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From the blog

The changing of the seasons


Trees have been given a new palette of colour, from vibrant reds to vivid oranges and flashes of brilliant yellows. With a sea of woolly hats, scarfs and gloves… but “hold on, hold on” I’m still in a t-shirt with the windows wide open! Ok, so it’s not autumn just yet but the changing seasons are fast approaching and before you realise it autumn has passed and winter is with us.

It’s not too long before we change the clocks. The nights are now longer than the days and remarkably trees are able to sense this loss of light due to chemical light receptors within their leaves. Trees can detect day-length changes by as little as half an hour and when the receptors are triggered the tree’s leaves begin to undergo chemical and physical changes that will produce all the vivid colours seen during the autumn season. 

So what is happening? (In a condensed version):

Chlorophyll is a green pigment that allows plants to absorb sunlight and turn it into food that can be stored for winter. While trees are in leaf they will create chlorophyll as fast as they use it up, allowing the leaves to stay green. But as the days shorten this process begins to slow down and the production of chlorophyll is reduced until, finally, it stops altogether. This is due to the fading light and decreasing temperatures, which trigger the tree’s eventual dormancy period.

As the chlorophyll draws back, other pigments begin to appear like carotenoids that will produce yellows and oranges. This pigment is present throughout the growing season, however it is masked by the stronger green pigment within the chlorophyll. Once chlorophyll has reduced the carotenoids then come into their own and give leaves a new burst of colour.

However, this is not the only pigment involved in this process. We have another called anthocyanins that produce reds and browns, not only adding colour but also helping to lower the leaf’s freezing point. This gives some vital protection from the cold, allowing the leaves to remain in place for longer, thus giving more time to absorb vital nutrients to send into storage for the winter months ahead.

The precise timing of the colour shift is genetically controlled while weather and soil moisture can affect the quality of the autumn colours. A summer drought can delay the changes in the leaves by a few weeks while a warmer spell will tone down the autumn colours.

As all of the above takes hold, head outside, take in as much colour as you can, collect those remaining apples from the trees and enjoy my favourite season: autumn.

Simon Belham is the Forest for Peterborough Project Officer.