Why do Leaves Fall in Autumn?

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Tree leaves contain two main chemicals which give them their colours.  The first is called chlorophyll.  It's produced in the leaf cells through the spring and summer and makes the leaves look green.

Tree leaves contain two main chemicals which give them their colours.  The first is called chlorophyll.  It's produced in the leaf cells through the spring and summer and makes the leaves look green.

The second is a chemical called carotene which is orange, but is normally hidden by the stronger green colour of the chlorophyll.  It's the same stuff you find in carrots which gives them their orange colour.  Both chemicals help the tree to produce the food it needs by a process called photosynthesis.

There are other chemicals involved.  Xanthophyll which is yellow, and anthocyanin which is red and purple.  These are all common pigments found in fruit and vegetables such as apples, beetroot and pears.  In addition, the sugars in the leaf are converted into another chemical called anthocyanin which has a reddish colour.

The shorter days and cooler nights in autumn trigger the trees to produce a spongy layer of cells at the base of each leaf where it meets the stem.  This layer is called the 'abscission layer' and it seals off the pipe work which connects the leaf to the tree.  This stops the flow of nutrients to the leaf, and without this flow the chlorophyll breaks down and the orange of the carotene shines through.  After this, it only takes a gentle wind to blow the leaves to the ground.