Question: Why do leaves change colour in autumn?

Benjamin Hall answered on 20 Jun 2013:

They die, essentially.

Not all plants do it though. And not all plants that do change leaf colour/shed their leaves do it in autumn.

The ones that do shed their leaves and change their leaf colour in autumn are deciduous trees living in temeperate to cold climates. Like Oaks and Horse Chestnuts in Britain.

The reason it happens is that they simply don’t need the leaves during the cold, dark winter months. Plants have leaves to capture sunlight and carbon dioxide, which they convert into sugars to fuel their growth. By autumn, the plants have produced their fruit so simply don’t need as much sugar and keeping the leaves would cost more than they’re worth! There’s also a strong suggestion that shedding leaves recycles nutrients into the soil providing more nutrients for the seedlings the tree or plant has just dropped.

What happens is that two special layers of cells form at the base of the leaf which allows them to break off! This is the same mechanism that allows fruit to fall off as well. So some leaves change colour and turn yellow and brown becuase they’re dying.

What may also happen is that as the days get shorter and there is less sunlight, the plant just produces less chlorophyll. So the leaf begins to lose its green colour and turn yellow. Yellow attracts bugs like aphids and plants don’t want this! So some have evolved to inject pigments into the leaves which give us the beautiful reds and oranges we see in autumn leaves!