The Science of Autumn

It’s that time again – the nights are drawing in, the temperature is falling and the birds are beginning to migrate. As the signs of the changing seasons start to appear, as lovers of all things science-related, we couldn’t resist looking into the reasons behind one of our favourite phenomena of this time of year – the changing colours and falling of autumn leaves.

It is often thought that this is down to the change in temperature, and, although this does play a part, there are other factors which contribute to the change.

As most of us know, leaves need light to grow and, as the days get shorter, leaves have less access to sunlight. Throughout the summer, leaves produce chlorophyll almost constantly. Orange and yellow pigments are always present in a leaf, even during the summer time. However, the presence of the green chlorophyll masks these pigments, which is why the leaf appears as green. As the production of chlorophyll slows, the orange and yellow colours we associate with autumn begin to become visible.

As the days get shorter, the cells at the point where the leaf and the stem meet begin to divide rapidly and a thick layer of cells is formed. This layer – called the abscission layer – starts to block minerals and carbohydrates being transported from the tree itself to the leaves. As a result, the production of chlorophyll stops. As we move towards winter, the abscission layer begins to thicken and the cell connections weaken, causing the leaves to break off and fall to the ground.

While sunlight – or the fact that it is less available at this time of year – has the biggest effect on the leaves during Autumn, temperature and soil moisture do also play a part. If the temperature is lower than normal during Autumn, chlorophyll will be destroyed more quickly, resulting in faster changes in leaf colour. Moreover, if it has been a particularly dry summer and there is low moisture content in the soil, the abscission layer will form more quickly, causing the leaves to fall to the ground earlier.

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