioxide are demonstrably good for plants as one of their three baseline requisites (with water and sun). This will likely delay the onset of color change as leaf life is extended.  Geographically, cool weather trees like sugar maples will migrate northward, granting Canadians exclusive rights to the maple leaf of their flag.  Plants and animals find their niche through trial and error. Chance mutations lead each organism down a circuitous path to a survivable place in the ecosystem, to eat and reproduce before being eaten. The big brain of Home sapiens is simply an evolutionary adaptation that worked perhaps too well. And that is the glory of nature. Which is why leaves turn red in the autumn … which follows summer as the earth continues on its annual orbit tilted just enough for seasonal variance.
1. Thoreau, H. “Autumnal Tints” The Atlantic Monthly October 1862. Available at https://archive.vcu.edu/english/engweb/transcendentalism/authors/thoreau/autumnal.html
2. Little, E. The Audubon Filed Guide to North American Trees Eastern Region. Knopf, New York, 1996. pp 375-411.
3. Wilson, C. and Loomis, W. Botany, Fourth Edition, Holt, Rhinehart and Winston, New York, 1967, pp 37-110.
4. Kricher, J. and Morrison, G. A Field guide to Eastern Forests of North America, Houghton Mifflin Co. Boston, 1988. Pp 6-36.
5. Lee, D. and Gould, K. “Why Leaves Turn Red,” American Scientist Volume 90, 2002 pp 524-531. A seminal article on international studies to determine what causes plants to make anthocyanin. A publication of Sigma Xi.
6. Archetti, M., “Evidence from the domestication of apple for the maintenance of autumn colours by coevolution”. Proceedings of the Biological Sciences, 22 July 2009, Volume 276 Number 1667 pp 2575-2580.
7. Hamilton, W., Brown, S. P. “Autumn tree colours as a handicap signal”. Proceedings of the Royal Society B: Biological Sciences. 22 July 2001, Volume 268 Number 1475 pp 1489–93.
8. Taylor, G. et al “Future atmospheric CO2 leads to delayed autumnal senescence”. Global Change Biology. 29 October 2007, Volume 14 Number 2 pp 264–75.
9. Long, K. “Climate change affects fall foliage” Washington Post, 20 October 2020