Why do leaves change colour in the autumn? It’s a simple question with a simple answer, you might say – they start to die. But there is a bit more to it than that, if you’re interested in the science, and how it can produce some of nature’s most picturesque scenery.
Every autumn the leaves from deciduous trees change colour before falling to the ground. This is due to the fact the leaves contain many chemical pigments, the most important being chlorophyll. Chlorophyll makes leaves green and helps in the process of photosynthesis, which attracts sunlight to the tree, helping them grow. Leaves also contain the chemical carotene, which has a yellow colouring. Carotene lives in the leaves all year, but is masked by the green of the chlorophyll.
The process of leaves turning from green to yellow, red or brown, is dependent on the climate. When autumn approaches and the warmer temperatures of summer begin to dip, the chlorophyll within the leaves begins to break down. Other pigments that live beneath the chlorophyll, such as the carotene, come forward.
Chlorophyll is dependent on water as well as sunshine. As the climate cools and the tree draws colder water up through its roots, the tree prepares for winter. It does this by growing a thin layer of cells over the water tubes in its leaves, closing them up in preparation. Without a regular supply of water, the green chlorophyll starts to disappear and the other colours in the leaf, such as the yellow carotene, can be seen. In some trees, when the leaf cells build, the water blocking wall which seals the tubes in the leaf’s stem traps sugar inside the leaf. This turns the sap and therefore the leaf red, or even purple.
The final part of the process before a leaf falls is when the water within the tree dries up completely. This dehydration kills any remaining green chlorophyll, as well as the yellow and red pigments. Consequently, the leaves turn brown and start to die, becoming dry and crunchy before they fall from the tree.
All in all, it’s quite a complicated and intricate process that provides us with this often beautiful time of year. When it isn’t raining at least!
Dr Kathryn Bates is a graduate of archaeology and history. She has excavated across the world as an archaeologist, and tutored medieval history at Leicester University. She joined the administrative team at Oxford Open Learning twelve years ago. Alongside her distance learning work, Dr Bates is a bestselling novelist, and an itinerant creative writing tutor for primary school children.