Although scientists understand the chemistry behind autumn colours, the reason trees evolved them remains a lively debate.
Driving north through Duchess County, New York, in mid-October, golden yellows and fiery reds flank the road. Sunlight dapples the leaves, making the colours explode like fireworks. The vibrant display signals that colder months are approaching.
Further north in Maine, autumn colours appear more muted. “Some plants are changing early and undergoing senescence quickly because they are drought-stressed,” says Amanda Gallinat, an ecologist at Colby College in Waterville, Maine. From her window, she sees many leaves already turning brown.
Across the Atlantic, the UK experiences similar colour changes every autumn. Here, yellows dominate over reds due to native species like beech, sycamore, and oak. “The most colourful trees in the UK are usually introduced species,” says David Wilkinson, evolutionary ecologist at the University of Lincoln. Sumach, for instance, produces vivid red leaves, but it originates from the Mediterranean, Asia, and North America.
In Japan, maples transform landscapes with red, purple, and yellow shades, drawing thousands of tourists. Globally, “leaf peeping” has become a popular attraction as millions travel to photograph seasonal leaf colours.
Yet despite this attention, scientists still debate why trees undergo these transformations. Biologists understand how green chlorophyll breaks down, revealing carotenoids or forming anthocyanins. But the evolutionary purpose of vivid autumn colours remains unclear.
Understanding Leaf Colour
Leaves that turn yellow were always yellow. Carotenoids produce the colour, revealed as chlorophyll breaks down and nutrients return to the tree. Red or purple leaves form through chlorophyll loss combined with anthocyanin production, a pigment also found in fruits and vegetables.
Genetic studies suggest autumn pigments appeared late in tree evolution, after chlorophyll reabsorption had already begun. Some species produce multiple colours, and sometimes a single tree displays several shades. After changing colour, leaves fall within days or weeks.
Theories Behind Red Leaves
One theory suggests red pigments evolved as a defence against pests. Another proposes the photoprotection theory, where pigments act like a sunscreen. Anthocyanins function as antioxidants, shielding leaves from sunlight damage during senescence. This protection seems crucial in species like red osier dogwood.
Sunlight can damage leaves more during northern autumns. “Colour change is really a northern hemisphere phenomenon,” says Susanne Renner, evolutionary biologist at Washington University in St Louis. She notes that relatively few northern species change colour.
A global analysis of 2,368 temperate deciduous trees found 290 turn red and 378 turn yellow. Red colours evolved independently at least 25 times, while yellow appeared 28 times, supporting the idea that autumn pigments benefit trees.
Eastern North America and East Asia host the most red-leaf species. North America has about 89, East Asia 152, while northern Europe has only 24. Temperature alone cannot explain the pattern. Higher solar radiation, abrupt temperature changes, and shorter growing seasons may contribute to brighter reds in eastern North America.
Renner explains that sunlight triggers oxidative stress in leaves lacking chlorophyll. Anthocyanins block and refract this light, protecting cells until winter. Yellow carotenoids offer some protection, though not as strong.
Cold temperatures also increase anthocyanin production, shielding leaves from frost. Yet Wilkinson emphasizes that no single study proves photoprotection fully. Instead, multiple studies collectively support the theory.
Leaves and Insects
The co-evolution theory argues that red leaves signal insects to stay away. Aphids, locusts, and caterpillars feed on leaves, so a warning signal could reduce infestations. However, many insects cannot perceive red like humans. To them, red appears grey or black.
Some researchers propose that insects avoid red leaves because they appear dead, a concept known as the camouflage theory. Yellow leaves attract aphids by mimicking green, and some studies link leaf yellowness to higher insect colonisation.
Renner doubts the co-evolution theory, while Gallinat sees some merit. Wilkinson suggests both photoprotection and insect signalling might play roles simultaneously.
Other Theories and Human Influence
Some scientists argue that anthocyanins may help break down carbohydrates rather than protect leaves from sunlight. Humans may also influence leaf colours by selecting trees with vivid autumn shades over centuries. Urban environments with warmer temperatures could similarly affect pigment intensity.
Climate change already impacts leaf colour timing and brightness. In Maine, drought and warmer autumns produce duller, inconsistent colours. Gallinat notes that shallow-rooted trees, which often have brilliant autumn hues, are most vulnerable to these changes.
The mystery of why leaves turn red, yellow, and purple remains unsolved, but researchers continue to explore the interplay of evolution, climate, and human influence.
