Why do leaves change colour in autumn?

, 20 October 2021
Why do leaves change colour in autumn?
Halnaker © Emma Varley

By Charlotte Owen

WildCall Officer

With the changing of the season, we are treated to a spectacular show of autumn colour as the natural palette shifts from leafy green to a golden blaze of red and yellow. This dramatic display can be incredibly beautiful but it also serves an important function.

Deciduous trees prepare for winter by shedding their leaves, partly to conserve water - which can be in short supply when the ground is frozen - and partly to reduce the risk of being blown down or damaged by winter storms. Growth is halted and the trees enter a dormant state to ‘sleep’ through the worst of the wintry weather.

Autumn beech trees Roger Wilmshurst
The reason the leaves change colour before they fall is linked to chlorophyll production - the green pigment that’s vital for photosynthesis. Each leaf is like a solar panel, using the energy from sunlight to produce the sugars that fuel new growth, and chlorophyll levels are regularly topped up throughout the spring and summer to make the most of the growing season. But as the days shorten and there is less solar energy available, chlorophyll production slows down and eventually stops altogether. As the green fades, the leaf pigments that were previously masked by chlorophyll are gradually revealed: the yellows and oranges are produced by carotene pigments, while anthocyanins create reds, pinks and purples.

The drop in temperature also triggers structural changes within each leaf, so that a wall of cells is built across the base of each stem. This will eventually sever the leaf to send it drifting to the ground but it also blocks the usual flow of sugars out of the leaf. With nowhere to go, the excess sugar is converted into more anthocyanin pigments, producing a deeper red blush. The colours are most impressive when the weather is favourable: a combination of low (but above freezing) temperatures, dry weather and plenty of sunshine all serve to reduce chlorophyll levels and enhance the conversion of leftover sugars, producing more intense shades of red.

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