Why Do Leaves Change Color in the Fall?

It looks as if leaves are painted new colors every autumn. In fact, most of the yellow and orange was in the leaf all summer. It was simply hidden. The red is the one color that is usually made fresh in the fall. Understanding that difference explains almost everything about fall color, including why some years are spectacular and others are dull.

1. Three kinds of pigment

The USDA Forest Service describes three groups of pigments involved in autumn color:

Chlorophyll is the pigment that powers photosynthesis, the process plants use to turn sunlight into sugars for food. It is also why leaves look green.

Carotenoids are present in the leaf the whole growing season, alongside the chlorophyll. The National Park Service notes that one of them, carotene, is the same pigment that makes carrots orange, and another, xanthophyll, is the same pigment that makes egg yolks yellow.

Anthocyanins are different. According to the Forest Service, most of them are produced in the autumn, in response to bright light and extra sugars inside the leaf cells. They dissolve in the watery liquid of the leaf cells.

2. Why the green disappears

All summer, a leaf constantly makes chlorophyll and breaks it down, so the supply stays high and green covers up every other color.

The Forest Service explains that the timing of color change is set mainly by the increasing length of night. Temperature and rainfall vary from year to year, but the steady lengthening of nights in autumn is reliable, which makes it a dependable signal.

As nights get longer, chlorophyll production slows and then stops, and the remaining chlorophyll is destroyed. The National Park Service describes trees sending an enzyme into each leaf to take apart the chlorophyll. With the green gone, the yellows and oranges that were there all along become visible.

The process starts earlier than most people notice. The Park Service points out that in northern latitudes, day length in July is already about 30 to 45 minutes shorter than at the June solstice, and trees begin ramping down photosynthesis and pulling nitrogen back out of their leaves. By the end of September, more than half of a leaf's nitrogen has been moved into the tree's woody tissues, where the tree uses it to make compounds, such as sugars and proteins, that help protect its cells from freezing damage.

3. Where the red comes from

Red needs extra sugar trapped in the leaf, plus sunlight.

In early autumn, a layer of cells starts forming at the base of each leaf stem. The Forest Service explains that as this layer develops, the veins carrying fluid in and out of the leaf gradually close. Sugars made in the leaf during the day get stuck there, and that trapped sugar, combined with bright light, drives the production of anthocyanins.

This is why sun matters so much. The Park Service notes that sugar maple leaves can stay yellow in full shade and turn shades of red in the sun, even on the same tree.

4. Why some autumns are brighter than others

Because carotenoids are always present, yellows and golds are fairly steady from year to year. The reds are what change. According to the Forest Service:

• The best displays come from a series of warm, sunny days with cool, crisp, but not freezing nights. Sunny days make lots of sugar, and cool nights help keep it in the leaf.
• A warm spell in fall lowers the intensity of the colors.
• A late spring or a severe summer drought can delay fall color by a few weeks.

The Park Service adds two more patterns. Cloudy, rainy autumns bring fewer reds. Autumns that are too cold can produce hard frosts that rupture plant cells and make leaves drop early.

5. Why different trees turn different colors

Each species has its own mix of pigments. The Forest Service lists some typical colors:

• Oaks: red, brown, or russet
• Hickories: golden bronze
• Aspen and yellow-poplar: golden yellow
• Dogwood: purplish red
• Beech: light tan
• Sourwood and black tupelo: crimson
• Red maple: brilliant scarlet; sugar maple: orange-red; black maple: glowing yellow

Some species show very little color. Striped maple becomes almost colorless, and elm leaves often just shrivel and fall a dull brown. The Park Service adds that tannins contribute to the browns of oaks.

6. What finally makes the leaf fall

The same layer of cells that traps sugar also prepares the leaf to drop. Once that separation layer is complete and the connecting tissues are sealed off, the Forest Service explains, the leaf is ready to fall. Before that happens, the Park Service notes, the tree takes back the last nutrients it can from the leaf into its branches and trunk.

7. A changing season

The Park Service highlights one concern for the future: of the three main factors behind a good foliage season (day length, temperature, and precipitation), only day length is unaffected by a warming climate. It cites a study of 212 US locations that found fall nights warmed by an average of 2.7 degrees Fahrenheit from 1970 to 2023. Since cool nights help produce the brightest reds, warmer nights can mean less colorful foliage seasons.

8. Key takeaways

• Yellow and orange pigments (carotenoids) are in leaves all summer, hidden by green chlorophyll.
• Lengthening autumn nights trigger trees to stop making chlorophyll, which reveals the yellows and oranges.
• Most reds (anthocyanins) are made in autumn from sugar trapped in the leaf, with help from bright light.
• Warm, sunny days with cool but not freezing nights produce the brightest reds.
• A layer of cells at the base of each leaf stem traps sugars and eventually lets the leaf fall.