Why do trees change colour in autumn?

Some trees (deciduous trees) lose their leaves every autumn. 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.

The most efficient energy receptors in the leaves are called chloroplasts. They give the leaves the natural green colour. These sit beneath the leaf surface and absorb the light energy for the process of photosynthesis. 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 and so the leaves turn from green to red, or to orange or yellow, or possibly to muddy brown.

Many of the coloured chemicals 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 tree begins to recall the resources.

Why do trees change colour in the autumn?Why do trees change colour in the autumn?

Article written by Steve Cox on November 16, 2009