We all know that Leaves are green in Summer, change to beautiful reds and oranges in Autumn, fall off in Winter, only to bud again in Spring. But most people don’t know why these seasonal phenomena occur, and it’s all down to science!

Trees undergo a process called photosynthesis, which is where they take in carbon dioxide (one of the gases we breath out), water and light, to produce oxygen (one of the gases we breath in) and glucose (sugar). This process is important for us, as it gives us oxygen, which we need to survive, though it also removes carbon dioxide from the atmosphere, reducing the effects of climate change and global warming. However, this process is mainly beneficial to the tree.

It absorbs sunlight and solar energy through chloroplasts, which are the parts of a plant cell that contain chlorophyll, a plant chemical, which gives the leaves their green colour and allows them to absorb sunlight and convert this into energy. This energy allows the plant to grow, and the more sunlight energy there is, the more a plant can grow.

In Summer, there is more sunlight, and in Winter the days are shorter and colder, so there is less sunlight for the trees to use for photosynthesis. Because of this, the chloroplasts in the leaf begin to break down to take the nutrients into more important parts of the tree like the roots or the trunk and can preserve moisture in these places. Leafless trees can also deal with Winter storms better, as the wind can move through the branches easily without the tree falling down. This process means that as the chlorophyll breaks down, the green colour begins to fade. Other hormones play roles in this too, and they also help to change the colour of the leaves to rusty oranges and reds. As the leaves lose their chloroplasts, the leaves are no longer needed by the tree, so they die and fall off. This happens during Winter, and allows the tree to make room for new buds next Spring.

It's important to note that only deciduous trees like oaks and hazels undergo this process, and coniferous trees like pines, do not.

After these deciduous trees have dropped their leaves, they do not produce oxygen and absorb carbon dioxide, though they make up for this in Summer, but aspen trees, however, can photosynthesise through their bark! This means that aspens can keep growing during Winter, whereas most trees just have to wait until Spring and Summer before they can get any taller.

So, while you’re out and about this Winter, see how many trees you can spot which's leaves have dropped, and there will likely be a huge pile of leaves surrounding them!