Why do leaves change color in the fall?
- Meghan McMahon
- Oct 10
- 3 min read
What’s your favorite part of fall? Picking apples? Carving pumpkins? Trick-or-treating? Those are all fun fall activities, and watching as the leaves change color each autumn is another one of the hallmarks of the season.
The yellows and oranges and reds and purples and browns that paint our forests each fall are the result of pigments. Pigment is also why the leaves start off as green. They are green in the spring and summer because of the presence of a pigment called chlorophyll. Chlorophyll is what makes the leaves green, and it also helps the trees make energy through photosynthesis.
Have you heard of photosynthesis? It’s how plants make energy. They use carbon dioxide, water and sunlight to make energy in the form of glucose and oxygen. Chlorophyll is important in photosynthesis because it absorbs the energy from the sun.
As summer continues on, the days begin to get shorter as the sun rises later and sets earlier. The angle of the sun also changes as we move closer toward fall. The leaves on our trees are sensitive to these changes.
Less sunlight signals the leaves to stop producing chlorophyll. When the chlorophyll begins to break down in the leaves without being replaced, the green color begins to fade away. When that happens, it allows the colors of other pigments to become visible.
Words to know
Hallmark: To designate as distinctive.
Pigment: The natural coloring matter of plant or animal tissue.
Variance: The fact or quality of being different or inconsistent.
Vibrancy: Striking brightness of color.
The various leaf colors we see in fall are created by these different pigments. Shades of yellow are created by pigments called xanthophylls, while shades of orange come from pigments called carotenoids and reds and purples are from anthocyanins.
Each tree produces the same color leaves each fall, but the vibrancy can vary from year to year based on the weather in late summer and early fall. The brightest displays happen in years with stretches of warm, sunny days with cool but not freezing nights. These are the optimum conditions because the sunny days allow the leaves to produce sugars and the cool nights prevent the sugars from leaving the leaves.
This weather pattern can make red and purple leaves appear brighter because anthocyanins — the pigments that create red and purple leaves — are most affected by sunlight and sugar production. There is less variance from year to year in the appearance of yellow and orange leaves because the pigments causing those colors are always present.
Other weather and environmental factors, such as rainfall, temperature and even food supply, can affect how leaves change color. However, none of these factors is nearly as important as the decreasing amount of sunlight.
Want to know what colors you can expect to see in your neighborhood? Oak leaves turn red or brown, while hickory leaves will turn a shade of golden bronze. You'll see yellow leaves on aspen, birch and poplar trees, and beech trees typically have tan-colored leaves in fall. The most color variation occurs among different maple tree species. Red maples have bright red leaves, while sugar maple leaves are an orangish-red color and black maple leaves turn yellow.
Once fall color fully sets in, it doesn't stick around for too long. After a few weeks, the leaves will begin to fall from the trees. Leaves are too delicate to survive winter, and if they remained on the trees it would make the trees themselves more vulnerable to damage from winter weather.
To avoid this, the trees will create a seal between the leaves and their branches. Once the seal is formed, the leaves no longer receive fluid from the tree. After they are cut off from fluid, they eventually fall from the tree, leaving the tree bare until spring returns and the process begins all over again.
