Pick a tree, then move the date from summer to late autumn (or press Play). The leaf’s color is worked out from the mix of pigments in it. Change the latitude (which sets the day length), the night temperature and the sunny days, and see how the color changes.
Season strip: the leaf’s color on every day from 1 July to 1 December, the day length (orange line) and the key dates. Tap it to jump to a date.
The slider sets the night temperature on 15 October; in the model, nights follow a seasonal curve (about 6 °C colder each month in October, summer nights between 12 and 22 °C).
Bars: relative amounts in this simplified model (100% = a full summer load of chlorophyll, or a very red leaf).
Choose the sugar maple and set the weather so that it makes as much red anthocyanin as it can. Then press Check.
Two autumns gave dull sugar maples. Load each year, play the season, and decide what went wrong. Compare with a good year.
Year A: what went wrong?
Year B: what went wrong?
Look at an aspen leaf in July, then move the date to October. Which pigment was already there in July, hidden under the green?
Move the date slider to the first day when the night is longer than the day at your latitude. (Watch the day/night bar at the top of the picture.)
Show the white pine in October. Its needles stay green while the maples change. Why?
Chromatography separates a mixture. Crush a leaf onto a pencil line on a strip of paper, stand the strip in a solvent, and the solvent soaks up the paper carrying the pigments with it. Each pigment moves a different distance, so a green leaf turns out to hold yellow and orange pigments too. The leaves here come from the Leaf Season Lab model.
Rf = distance moved by the pigment ÷ distance moved by the solvent front, both measured from the pencil line. The virtual lab uses a typical non-polar laboratory solvent mixture (for example petroleum ether with a little acetone). Its Rf values are typical / illustrative: real values depend on the paper, the solvent and the temperature, but in this kind of solvent the order (carotene, xanthophylls, chlorophyll a, chlorophyll b) is the usual one, and the water-soluble anthocyanin stays near the pencil line. Stop the run early and measure again: the Rf values stay the same.
Rubbing alcohol and acetone are polar solvents, and 70% rubbing alcohol is about 30% water, so the result differs from the virtual lab: the bands are wider and can overlap, their order can differ, and red anthocyanin (which dissolves in alcohol and water) can move partway up the strip or smear. You will still see that a green leaf holds yellow pigments.
Isopropyl alcohol and acetone are lab chemicals. They are flammable and poisonous.
Learn the parts that matter when a leaf changes color and falls. Pick Basic (6 labels) or Advanced (10), then try the blank diagram, the label quiz and match the jobs. On a phone, swipe the diagram sideways.
Simplified drawing: the cells are not to scale, and real abscission zones differ between species. In many red leaves the anthocyanin is in the cells near the upper surface (the epidermis or the palisade layer).
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back.
Each line describes the job of one part. Tap a part name, then tap the box next to the job it does. Tap a filled box to take the word back.
The cards are made from the same pigment facts the lab uses. Tap a card, then tap the pigment it belongs to. Press Check when all 15 are placed.
A worksheet with an answer key: label the leaf diagram, graph the day length at 45°N, fill in a pigment table, work out Rf values from a chromatogram of a red leaf, predict the color in three kinds of autumn weather, and explain. The numbers come from the same model as the lab.
For NGSS HS-LS1-5 (photosynthesis turns light energy into stored chemical energy): which light each pigment absorbs, what carotenoids do, what the tree gets back from a dying leaf, and why some trees make red pigment at all.
A pigment absorbs some wavelengths of light and reflects or transmits the rest; the color we see is the light that is not absorbed. Tick the pigments and move the wavelength probe.
Illustrative curves with peaks near the textbook values for pigments dissolved in a solvent: chlorophyll a about 430 and 662 nm, chlorophyll b about 453 and 642 nm, β-carotene about 425–480 nm, anthocyanin about 520–530 nm. In a living leaf the peaks are broader and shifted a little.
Chlorophylls a and b absorb blue and red light strongly and absorb little green, so a summer leaf reflects and transmits green. Carotenoids absorb blue and blue-green light (about 400–500 nm), so they look yellow or orange. Anthocyanins absorb green light, so they look red or purple. When chlorophyll breaks down in autumn, the light that it used to absorb is no longer taken away, and the colors of the remaining pigments show.
Because chlorophyll a and b and the carotenoids absorb slightly different wavelengths, together they catch more of the light that reaches the leaf. The photosynthesis page shows how that light energy is stored in sugar.
Accessory pigments. Carotenoids sit in the chloroplast membranes next to chlorophyll. They absorb light that chlorophyll absorbs poorly and pass the energy on to chlorophyll.
Photoprotection. Strong sunlight can give a leaf more energy than it can use. Carotenoids take up the extra energy and give it off safely as heat, and they protect chlorophyll from reactive forms of oxygen that would damage the leaf. Some xanthophylls are changed into other xanthophylls in bright light to do more of this.
A leaf is full of valuable nitrogen, much of it in the proteins of the chloroplasts. In autumn the tree breaks down chlorophyll and these proteins and moves the nitrogen (and other nutrients such as phosphorus) into its twigs, bark and roots, to use for new leaves in spring. This is called nutrient resorption. In one study of red-osier dogwood, about 57% of the leaf’s nitrogen was moved out before the leaves fell (Feild, Lee & Holbrook, 2001).
Chlorophyll itself is taken apart into colorless products. This has to be done carefully: loose chlorophyll in the light can make damaging forms of oxygen.
Making anthocyanin costs the tree sugar and energy, in leaves that are about to fall. Why do it? Scientists have proposed several hypotheses, and the question is not settled:
Many trees, such as aspens and birches, make little or no autumn anthocyanin, and they do fine. The answer may be different for different species.
| Pigment | Color | When is it in the leaf? |
|---|---|---|
| Chlorophyll a and b | Green | Made all summer; breaks down in autumn |
| Carotenoids (carotenes and xanthophylls) | Yellow and orange | All summer, hidden by the green; revealed in autumn |
| Anthocyanins | Red and purple | Made new in late summer and autumn by some species, from sugar in the leaf |
| Tannins | Brown | Show when the leaf cells die (as in many oaks) |
Imagine a wall painted yellow and then painted over in thick green paint. If the green slowly washed away, the yellow would show. A leaf is a little like that. All summer it is packed with green chlorophyll, which it uses to catch sunlight and make food. Under the green there are yellow and orange pigments too. In autumn the green goes, and the yellow and orange show through.
How does a tree know that autumn is coming? Mostly from the length of the night, which follows the calendar every year, while the weather can be warm one week and cold the next. When the nights get long enough, the tree stops making new chlorophyll and breaks down the old chlorophyll. Then the carotenoids show: yellow in aspens and birches. Some trees, such as red maples, also make brand-new red anthocyanins from sugar trapped in the leaf. At the base of the leaf stalk, a layer of cells, the abscission layer, forms and slowly blocks the veins. Finally the leaf breaks off, and a corky layer seals the scar on the twig.
Red anthocyanin is made from sugar. Sunny days let the leaf keep making sugar by photosynthesis while some chlorophyll is left. Cool nights (but not freezing) and the blocked veins keep that sugar in the leaf, and cool, bright conditions favor making anthocyanin. That is why sunny days with crisp, cool nights give the brightest reds. A warm, cloudy autumn gives less sugar and less red. An early hard freeze kills the leaves before they finish, so they turn brown and drop. Drought and strong winds can also end the color early. Evergreens such as pines keep most of their needles all winter; each needle lasts for a few years.
Chlorophyll absorbs blue and red light and reflects green; carotenoids absorb blue and blue-green; anthocyanins absorb green. Carotenoids are accessory pigments and protect the leaf from too much light. Before the leaf falls, the tree takes back nutrients such as nitrogen from the broken-down chloroplast proteins. Why some trees spend energy on red pigment is still debated; protecting the leaf from strong light while it is taken apart is the best-studied hypothesis. In chromatography, the pigments separate because they dissolve in the solvent and stick to the paper to different degrees.
In the Leaf Season Lab, put the sugar maple on 15 July and look at the bars: the carotenoids are already there. Play the season with 100% sun and 4 °C nights, then again with 20% sun and 14 °C nights, and compare the season strips. Then switch on the early hard freeze. In the chromatography lab, run the summer leaf and the red leaf and find which band only the red leaf has.
The timing of leaf color is part of phenology: the study of when seasonal events happen in living things.
| Area | Why leaf color matters |
|---|---|
| Forests and ecology | Autumn color and leaf fall mark the end of a tree’s growing season. The longer leaves stay green and working, the more sugar the tree can make, so foresters and ecologists track when the change happens. Fallen leaves also feed the soil when decomposers break them down. |
| Citizen science | In the USA National Phenology Network’s program Nature’s Notebook, volunteers watch the same plants through the year and report stages such as “colored leaves”. Scientists use these records to study how the timing changes from place to place and year to year. |
| Climate and timing | Scientists are studying how a warming climate shifts autumn. In many places warmer autumns delay color and leaf fall, but an earlier spring, drought or heat can make leaves change earlier. The result depends on the place and the species, so it is an active research question. Satellites that measure the “greenness” of forests help track it. |
| Fall color reports | The US Forest Service and many states post fall color reports each autumn, because visitors travel to see the colors. The reports follow the same science as this page: day length starts the change, and the weather decides how bright it is. |
| Chemistry and food | The same pigment families color many foods: anthocyanins make red cabbage, blueberries and cherries red or purple, and carotenoids make carrots and corn orange and yellow. Chromatography is a standard lab method for separating mixtures. |
Related pages: Photosynthesis, the plant cell, chromatography in chemistry, cellular respiration.
In autumn the nights get longer, and that signal tells trees to stop making chlorophyll, the green pigment used for photosynthesis. As the chlorophyll breaks down, yellow and orange carotenoids that were in the leaf all summer show through. Many trees also make new red and purple anthocyanins from sugar trapped in the leaf. At the same time, a layer of cells at the base of the leaf stalk gets ready to cut the leaf off.
Key takeaway: the green breaks down and the hidden and new pigments show.Yellow and orange come from carotenoids, which almost all leaves have. Red and purple come from anthocyanins, which only some species make, and only in autumn. Aspens, birches and hickories make little or no anthocyanin, so they turn yellow; red maples, sweetgums and dogwoods make a lot, so they turn red. Sugar maples make some, so they range from orange to red depending on the weather.
Key takeaway: yellow is always there; red is made new by some trees.Carotenoids. They come in two groups: carotenes (orange-yellow, like the beta-carotene in carrots) and xanthophylls (yellow). They help catch light and protect the leaf from too much light. They are in the leaf all summer, but the much larger amount of green chlorophyll hides them until it breaks down in autumn.
Key takeaway: carotenoids, which were there all along.Not mainly. The main trigger is the length of the night, which changes the same way every year. Temperature changes the timing and the colors: cool nights speed up the change and help make bright reds, while a hard freeze can kill the leaves before they finish changing, so they turn brown and drop. A warm autumn can delay the change.
Key takeaway: night length starts it; temperature tunes it.A wet enough growing season followed by an autumn of warm, sunny days and cool but not freezing nights. Sunny days let the leaves make sugar, and cool nights and the closing veins at the base of the leaf keep the sugar in the leaf, where it is turned into red anthocyanin. Cloudy, warm or rainy autumns give duller colors, and drought, an early hard freeze or strong winds can end the color early.
Key takeaway: sunny days and cool, not freezing, nights.In winter, broad leaves would lose water when the ground is frozen, and they could be damaged by freezing, while short days give them little light. So deciduous trees drop them. Before that, the tree takes back useful nutrients such as nitrogen. An abscission layer forms at the base of each leaf stalk; its cells weaken until wind or rain breaks the leaf off, and a protective corky layer seals the leaf scar.
Key takeaway: the abscission layer cuts the leaf off to get the tree through winter.Evergreens such as pines, spruces and firs keep their needles for more than one year. Needles are narrow and have a thick waxy coating, so they lose little water in winter, and they can photosynthesize whenever the weather allows. Evergreens do lose leaves, just not all at once: each autumn the oldest needles turn yellow and fall.
Key takeaway: their needles last for years and lose little water.A way to separate the pigments in a leaf. Leaf juice is put on a pencil line on paper, and the end of the paper is stood in a solvent. As the solvent moves up the paper, each pigment travels a different distance. In a non-polar laboratory solvent, carotene goes farthest, then xanthophylls, chlorophyll a and chlorophyll b, and the water-soluble red anthocyanin stays near the line. In class, rubbing alcohol or acetone on a coffee filter also works; these polar solvents give wider bands, the order can differ, and the red pigment can move partway up. The Rf value of a pigment is the distance it moved divided by the distance the solvent moved.
Key takeaway: it separates the pigment mix on paper.Brown comes from tannins and other compounds that are left when the leaf cells die and the other pigments break down. Many oaks turn brown or russet. Leaves also turn brown when a hard freeze or drought kills them before they finish changing color.
Key takeaway: tannins show when the leaf cells die.