- Leaves contain several different coloured pigments – green chlorophyll, red anthocyanin, yellow xanthophyll and orange carotenoid
- There is so much more chlorophyll compared to the other pigments, that the leaves appear green.
- Autumn’s cold weather and reduced daylight hours prompt trees to enter a state of hibernation
- Chlorophyll makes energy for the tree, but needs sunlight to function.
- It also costs the tree a lot of energy to make its chlorophyll
- So, they stop producing chlorophyll in order to save energy
- Once the there is no more chlorophyll, we see the other pigments shine!
Are you ready to learn more? Let’s go!
Why do trees have leaves?
To understand why leaves change colour, we must first understand why trees have leaves in the first place.
Leaves are basically the kitchens of plants and trees. They take the main ingredients a tree needs to live (sunlight and carbon dioxide), combines them with water and minerals (absorbed by the trees’ roots), and uses sunlight to cook them up into oxygen and sugars. These sugars can then be used as an energy source by the tree.
Why are leaves green?
In order to acquire the sunlight it needs to cook its food, trees have a special pigment (called chlorophyll) inside their leaves. Chlorophyll absorbs the sunlight and uses it as a catalyst (a helper) in this energy-making process. (This process is called photosynthesis.)
Chlorophyll is green – and there is LOTS of it in every leaf. So, most leaves look green. But, chlorophyll is not the only pigment present inside leaves. There are actually several pigments, each with their own special function. The most important of these, are chlorophyll (green), carotenoids (orange), xanthophyll (yellow), tannins (brown) and anthocyanins (red). But there is so much chlorophyll compared to the others, that it completely masks them to the point that we cannot see the other colours. That is, until the arrival of autumn.
What makes leaves change colour in autumn?
As autumn sets in, the temperatures drop and the nights draw in, reducing daylight hours. This means less cooking or food-making time for the tree, which means less photosynthesis. The amount of energy created by the chlorophyll drops significantly – even though the tree is still using up energy to make it. (This change in temperature and daylight, is called photoperiodism). It makes no sense for the tree to be wasting energy in making chlorophyll if it is not getting anything back. So, it stops making chlorophyll.
(Remember, there is more than one pigment present in tree leaves. It is just the vast quantity of chlorophyll that makes us see leaves as green. So, it makes sense that once the tree stops producing chlorophyll, we will start to see the other pigments present in the leaves.)
But even these pigments compete with each other. Remember, xanthophyll is yellow, carotenoids are orange and anthocyanins are red. So, if there are more carotenoids and xanthophyll than anthocyanins, we see the leaves as orange or yellow. But if there are more anthocyanins than carotenoids, then the leaves appear red.
How does weather affect leaf colour?
Like chlorophyll, carotenoids and anthocyanin are also dependent on temperature and daylight hours – especially anthocyanin. So, sometimes, if the season is cold and bright, red leaves become much more vibrant (because the tree is producing lots of anthocyanin). Whereas, if the season is cloudy and overcast, the leaves look duller and not as bright (because it is not producing as much).
Do all trees lose their leaves in autumn?
Although photoperiodism will affect almost all trees, not all trees lose their leaves in response to it. Conifers are called evergreens because they do not shed their leaves in the autumn (although, larch does). Whereas deciduous trees almost always shed their leaves (although, holly and holly oak are exceptions).
Also, not all deciduous trees shed their leaves at the same time. Some trees, like ash, change colour and drop their leaves really early in the season. While others, such as oak, beech, sycamore and hornbeam take time to respond and hold on to their leaves for much, much longer.
Why are autumn trees different colours?
If you look around you in the autumn, you will notice a wide array of autumn shades. Some trees are red, some are yellow, some orange and others are brown. While the colour of each tree is dependent on the ratio of its pigments, certain trees tend to turn certain colours. For example, hornbeam and maple often turn red, while field maple and ash are usually yellow. Hawthorn often turns to orange and oak, with its tannins, turns a tan-brown colour.
So, what makes leaves change their colour in the autumn? It is all down to cooler weather, shorter daylight hours and pigments! Once the autumn kicks in and the days get shorter, the trees cannot justify wasting energy to produce chlorophyll, because it cannot help the tree make energy in return. So, they stop making it, allowing all the other pigments present in the leaf to step into the dimming light and shine!
Your Autumn Nature Walk Task
So, now it is your turn! Have a good look around while you are out and about this week.
- What trees can you see?
- What colours are they turning?
- Are all trees of the same type turning the same colour?
- Are the leaves within a single tree all the same colour?
- Finally, did you enjoy this tutorial? If so, we would love 5 stars!
Have fun with your nature walk x
P.S. If you would love to learn more, you can have a look at this BBC documentary about why leaves change colour in the autumn. You can also learn how to preserve your leaves for autumn crafts in our tutorial here x