COLOUR STORY: Why do leaves change colour in Autumn?

Autumn is a special time for colour lovers. Red sunset skies, warm low lying light, cobwebs, fog and of course Autumnal foliage. Did you ever wonder about the science behind this seasonal colour show? Why do leaves change their colour in Autumn and why are some years more spectacular than others?

The first thing to understand about Autumnal colour is that leaf colour comes from pigments. These pigments are natural substances produced by leaf cells and their role is to aquire food for the trees.

The 3 pigments that colour leaves are…

Chlorophyll (green)
Carotenes (yellow and oranges)
Anthocyanins (reds and pinks)

And each pigment has a role to play…

“Chlorophyll is really important for plants to make food using sunlight. During spring and summer when there is plenty of sunlight, trees plants make a lot of chlorophyll.”

We all know that in Spring and Summer leaves are green. And you probably already know that the pigment that causes leaves to be green is called chlorophyll.

Chlorophyll is really important for plants to make food using sunlight. During spring and summer when there is plenty of sunlight, trees plants make a lot of chlorophyll.

However, in later Summer and early October autumn when the days get shorter and the weather starts to get colder, many trees and plants stop making chlorophyll. And instead of producing, they start to break down chlorophyll in the plant into smaller molecules. This is because it takes a lot of energy to make chlorophyll. If the plants break down the chlorophyll and move it out of their leaves before the leaves fall, plants save energy.

As chlorophyll is moved and stored away, the other pigments in the leaves start to show their colours. This is why leaves turn from green into yellow or red in Autumn.

“When the weather starts to get colder and days get shorter , some trees and plants stop making chlorophyll ... This dissipating chlorophyll is why leaves turn yellow or red in autumn.”

The yellow and orange pigments in leaves are called carotenoids.

The red and pink pigments are called anthocyanins. Anthocyanins also protect leaves from being eaten or getting sun burned.

And why is autumn colour more spectacular some years?

The depth of colour is influenced by the blend of chemical processes and weather conditions.

Cold nights: low temperatures destroy chlorophyll so the green leaf fades to yellow, but if temperatures stay above freezing, anthocyanin production is enhanced and the leaves take on a red colour.
Dry weather: sugars become concentrated in the leaves, more anthocyanin is produced and consequently, leaves are redder.
Bright sunny days: although the production of new chlorophyll stops in autumn, photosynthesis can still occur on sunny autumn days, using the remaining chlorophyll. Sugar concentration increases, more anthocyanin is produced and the leaves are redder

What are the benefits of leaf fall?

In the winter, the weather is cold and usually, there isn't much daylight (which helps is needed to produce chlorophyll). It would take a lot of energy and water for trees to keep their leaves healthy. In contrast, a tree without leaves is in a state of dormancy and needs less energy to remain alive.

Another benefit of shedding leaves is that trees can preserve the moisture in their branches and trunk, instead of drying out and dying.

Leafless trees are also better able to tolerate winter storms as strong winds can move through the branches more easily.

So, instead of trying to keep their leaves, many trees drop their leaves and seal the spots on their branches where the leaves had been attached.

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