Each fall, forests across the world trade their summer greens for shades of yellow, orange, red, and brown.
The transformation is one of the most familiar signs of the changing seasons, but there’s more happening inside those leaves than meets the eye. Long before the first leaf reaches the ground, trees are already responding to season changes.
Which trees change color?
The colorful leaves we associate with fall primarily come from deciduous trees, which shed their leaves each year. These include maple trees, oaks, beeches, and birches as examples. As these trees prepare for winter, changes within their leaves produce the familiar colors of autumn before their leaves eventually fall.
Evergreen trees follow a different strategy. Many conifers, including pines and spruces, keep their needles for multiple years rather than shedding them all at once each fall. Their needles have adaptations that help them conserve water and withstand cold conditions, which allows the trees to remain green through winter.
What makes leaves green?
Throughout spring and summer, most leaves appear green because they contain large amounts of chlorophyll. This pigment plays an important role in photosynthesis—the process plants use to capture energy from sunlight and turn carbon dioxide and water into sugars.
During the growing season, trees continually produce chlorophyll. Its strong green color masks other pigments that are also present in the leaves.
As summer comes to an end and daylight hours decrease, trees receive a reliable signal that the seasons are changing. Chlorophyll production slows and eventually stops. The chlorophyll already in the leaves begins to break down and causes the green to gradually fade. This is when fall’s other colors begin to show.
Where do fall colors come from?
Some of the colors we associate with autumn have been inside the leaves throughout the growing season. Carotenoids, a group of pigments responsible for yellow and orange hues, are present during spring and summer but are usually overshadowed by chlorophyll. As the green disappears, these pigments become easier to see.
Red and purple leaves work a little differently. These colors come from a pigment called anthocyanin, which many plants produce as their leaves age in the fall. Their production can be influenced by factors including light, temperature, and the sugars within a leaf. The amount of anthocyanin present, along with the other pigments that remain, helps determine whether a leaf appears red, purple, orange, or somewhere in between.
As leaves continue to age, they may eventually turn brown. Unlike the bright yellows and reds created by specific pigments, browning is generally associated with the breakdown and death of leaf cells. Tannins and other compounds within the leaf can also contribute to the brown shades that become more noticeable late in the season.
Why are some fall seasons more colorful than others?
Shorter days help set the seasonal process in motion, but weather can influence the timing, intensity, and duration of autumn color. Sunny fall days paired with cool, but not freezing, nights can favor the anthocyanin in some leaves, which contributes to vivid reds and purples. Temperature and moisture throughout the growing season can also affect when colors appear and how long they last.
Other conditions can cut the display short. Drought may cause leaves to change or fall earlier than usual, while an early or hard freeze can damage leaves before they reach peak color. Strong winds and heavy rain can also bring colorful leaves to the ground sooner. Cold weather alone, then, isn’t responsible for autumn’s transformation. The conditions throughout the season help determine what the display ultimately looks like.
Why do trees lose their leaves?
The changing colors are only one part of a tree’s preparation for winter. Dropping leaves help trees conserve resources during a season when there is less sunlight available for photosynthesis, and water can be more difficult to access. As fall progresses, a specialized layer called the abscission layer develops where the leaf stem meets the branch.
This gradually limits the movement of water and nutrients between the tree and leaf. Eventually the connection weakens enough for the leaf to fall. The tree remains alive throughout winter, relying in part on energy stored during the growing season. When temperatures rise and days lengthen again in spring, new growth begins.
See the season change for yourself
Experience fall color beyond your own backyard at the Museum’s preserves. Explore these protected landscapes on a guided field trip or nature walk, where you can see seasonal changes up close and learn more about the plants, wildlife, and habitats found throughout Northeast Ohio.
