Why Do Leaves Change Colour in Autumn? The Science Explained

Leaves change colour in autumn because trees stop making chlorophyll, the green pigment used for photosynthesis, as nights grow longer. As the green fades, yellow and orange pigments that were there all along are revealed, and some trees also make new red and purple pigments. The timing and brightness of the display depend on day length, temperature, rainfall and the species of tree. Here is the science behind autumn colour, explained step by step.

Why Are Leaves Green in the First Place?

Leaves are green because they are packed with chlorophyll, the pigment that captures sunlight for photosynthesis, the process plants use to turn carbon dioxide and water into sugars. Chlorophyll absorbs mostly red and blue light and reflects green light, which is why healthy summer leaves look green.

Chlorophyll is constantly broken down and replaced during the growing season. As long as a tree keeps making fresh chlorophyll, its green colour hides the other pigments in the leaf.

What Triggers the Colour Change?

The main trigger is day length. As autumn arrives, nights get longer, and many trees respond by preparing to shed their leaves for winter. According to the US Forest Service, as night length increases, chlorophyll production slows and the existing chlorophyll is eventually destroyed, unmasking the other pigments.

At the same time, the tree grows a layer of special cells, called the abscission layer, where each leaf stalk joins the twig. This layer gradually seals off the leaf, cutting the flow of water in and sugars out, and eventually allows the leaf to drop cleanly.

Which Pigments Create Autumn Colours?

Three groups of pigments are responsible for almost all the colours you see:

• Chlorophyll (green): dominates in spring and summer and breaks down in autumn.
• Carotenoids (yellow, orange and brown): the same family of pigments that colour carrots, corn and bananas. They are present in leaves all summer but hidden by chlorophyll. When the green fades, they show through.
• Anthocyanins (red, crimson and purple): the pigments that colour cranberries, red apples and blueberries. Unlike carotenoids, most are made fresh in autumn, from sugars trapped in the leaf.

Brown colours come from tannins, waste products that remain once the other pigments have broken down. They are especially noticeable in oaks.

Why Do Some Trees Turn Red and Others Yellow?

It depends on the species and its chemistry. Trees that produce lots of anthocyanins in autumn turn red or purple; those that do not show mainly the yellow and orange of carotenoids. The Forest Service lists typical colours for common North American trees:

• Red maple: brilliant scarlet
• Sugar maple: orange-red
• Black maple: glowing yellow
• Oaks: red, brown or russet
• Hickories: golden bronze
• Aspen and tulip poplar (yellow poplar): golden yellow
• Dogwood: purplish red
• Sourwood and black tupelo: crimson
• Beech: light tan

Even within a species, colours vary. A sugar maple on the sunny edge of a field may turn deep red, while one in the shade of a forest stays yellow-orange.

Why Do Trees Make Red Pigments at All?

Scientists are still debating why trees spend energy making anthocyanins just before dropping their leaves. Leading explanations include:

• Sunscreen: anthocyanins may protect the leaf from light damage while the tree withdraws valuable nutrients, especially nitrogen, before the leaf falls.
• Antifreeze and stress protection: they may help leaves cope with cold and drought.
• Signalling: some researchers have suggested bright colours may warn insects such as aphids that a tree is well defended, although this is less widely accepted.

How Does Weather Affect Autumn Colour?

Weather does not start the colour change, but it strongly affects how vivid it becomes. The best displays usually follow:

• A warm, wet spring and a summer without severe drought: healthy trees with plenty of leaves.
• Sunny autumn days: allow leaves to keep producing sugars.
• Cool but not freezing nights: slow the flow of sugars out of the leaf, which encourages anthocyanin production and brighter reds.

By contrast, an early hard frost can kill leaves before colours develop, drought can cause leaves to brown and drop early, and long spells of warm, cloudy weather can produce duller colours. The State University of New York College of Environmental Science and Forestry’s guide to autumn leaves explains these effects in more detail.

Why Don’t Evergreen Trees Change Colour?

Most conifers, such as pines, spruces and firs, keep their needles through winter. Their needles have a thick waxy coating and a small surface area, which reduces water loss and helps them survive freezing conditions. They do shed old needles, but gradually rather than all at once. A few conifers, such as larches and the bald cypress, are exceptions: they turn yellow or rusty orange and drop their needles each autumn.

Is Climate Change Affecting Autumn Colours?

Studies in North America and Europe have found that in many places autumn leaf colour and leaf fall are, on average, arriving later than in past decades as temperatures warm. Changes in drought and heat can also shorten or dull the display. Because each species responds differently, scientists expect some areas to see shifts in which trees dominate the show over the coming decades.

What Happens to Leaves After They Fall?

Fallen leaves are not waste. On the forest floor they break down into nutrient-rich soil, feed fungi and insects, and give shelter to amphibians and small mammals through winter. The Forest Service suggests putting them to use at home too:

• Mulch: shred leaves and spread them around beds and shrubs to keep in moisture and suppress weeds.
• Compost: leaves are rich in carbon and balance nitrogen-heavy kitchen scraps.
• Leaf mould: pile leaves in a quiet corner and let them rot slowly into a crumbly soil improver.
• Wildlife habitat: leave some areas of leaf litter undisturbed for pollinators such as native bees and butterflies, which rely on it to complete their life cycles.

When Do Leaves Change Colour Where You Live?

Timing depends mainly on latitude and altitude. Colour usually appears first in the far north and at high elevations, often in mid to late September, then moves south and downhill over the following weeks. In much of the northern and central US and southern Canada, peak colour typically falls between late September and late October, while southern states may not peak until early November. A dry or unusually warm autumn can shift these dates by a week or more.

Where Are the Best Places to See Autumn Leaves?

In the US, New England is famous for its sugar maples, with Vermont, New Hampshire and Maine often peaking from late September to mid-October. The Great Smoky Mountains, the Blue Ridge Parkway, Michigan’s Upper Peninsula and the Colorado Rockies, with their golden aspens, are also popular. In Canada, Quebec and Ontario offer spectacular maple displays. Many states publish weekly foliage reports during the season to help visitors time their trips.

Curious about other natural phenomena? Read our explainers on how to see the northern lights, the difference between hurricanes and tornadoes and DNA and RNA.

Autumn Leaves at a Glance

• Trigger: longer nights cause trees to stop making chlorophyll
• Yellow and orange: carotenoids, present all year but hidden
• Red and purple: anthocyanins, mostly made in autumn
• Brown: tannins left behind
• Best conditions: sunny days and cool, frost-free nights
• Why leaves fall: the abscission layer seals off and releases each leaf

Featured image: Acer saccharum Equinox Mountain Vermont, by Famartin / Wikimedia Commons, CC BY-SA 4.0. Cropped, colour-graded and combined with MediaPavilion branding.

Sources

• US Forest Service — The joy and science of fall colors (accessed 27 October 2026).
• SUNY College of Environmental Science and Forestry — Why leaves change color (accessed 27 October 2026).