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Why do leaves change colour in autumn?

The days are getting shorter and deciduous trees are turning red, orange, yellow and brown - it must be autumn! The Geeky Gardener explains the biology of changing colours.

Autumn leaves

Turning red: when the chlorophyll level in some trees has degraded down to half its usual level, the leaves start making anthocyanins to produce vibrant red colours (Source: lubilub/iStockphoto)

In the US, they call it leaf peeping, in Japan, momijigari. It's when people flock to parts of the country to see leaves change colour in autumn.

We seem to be fascinated by this familiar marker of the seasons, whether we experience it in the Northern Hemisphere, in cool-climate regions of Australia or in our own backyards.

You'll probably have a deciduous tree near you that's changing colour right now.

Chances are that it will be an introduced species that's putting on a show of yellow, orange, brown or red. (Although there are a handful of Australian deciduous natives such as the deciduous beech, most tend to change colour and shed their leaves at other times of the year, or lose leaves gradually.)

To understand autumn leaf colour we need to start with chlorophyll, the pigment that makes leaves look green most of the year.

We know that chlorophyll is critical to life on Earth because of its role in capturing sunlight to drive photosynthesis. Yet, chlorophyll is inherently unstable and degrades. So, because of its vital role, leaves make more chlorophyll to compensate. Photosynthesis continues, leaves still look green and no-one suspects a thing.

But the shortening days of autumn leads leaves to stop making chlorophyll, meaning that the degraded chlorophyll isn't replaced.

With the overwhelming green of chlorophyll no longer around, the leaves' orange-yellow carotenoid pigments are unmasked. They were there all along but we just couldn't see them.

That tells the story of orange or yellow leaves, for instance, in the large and slow-growing Ginkgo biloba or the golden ash.

But for brown autumn leaves, like those of the English oak, the process is different. Browns are down to tannins in the leaves or the polymerisation of quinones. Brown is also a marker that cells have died.

On a brighter note, for trees that turn red or purple in autumn, like the Japanese maple or claret ash, the process is different still.

When their chlorophyll has degraded down to half its usual level, their leaves start making anthocyanins, the same group of pigments that turn apples and strawberries red.

Whichever colour your nearest tree is turning, you can never guarantee that it will have the same display next year, or the same display as the one down the street or across the state. That's because environmental factors like sunlight, temperature, moisture levels and nutrient levels are all thought to affect the intensity and timing of any colour changes.

Genetic differences between species are also thought to play a part in the variety of displays.

While scientists have got a fair idea of how leaves change colour in autumn, they still don't know why.

Why would trees invest all that energy, especially to make anthocyanins, if the leaves are just about to fall off anyway? Why not fall off when leaves are green save all that hassle?

Two broad hypotheses are outlined in a paper in the journal Trends in Ecology and Evolution.

One, the photoprotection hypothesis, says that leaf pigments act as a type of sunscreen to protect leaves from light damage when it gets colder.

The other, the co-evolution hypothesis, suggests that red pigments warn insects that the leaves are low in nutrients, contain toxic chemicals and are about to drop. The trees evolved to avoid attack and the insects evolved to prefer green leaves, which is where the co-evolution part comes in.

Associate Professor Mark Hovenden, a plant scientist from the University of Tasmania, says both hypotheses may be true.

"Producing pigments that provide both photo-protection and anti-herbivory [via co-evolution] might have benefits," he says.

Why plants produce these pigments might also depend on the plant, its habitat and what its predators are, he adds.

As to which hypothesis will turn out right, he says: "We'll never really know".

But for now, it's time for a bit of leaf peeping. And if you're feeling energetic, some raking too.

More linksFact sheet: the colour of Autumn (Gardening Australia)

Try this: Autumn leaf paper (CSIRO science by email)

About the authorAnna Evangeli writes about the science of gardening at her award winning blog The Geeky Gardener. She is a former news editor at ABC Science and science correspondent for ABC Gardening Australia magazine, and has degrees in biochemistry and journalism.

Tags: biology, botany

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Published 01 April 2014