The science behind autumn colour

by Jo Saxton October 08, 2020

During autumn, nature treats us to the most spectacular burst of colour before winter lays its icy fingers on the land. But why do leaves change colour?

Leaves are essentially nature’s ‘food factories’. Inside the leaves are cells which contain various colour pigments which help the plant to make food via photosynthesis. These pigments reflect and absorb light at different wavelengths, which makes the leaves appear certain colours. The main pigments that colour leaves are chlorophylls (green), carotenes (yellows) and anthocyanins (reds and pinks).

Autumn leaves showing a spectrum of colours

Some years, the autumn colours are more beautiful than others. This is due to a combination of weather conditions during autumn and chemical processes in the plants themselves.

For example, very cold autumn nights destroys chlorophyll and so leaves fade from green to yellow. However if temperatures are mild, anthocyanin is produced in larger amounts and so the leaves take on a more red colour.
During dry autumn weather sugars are more concentrated in the leaves and more anthocyanin is produced, resulting in redder leaves.
If we have sunny autumn days, photosynthesis can still occur and more anthocyanin is produced and the leaves are redder.

Green is usually the first pigment to fade, revealing yellow, orange and red leaves over time, and then the leaf turns the brown colour of waste by-products.

Red maple leaves

But why do some plants lose their leaves?

At the end of summer, the days get shorter which triggers a chemical response in the plant. During winter, light and water levels are often too low for plants to photosynthesise and so they shed their leaves to preserve moisture. The plant goes into a state of dormancy and so needs less energy to survive. It will live off food reserves stored from activity during the spring and summer.

A tree without leaves is often better able to tolerate severe winter storms, as the strong winds can more easily move through the branches.

Fallen leaves at the foot of a tree