Why Leaves Change Colour in Autumn
Every autumn, forests in the cooler parts of the world seem to catch fire with colour. Leaves that were bright green all summer turn gold, orange, and deep red in just a few weeks. It looks like magic, but it is simple chemistry. Leaves are green because they contain a pigment called chlorophyll, a substance that gives something its colour. Chlorophyll captures sunlight and uses its energy to make food for the tree, a process called photosynthesis. Because a leaf makes so much chlorophyll, its strong green hides every other colour underneath. But green was never the only colour there.
As autumn approaches, days grow shorter and nights grow colder, and trees respond by shutting down. A tree cannot feed water all winter to thin, delicate leaves that could freeze. So it grows a thin wall of cells where each leaf joins the branch, slowly blocking the flow of water and nutrients into the leaf. Without fresh supplies, the leaf can no longer make new chlorophyll, and the chlorophyll already there quickly breaks down and disappears. As the green fades, other colours, hidden all along, begin to appear.
Those hidden colours come from carotenoids, the same pigments that make carrots orange. Carotenoids stay in the leaf all year, but chlorophyll is normally so much stronger that we never notice them. Once the green disappears, the yellows and oranges finally shine through, like paint that was always there under the green. Red and purple leaves work differently: their colour, called anthocyanin, is not hidden at all. It is made fresh in autumn, from sugar trapped inside the leaf once the cell wall blocks its exit. Scientists still debate why trees bother making this red pigment, but one idea is that it acts like sunscreen, protecting the leaf's last useful cells while the tree reclaims their nutrients.
This is why the brightest autumns follow warm, sunny days that keep sugar production high, followed by cool nights that trap the sugar inside the leaf. Rain, wind, or an early frost can cut a colourful season short by knocking leaves down before their pigments fully develop. Eventually every leaf falls, whatever colour it turned, but by then the tree has already pulled back most of the nutrients it can reuse, leaving an empty, dry leaf behind. The bright display we enjoy each autumn is really the last stage of a tree quietly preparing for winter.
Vocabulary
- pigment
- a natural substance that gives something its colour — Red pepper skin contains a pigment that turns from green to red as it ripens.
- chlorophyll
- the green substance in a plant that helps it use sunlight to make food — Plants that grow in the dark cannot make enough chlorophyll, so their leaves stay pale.
- photosynthesis
- the way a plant uses sunlight, water, and air to make its own food — During photosynthesis, a plant turns sunlight into the energy it needs to grow.
- delicate
- thin and easily damaged — The butterfly's wings were so delicate that a light touch could tear them.
- nutrients
- the substances in food or soil that a living thing needs to grow and stay healthy — Fruit and vegetables are full of nutrients that keep your body healthy.
- breaks down
- to change into simpler parts and stop working — Old newspaper left outside breaks down quickly in the rain.
- fades
- to slowly become weaker or less bright until it disappears — The colour in her favourite shirt fades a little more every time she washes it.
- carotenoids
- natural yellow and orange substances found in some plants, such as carrots — Sweet potatoes get their bright orange colour from carotenoids.
- anthocyanin
- a red or purple substance that a plant makes, especially in its leaves or fruit — The deep purple skin of a blueberry comes from anthocyanin.
- shine through
- to become clearly visible after something that was covering it is gone — Even under the thick clouds, a little sunlight began to shine through.
- reclaims
- to take something back so that it can be used again — The company reclaims unused office equipment at the end of each year.
- frost
- a thin white layer of ice that forms outside on very cold nights — A light frost covered the grass on the cold October morning.