Why Do Autumn Leaves Turn Red and Yellow?
– Hidden Colors and Newly Made Colors
As autumn deepens, mountains and street trees that were solid green all summer turn red and yellow. Ginkgo leaves go a bright yellow, while maples glow a fiery red. It is the same "autumn colour" phenomenon, so why does the colour differ from tree to tree?
Summer leaves all look much the same shade of green. But in autumn, ginkgo turns yellow, maple turns red, and some trees turn brownish, each species changing to its own colour.
In fact, these two kinds of colouring are thought to happen through entirely different mechanisms. Yellow leaves and red leaves are believed to owe their colour to different substances.
So what is actually going on inside a leaf?
The yellow pigment (carotenoid) is thought to be present in the leaf all summer, but hidden behind the green pigment.
The red pigment (anthocyanin) is said to be newly made inside the leaf once autumn arrives.
Let's look at this difference between a "hidden colour" and a "newly made colour" one at a time.
Why Summer Leaves Are Green: The Pigment Called Chlorophyll
Summer leaves look green because they contain a lot of a green pigment called "chlorophyll". Chlorophyll is essential for photosynthesis, and its job is to absorb sunlight. Leaves do hold other pigments too, but the green of chlorophyll is so strong that the other colours are hard to notice.
Yellow Is a "Hidden Colour": How Yellow Leaves Work
As autumn approaches and temperatures fall, trees begin getting ready to drop their leaves for winter. At this point, the chlorophyll in the leaves is thought to break down and shrink little by little. Then the yellow pigment called "carotenoid", which the chlorophyll had been concealing, starts to stand out. Carotenoid was in the leaf all summer, unchanged, so it is not newly made. Yellowing in ginkgo and similar trees is thought to be this "hidden colour comes into view" type of change.
A colour that was already there simply becomes visible as the green veil thins.
Red Is a "Newly Made Colour": How Red Leaves Work
Red autumn leaves, such as those of maples, are said to arise through an entirely different mechanism. In autumn, a wall-like tissue called the "abscission layer" starts to form at the base of the leaf, blocking the passage for water and nutrients. Because of this layer, sugars made in the leaf find it harder to flow toward the branch and build up inside the leaf, it is thought.
From the sugar that accumulates in the leaf, a red pigment called "anthocyanin" is said to be newly made once autumn has come. In other words, the red of autumn leaves is an entirely new colour that did not exist in the leaf during summer.
The leaf is going to fall anyway, so why spend energy making a red pigment? Several explanations have been proposed. One is that the pigment works like a "sunscreen", protecting the inside of the leaf from strong light. Another is that it acts as a signal to leaf-eating insects that "this leaf has few nutrients left". No clear conclusion has been reached yet, it is said.
Things You Can Check Yourself
- Pick one maple or other tree that turns red
- Compare leaves on the sunny side with leaves inside the tree or in shade
- Check which ones have turned a deeper red
- Watch the same tree every few days, and also note the order and speed at which the colour progresses
Anthocyanin is said to be made more readily where strong light hits, so sunny leaves may tend to turn a brighter red.
Summary
Red and yellow autumn leaves look alike but arise through entirely different mechanisms. Yellow comes from carotenoid, a pigment that was in the leaf all summer and becomes visible as chlorophyll breaks down. Red is thought to come from a pigment called anthocyanin, newly made in autumn as sugar builds up in the leaf behind the abscission layer.
Yellowing is a "hidden colour appears" phenomenon; reddening is a "new colour is made" phenomenon. Two entirely different kinds of chemistry are hiding in the same autumn scene.
A tree that has dropped its leaves sleeps through the winter. How that sleep ends is covered in Why Cherry Trees Bloom All at Once Every Year.
For Those Who Want to Know More – Terms, Numbers and Links to TextbooksFrom middle-school science to topics under active research, with the level of each part clearly marked
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Biology"
- High school+High-school "Biology", or material treated as advanced or as a textbook sidebar
- UniversityNot taught in high school; university-level plant ecology
- ResearchNot yet taught even at university as settled fact; researchers are still investigating it
Middle schoolTerms: Words Around Autumn Colour
- Chlorophyll: The green pigment that carries out photosynthesis.
- Carotenoid: A yellow or orange pigment. Present in summer leaves too.
- Anthocyanin: A red or purple pigment. Newly made once autumn arrives.
- Abscission layer: Tissue at the base of a leaf that blocks the passage for water and nutrients.
High schoolChecking with a Formula: A Rough Calculation of Chlorophyll Decline
Let's calculate how chlorophyll breaks down and shrinks using a simplified rule-of-thumb model (the real rate of decline varies with tree species and climate).
Chlorophyll remaining = previous amount × 0.5 (each time a set number of days passes)
| Multiply by 0.5 | Based on a rule-of-thumb model in which the amount roughly halves after each set number of days |
| After 5 days | 100 × 0.5 ＝ 50 |
| After 10 days (halved again) | 50 × 0.5 ＝ 25 |
| After 15 days (halved again) | 25 × 0.5 ＝ 12.5 |
| Result | After 15 days it falls to about one eighth (12.5%) of the starting amount |
If chlorophyll declines at about this pace, the green fades steadily and the hidden yellow stands out.
* This calculation is a simplified rule-of-thumb model for picturing how the colour change progresses. The real rate of chlorophyll breakdown is said to vary greatly with temperature, hours of sunlight and other factors.
High school+Temperature as the "Switch" That Starts the Colour Change
Temperature is said to play a large part in when autumn colour begins. In general, colouring is said to progress more readily once the daily average temperature falls below a certain threshold (roughly 8 to 10 °C). There is also a known tendency that the larger the gap between day and night temperatures, the more readily anthocyanin is made, and the more vivid the red tends to be.
UniversityHypotheses About the Ecological Meaning of Autumn Colour
In plant ecology, research is carried out on several hypotheses about what biological meaning there is in leaves turning red. Proposals include the "photoprotection hypothesis", in which the red protects leaves from oxidative stress caused by strong light, and the "signalling hypothesis", in which the leaf shows its strong defences to pests such as aphids that arrive in autumn.
ResearchWhat Is Still Unclear
- Why a tree spends energy making a red pigment, and what its biological significance is, remains a topic still debated and studied in plant ecology.
- Long-term observation and research is under way in many places on how climate change will alter when autumn colour begins and how vivid it is.
- Researchers are also looking at how genetic and environmental factors relate to why the vividness of colour differs between individual trees of the same species.
Even the autumn scenery we see every year hides a rich topic, where pigment chemistry meets ecology, and where research still continues.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: how plants function | Basic terms: chlorophyll, carotenoid, anthocyanin |
| High school | Basic Biology: breakdown of pigments | Rough calculation of chlorophyll decline |
| High school+ | Biology: responses to the environment (advanced) | How temperature and day–night gaps relate to colouring |
| University | Plant ecology | Hypotheses about the ecological meaning of autumn colour |
| Research | Plant ecology and climatology (ongoing research) | Testing the role of the red pigment; changes due to climate change |
- Explanations of the breakdown of chlorophyll and carotenoids in plant physiology textbooks and materials.
- Explanations of anthocyanin synthesis and abscission layer formation in plant ecology materials.
- Research reviews on the biological significance of autumn colour (photoprotection and signalling hypotheses) in plant ecology.
- Explanations of how temperature relates to the timing and colouring of autumn leaves in climatology materials.
- Explanations of differences in colouring between tree species in forestry and dendrology materials.
* The progress of colouring and the temperature thresholds are said to vary with tree species, region and the climate of the year.
* This article is a general-audience science explainer. When you go out to see the autumn leaves, take care on mountain paths and underfoot, and follow the rules in places where picking plants is prohibited.