The Science of Autumn Leaves

img_2515Autumn has well and truly arrived as deciduous trees hold on to their remaining leaves of yellow, orange, brown and red, while the fallen leaves form mounds below the tree’s soon to be bare branches. The once lush green summer landscape has turned into a vision of autumn colour. But why does this happen?

It is well known that plants absorb carbon dioxide and water, meaning through the process of photosynthesis they can produce glucose (the plant’s ‘food’) and oxygen. It is the chlorophyll, which causes the green leaf colour, which helps absorb the sun’s energy allowing the process of photosynthesis to occur. However as the weather turns colder and the day length shortens less chlorophyll will be present as there is not a sufficient amount of sunlight for photosynthesis to take place. Remaining chlorophyll is broken down and the carotenoid pigment, responsible for the autumn colours of brown, yellow and orange, appears in the leaf. Although this pigment has always been present, it is no longer concealed by the green chlorophyll. On the other hand red leaves are made by the pigment anthocyanin, which is not present in the leaf throughout the year but only occurs in the autumn months.

The trees are saving energy by discarding leaves which will not survive the winter months. The detachment of plant parts by natural processes is called abscission and in trees this occurs when cell layers form between the branch and leaf stem, causing the leaves to detach and drift to the ground.

This is the time of year when you see huge piles of leaves along pavements, dogs strolling through them, children throwing them about and gardeners attempting to restore order by blowing them here there and everywhere with a leaf blower. But as days go by and summer once again appears, there are no longer leaf carpets across the United Kingdom as decomposition of the tree leaves would have occurred, helped by fungi and bacteria, allowing energy to continually cycle through the system.

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