Why Leaves Change Colour in Autumn

Every year, forests in many parts of the world turn from green to gold, orange and red. The change feels sudden, but it is the end result of chemistry that has been under way all season. Leaves do not suddenly gain new colours so much as lose the green that was hiding the others.

Key takeaways

• Green comes from chlorophyll, which captures sunlight for photosynthesis.
• As days shorten and temperatures fall, many trees stop making chlorophyll and the green fades.
• Yellow and orange come from carotenoids that were present all along.
• Reds and purples come largely from anthocyanins, which many trees make in autumn.
• Weather, species and soil all shape how bright a season looks.

Green is a working pigment

During spring and summer, leaves are factories. Chlorophyll absorbs red and blue light and reflects green, so leaves look green. The plant uses the captured energy to turn carbon dioxide and water into sugars. Because chlorophyll breaks down in strong light, the tree keeps making more of it all season.

Why the green fades

As autumn approaches, shorter days and cooler temperatures signal deciduous trees to prepare for winter. The tree reduces chlorophyll production and begins pulling useful nutrients back from the leaves into the branches and trunk. Once the supply of fresh chlorophyll slows, the existing green pigment breaks down faster than it is replaced, and other colours become visible.

The yellows and oranges

Carotenoids are yellow, orange and brown pigments found in leaves all year, the same family of compounds that colours carrots and corn. They help capture light and protect leaf tissue. In summer they are masked by large amounts of chlorophyll. When the green recedes, carotenoids are revealed, which is why birches, aspens and many maples turn buttery yellow.

The reds and purples

Red colours work differently. Many trees produce anthocyanins in autumn, when sugars can build up in leaves as the veins narrow and sap movement slows. These pigments may help protect leaf cells while the tree recovers nutrients, though scientists are still discussing exactly why trees make them. Different species and even different individual trees produce different amounts, which explains why one maple can glow red while its neighbour stays yellow.

What makes a vivid year

Bright displays are often linked to a run of sunny days and cool, but not freezing, nights in autumn, which favour sugar build-up and anthocyanin production. A severe drought, an early hard frost or a very wet, cloudy autumn can dull colours or shorten the season. Because each year's weather is different, the same forest can look quite different from one autumn to the next.

Inside a leaf

A leaf is a thin, flat factory. Its outer layer, the epidermis, is covered by a waxy cuticle that limits water loss. On the underside of many leaves are tiny pores called stomata, each flanked by two guard cells that open and close to let carbon dioxide in and water vapour out. Inside, the mesophyll layer contains the cells with most of the chloroplasts. Veins carry water and minerals from the roots in tubes called xylem and carry sugars away in tubes called phloem. The branching vein pattern strengthens the leaf as well as supplying it.

Evergreen and deciduous

Trees have different strategies for coping with winter or dry seasons. Deciduous trees drop their leaves, which lets them avoid frost damage and water loss when roots cannot take up water from frozen ground. Evergreens keep their leaves, which are usually tougher, and can photosynthesise on mild winter days, but they must invest in leaves that survive cold. Before a leaf falls, the tree forms an abscission zone, a layer of cells at the base of the leaf stalk that weakens the connection and seals the scar.

Autumn colour around the world

Autumn colour varies with the trees. In north-eastern North America, sugar maples, red maples and oaks make spectacular displays, drawing visitors to see the foliage. In Japan, the red leaves of maples, known as momiji, are celebrated in autumn viewing traditions. In Europe, beeches, birches and field maples turn gold, copper and yellow. Aspens in the Rocky Mountains create golden hillsides. The mix of tree species, soils and weather in each region produces its own palette, and some regions, such as parts of Europe, have fewer red-turning species.

What happens to fallen leaves

Fallen leaves are not waste. They break down and return nutrients to the soil, feeding fungi, worms and countless small animals. Leaving leaves under trees and shrubs provides shelter for insects, hedgehogs and amphibians. If you need to clear lawns and paths, you can rake leaves into a pile, or compost them, since leaves rot into leaf mould, a dark, crumbly material that improves soil. Burning leaves releases smoke and is banned in many places.

Climate and timing

The timing of leaf colour and fall follows temperature and day length, and it varies each year. In many regions, studies show that spring leaf-out is happening earlier, and in some places, autumn colour is arriving later as autumns warm. Long records of leaf dates, called phenology, provide a sensitive indicator of climate change. Some citizen science projects invite people to record when trees leaf out and change colour in their own areas.

Frequently asked questions

Do leaves really "die" in autumn?

Leaf cells age and eventually form a layer where the leaf attaches to the twig, which seals the wound as the leaf falls. The tree itself stays alive and rests through winter.

Why do evergreens stay green?

Evergreens such as pines have tough, waxy needles that resist cold and drought, and they replace needles gradually over several years instead of dropping them all at once.

Do all trees change colour?

No. Colour change is most noticeable in deciduous trees in temperate regions. Many tropical trees stay green all year or shed leaves in response to dry seasons.

Why do leaves fall in autumn?

Shorter days and cooler temperatures trigger deciduous trees to form a layer at the leaf base that cuts the leaf off. This lets the tree avoid frost damage and water loss in winter, when it cannot photosynthesise efficiently.

What is leaf mould?

Leaf mould is the dark, crumbly material produced when fallen leaves decay, mainly through the action of fungi, over a year or two. It is an excellent soil conditioner and mulch, and it holds water in the soil.

How do trees know when to drop their leaves?

Trees respond mainly to the length of the day, which tells them that winter is coming, and to falling temperatures. Hormones inside the tree change in response, shutting down chlorophyll production and starting the shedding process.

Sources and further reading

• Autumn leaf color (Wikipedia)
• Smithsonian Institution