Why Do Leaves Change Colour in the Fall?
Fall is a favorite season for many because of how beautiful the trees are. Before the leaves fall, they transform from green into a variety of yellows, oranges, and reds. Those colours are often used for fall decorations during Halloween and Thanksgiving.
While you’re admiring the trees, have you ever stopped to wonder why the leaves change colour? How can green leaves become so many different hues? Let’s explore the colours of leaves and how their colour-changing process works.
What Colours Can Leaves Be?
Most leaves start out as green in the summer and then shift to yellow, orange, red, or brown in the fall. Here’s why the leaves might appear as those colours.
Leaves look green because of the chlorophyll inside them. Chlorophyll is used for photosynthesis to absorb energy from the sunlight. The plant stores the energy and later converts it to carbon dioxide and glucose with the help of the oxygen and water the plant takes in. Without chlorophyll, plants can’t properly feed themselves.
So, healthy plants in the summer look green, but they start to change colour when the conditions around them shift. That could lead to fall colors and then the colours of dead leaves.
Chlorophyll isn’t the only type of pigment in leaves. They also contain other pigments known as carotenoids, flavonoids, and anthocyanins, which represent other colours. Different species of trees contain different pigments.
Xanthophylls (a subclass of carotenoids) and flavonoids are items always present in some leaves. They can give the leaf a yellow pigment when there’s less chlorophyll. An example of xanthophylls is lutein, which gives egg yolks their yellow coloring.
Carotenoids can also make leaves appear orange when there’s less chlorophyll. Beta-carotene is a type of carotenoid that absorbs green and blue colours and reflects red and yellow colours, appearing orange as a result. It’s also the pigment that makes carrots orange.
Some carotenoids make leaves look red, but anthocyanins can help with that too. Anthocyanins are found in the skins of grapes and apples, giving the fruits’ exteriors dark colours. Those pigments are caused by a buildup of sugars. They can reduce the risk of leaves getting damaged by the sun or eaten by animals.
When a leaf dies, it turns brown. Even after leaves fall, they may remain one of the above colours as they cling to life, but they’ll eventually turn brown and crunchy. Dead leaves are always this color because they can’t make pigments anymore.
Why Do Leaves Change Colour in Autumn?
During warm months, the chlorophyll that makes the leaves green absorbs energy from the sun to feed the trees. The energy turns into sugar, which acts as food for the tree. When the seasons change, the temperature cools down and the days get shorter, meaning less sunlight for the plants.
The conditions in fall alert the trees to stop making chlorophyll to prepare for winter. With less chlorophyll, the leaves stop appearing green, and instead, the other pigments present in the leaves become more prominent. When the leaves change colour, it means they’re no longer producing food for the tree. The chlorophyll breaks down and fades as it gets deeper into fall.
Other pigments like carotenoids are always present in leaves, but chlorophyll covers them. Chlorophyll disappearing usually results in yellow, orange, or light red leaves, but dark red leaves are caused by a unique chemical change. The anthocyanins that cause the dark red or purple hue appear when sugars get trapped in the leaves, which is more common in certain trees, such as oaks and dogwoods.
When the weather gets to a certain point, trees will cut off the leaves so fluid and nutrients no longer reach them. Then, the leaves usually change from green to another colour and a seal forms between the tree and the leaf’s stem.
The seal helps the tree not expend as much energy in the winter so it can survive. Roots and branches of trees can put up with cold temperatures, but leaves aren’t as adaptable. Leaves will freeze in the winter, so losing them is part of the tree’s survival.
Once the leaves stop receiving enough support from the tree, they’ll drop to the ground. The whole process is known as “leaf abscission.” On the ground, they’ll shrivel up and die over time, becoming brown.
Conditions Affect Colour-Changing
The location, temperature, light, and water supply of the tree play a factor in what colour the leaves will be and when they’ll change. For example, the most colorful trees tend to exist in dry, sunny, or cool areas during autumn. Cloudy, damp, or warm environments might not change color as much.
The most beautiful autumn leaves appear when the following conditions are met:
- Spring is warm and wet
- Summer isn’t too hot or dry
- Autumn has warm, sunny days and cool nights
In areas with low temperatures just above freezing, dark red leaves are more common. Yet, if early frost hits, the red pigment will be weakened and may turn brown sooner. Severe frost kills leaves and makes them fall from trees sooner.
Trees close to artificial lighting like street lamps tend to change colour later in the year than trees far from civilization. It’s likely because artificial lights give them the illusion that the day is longer than it actually is.
How Do the Leaves Grow Back?
Luckily, plants can absorb the molecules that makeup chlorophyll when the conditions are ideal again. When temperatures warm up in the spring and the sun appears more often, trees will create new chlorophyll so they can regrow green leaves.
Trees use the molecules inside them to make chlorophyll in the spring so they don’t need to create it from scratch. Then, once the leaves grow back, the cycle begins all over again for as long as the tree stays alive.
Do All Leaves Change Colour?
Not all leaves change color and fall as the seasons change. Only deciduous trees lose their leaves in the winter. Coniferous trees and evergreens stay green year-round because they can continue their photosynthesis process through all seasons. Many of them have needles instead of leaves.
Here are a few trees that stay green all year:
- Pine trees
- Fir trees
- Spruce trees
- Cedar trees
- Palm trees
Leaf Colour Meanings
Like all colorful things, there’s some symbolism behind the famous fall colors we see in the trees. If you’re hoping to decorate or make art with these leaf colours, here are some meanings you can incorporate into the design.
Green Leaf Colour Meaning
Green is the colour of growth, harmony, and health because many things in the environment are green. In nature, green leaves are alive and healthy, so people who see them might feel revitalized, relaxed, and encouraged. Some people see green as a sign of envy or judgment, but it’s more commonly seen as a positive, hopeful symbol.
Yellow Leaf Colour Meaning
Even though yellow leaves mean summer is over, yellow is an overall optimistic color. It can mean happiness, positivity, and intellect. Seeing yellow leaves might inspire, amuse, or energize you. Yellow gives off feelings of warmth and creativity, but it can also be associated with caution and cowardice.
Orange Leaf Colour Meaning
Orange is a vibrant color that stands out among the sea of yellow leaves in fall. It’s commonly associated with youth, optimism, and enthusiasm. It encourages and uplifts those who look at it. People who love the fall season might see it as a sign of warmth, positivity, and creativity, while those dreading cold weather might think of it as superficial or impatient.
Red Leaf Colour Meaning
Red is a bold color that only certain leaves display. It can mean strength, energy, and passion. It can catch someone’s attention, motivate them, or make them proceed with caution. It has a mix of both positive and negative meanings, including everything from courage, desire, and confidence to anger, danger, and revenge.
Brown Leaf Colour Meaning
Lots of things are brown in nature, but when it comes to plants, brown usually means they’re dying or dead. In general, brown is a symbol of stability, honesty, and comfort. It can simplify things or make people feel grounded and protected. When it comes to leaf colours, people are more likely to associate it with negative aspects, like being dull and predictable, because they’re unhappy about winter approaching. Yet, people looking on the bright side might associate it with appreciation, support, and wisdom instead.
Enjoying the Beauty of Fall
Fall is a beautiful time of year, full of reds, yellows, and oranges. Walking through wooded areas as the leaves fall around you is one of the most magical experiences, which is why many people favor autumn above other seasons.
Now, whenever you see the leaves changing colours and landing on the ground, you’ll know the reason. The trees are making a change to welcome a new season, so even though it’s getting colder, it’s an exciting encounter. Enjoy the colours while they last because autumn goes by fast.