Why leaves change color in fall

Aerial view of wooded area with green, yellow, orange, and red trees along bright blue river.
View at EarthSky Community Photos. | Our friend Abhijit Patil captured this image via drone from northeastern Vermont on September 29, 2019. He wrote: “The brilliance is back for the season!” Thank you, Abhijit! Click in to find his forecast for autumn color in the area. Image via Abhijit Patil.

Throughout the spring and summer, the deep green color of chlorophyll, which helps plants absorb life-giving sunlight, hides any other colors present in the leaves of trees. The vivid yellows and oranges of fall leaves are there, but hidden. In the fall, trees break down the green pigments and nutrients stored in their leaves. The nutrients are shuttled into the tree’s roots for reuse in the spring. It’s then that the trees take on their autumn hues.

As leaves lose their chlorophyll, other pigments become visible to the human eye, according to Bryan A. Hanson, a professor of chemistry and biochemistry at DePauw University who studies plant pigments. Some tree leaves turn mostly brown, indicating that all pigments are gone.

Yellow, orange, and green trees under a blue sky with reflection in a calm mirror-like lake.
A 2017 photo of leaves popping with color, from Scott Kuhn in North Georgia.
Yellow trees against evergreens under cloudy sky with rough zigzag rail fence in foreground.
Autumn 2016 in the Colorado Rocky Mountains. Photo via Jessi Leigh.
Closeup of jagged edged red-yellow and green-yellow leaves with water droplets on them.
Autumn leaves at Hurricane Mountain in the Adirondacks, New York, 2014. Photo via John Holmes.
Half-green half-yellow withering leaf lying on gray weathered wood.
Autumn leaf in about mid-September 2011, from our friend Colin Chatfield in Saskatoon, Saskatchewan.
Dark red trees under blue sky with gibbous moon floating above.
Steven Arthur Sweet captured this image at Centennial Park in Toronto, Canada, in 2016.

Burgundy and red colors are a different story. Dana A. Dudle is a DePauw professor of biology who researches red pigment in plant flowers, stems and leaves. Dudle said:

The red color is actively made in leaves by bright light and cold. The crisp, cold nights in the fall combine with bright, sunny days to spur production of red in leaves – especially in sugar maple and red maple trees. Burgundy leaves often result from a combination of red pigment and chlorophyll. Autumn seasons with a lot of sunny days and cold nights will have the brightest colors.

Close-in view of red-orange maple leaves with black twigs.
2009 image via treehouse1977.

In some cases, about half of a tree’s leaves are red/orange and the other half green. Dudle says that results from micro-environmental factors – such as only half the tree being exposed to sunlight or cold.

Hardwoods in the Midwest and on the East Coast are famous for good color selections. Some of the more reliably colorful trees, Hanson notes, are liquidambar trees (also called sweetgum) that turn a variety of colors on the same tree, and sometimes the same leaf. Ash tree leaves often turn a deep burgundy color. Ginkgo trees, although not native to North America, will feature an intense yellow, almost golden, color.

A brilliant red tree against bare trees. Ground covered with red leaves.
A lone red tree against bare branches, in 2013. Photo via Daniel de Leeuw Photog.
Closeup of red maple leaves nearly hiding tree trunk in background.
Autumn in Sweden, 2013, from our friend Jörgen Norrland.

The colors are doing something for the plant, or they wouldn’t be there, said Hansen. But what is the colors’ purpose?

Scientists think that with some trees, pigments serve as a kind of sunscreen to filter out sunlight. Hanson said:

It’s an underappreciated fact that plants cannot take an infinite amount of sun. Some leaves, if they get too much sun, will get something equivalent of a sunburn. They get stressed out and die.

Fallen yellow and brown leaves lying in large puddle seen from ground level.
Image via Tosca Yemoh Zanon in London, 2013.

Another theory is that the color of a plant’s leaves is often related to the ability to warn away pests or attract insect pollinators. Hanson said:

In some cases, a plant and insect might have co-evolved. One of the more intriguing scientific theories is that the beautiful leaf colors we see today are indicative of a relationship between a plant and insects that developed millions of years ago. However, as the Earth’s climate changed over the years, the insects might have gone extinct, but the plant was able to survive for whatever reason.

Because plants evolve very slowly, we still see the colors. So leaf color is a fossil memory, something that existed for a reason millions of years ago but that serves no purpose now.

Open woodland scene with trees turning yellow and red.
Early October 2011 in Hibbing, Minnesota. Photo via EarthSky Facebook friend Rosalbina Segura.

Bottom line: Biologists discuss why leaves change color in the fall.

Read more from DePauw University

October 3, 2019

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