Why do trees turn different colours in autumn?

Every season has its particular glory and autumn brings the annual spectacle of leaf fall from deciduous trees.  The leaf colours vary.  Some species of tree turn red, some orange, some yellow and some a mixture of all these colours.  Some trees opt out and just fade to a crinkly brown.  Some years are better than others, too.  What is it that makes a good year for autumn colour?

During spring and summer the leaves are working hard converting water and carbon dioxide into packages of energy (carbohydrates or sugars) by using sunlight as an energy source. This process is called photosynthesis.

Beneath the leaf surface are energy receptors called chloroplasts. They absorb the light energy needed to power photosynthesis and they give the leaves the natural green colour. There are also other chemicals in the leaf that are of different colours.

Chloroplasts don’t last very long and there is a continuing need for new chloroplast molecules. At the end of the summer chloroplast production slows and the tree begins to recall all the resources it can back from the leaves into the twigs. Some of the first chemicals to be drawn back are those in the chloroplasts and so, the green-ness of the leaves declines. This allows the colour of the other chemicals in the leaves to show through.

Many of the coloured chemicals we see in autumn leaves are sugars that are produced in photosynthesis. In a good year, when the summer is warm and sunny, the sugar production is high. Not all of this sugar is shipped out to other growing points on the tree and so it’s still there when the green colour fades. Temperatures at the end of the summer appear to be crucial to the leaf colour display in autumn. After a build-up of sugars in the leaf a quick change of temperature prevents further movement of chemicals into the twigs. This leaves them in the leaf to show themselves as the green chlorophyll breaks down.

The height of all this colour changing is at the end of October and early November.  The autumn winds and cold weather eventually lead to the leaves falling but it can be spectacular while it lasts.  Autumn colour is best displayed in sheltered places, away from exposed edges of the land.  Hillier’s Arboretum in Romsey is well worth a visit and gardens such as Minterne House near Dorchester and Exbury Gardens in the New Forest.

This BBC article introduces some trees with significant autumn colour appeal – https://www.gardenersworld.com/how-to/grow-plants/trees-for-autumn-colour/.

Lime tree

A cider-coloured beech at Westonbirt Arboretum

Maple leaves. On the ground you can appreciate their texture as well as the colours

A tupelo ‘aflame’ at Hillier’s Arboretum

Article written by Steve Cox on November 12, 2021