The science behind autumn foliage colour

Trees, use a green pigment called chlorophyll to photosynthesise and produce sugars from the energy of the sun, using water and nutrients from the soil, providing the trees the energy they need to live and grow.

Leaves are at the centre of this process, exposing the largest area they can to sunlight to speed things up. Leaves also contain many other substances, some that are used in photosynthesis, and some created as by-products from it.

In autumn, trees that lose their leaves for winter go through a process to shut down photosynthesis and reclaim as many valuable chemicals as possible. The trigger for autumn colour to develop is a combination of day length and night temperature.

Chlorophyll is constantly breaking down and being replaced though summer, but the process slows in autumn. This reveals all those other chemicals in leaves that were hidden by the dominant green chlorophyll.

Some of these include flavonols, orange carotenoids and red to purple anthocyanins that vary in exact mixtures between species, resulting in varying degrees of yellow or red colour in the leaves.

As the days shorten, the amount of sugar generated by photosynthesis drops off, and hormones in the plant trigger the leaf sealing and shedding process.

As the tree becomes dormant, a compound called abscisic acid triggers a seal to develop at the base of the leaves, before they fall off.

This reduces water reaching the leaf and traps the chemicals remaining in the leaves. They gradually break down, changing the colour of each leaf before it drops to the ground. Being a gradual process, a tree will have leaves of many changing hues as autumn progresses.