Nature on our doorsteps: Why do trees change colour in autumn?

Rosaleen Dwyer is the County Heritage Officer at South Dublin County Council. Each week she writes about the natural heritage and beautiful biodiversity on our doorstep. 

While the beautiful colours of autumn brighten our days in October and early November, for trees there is a practical reason behind this colour change.

The beautiful reds of these Maples brighten up a carpark in Tallaght compressor

The beautiful reds of these maples brighten up a carpark in Tallaght 

During spring and summer, leaves contain a chemical called chlorophyll which acts to capture energy from the sun.

With this energy, the tree converts water and carbon dioxide from the air into food for itself, in the form of sugars and starch.

In autumn the days shorten and air temperatures drop, causing food production in the leaves to slow down. This is the signal for trees to prepare for winter.

Chlorophyll is a green-coloured pigment. Yellow and orange pigments also occur in leaves which similarly absorb sunlight.

This line of yellow ash trees is stunning along the Firhouse Road compressor

This line of yellow ash trees is stunning along the Firhouse Road 

Chlorophyll, however, is usually the most abundant, masking the other colours for most of the year.

In autumn, chlorophyll breaks down. This releases nitrogen and other nutrients which are recycled and stored in the main body of the tree over winter, ready for use in new growth in springtime.

As green chlorophyll breaks down and is no longer the dominant colour, the glory of the yellows and oranges of the other pigments is finally revealed.

Different tree species have different proportions of coloured pigments.

That is why we get such a wide range of browns, golds, reds, oranges and yellows in the leaves that finally fall.

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