Autumn Leaves: The science behind one of the most beautiful seasons

Hi everyone!

We know that at the time this post is being published it will still be August, and the height of summer here in the Northern Hemisphere, but within the next two months or so, autumn will well and truly be upon us. So, today, we thought we could talk a bit about the science behind this beautiful season.

Thinking of autumn automatically brings to mind images of crisp mornings, cosy evenings, and of the picturesque falling leaves with their hues of red, orange, yellow and gold. The change of colour of these leaves marks a change of seasons, and the decreasing temperature and light levels that goes along with that. But why do these leaves lose their greenness and fall from the trees?

Trees' leaves contain a chemical called chlorophyll which allows them to convert water and carbon dioxide into glucose, or sugars, through a process called photosynthesis. This is how they gain energy from sunlight. Chlorophyll is green in colour, giving the summer months, when trees photosynthesise the most, that beautiful fresh, green backdrop. However, as the light levels start to fade throughout the months of September and October, the tree receives less and less sunlight in order to gain energy from. During the summer, chlorophyll naturally breaks down and is replenished, but now this process slows down, meaning the chlorophyll isn't replaced as quickly or as much. This causes the green colour to start to disappear.

The chemicals that cause the classic orange and yellow shades associated with autumn, are actually always contained within the leaves, but are hidden by the stronger green colour produced by the chlorophyll. So, as the chlorophyll breaks down, these orange and yellow colours are revealed. Interestingly, each species of tree has slightly different colouring of leaf, giving a stunning wide array of shades. Not all species will produce red leaves, and the ones that do require specific conditions, in particular, an extremely dry and sunny year.

The next stage is 'the fall'. This is when the leaves die and drop off of the branches, allowing space for new growth in the spring. But, the trees' use for them is not over. The leaves coating the ground around the tree improve the soil's drainage and ability to retain moisture. They also host a variety of species of insects and fungi which decompose them.

The ideal conditions for this process is a long 'Indian' summer, which allows the leaves to keep producing energy for the tree through photosynthesis for as long as possible, followed by a short burst of cold temperatures which speed up the changing of colours and the fall, giving us the maximum amount of yellow and orange leaves. If we have a wet autumn, the process is slowed down, meaning many of the leaves will turn completely brown before falling off.

We hope you found this post interesting and helpful, and will see you again next week with another post,

Your Green World Blog Team💚

Sources:
https://www.kew.org/read-and-watch/why-do-leaves-change-colour
https://www.forestryengland.uk/blog/why-do-leaves-change-colour-autumn

Images:
Autumn Leaves, owned by Hideo (on flickr.com), is licensed under CC BY-NC-ND 2.0.
Autmn Beech Leaves, owned by Jonas Strömtedt (on flickr.com), is licensed under CC BY 2.0.

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