The autumnal colours of our deciduous trees in the northern hemisphere are a signal that winter is fast approaching. The green colour of leaves is due to the presence of a pigment called chlorophyll, which is used for photosynthesis. Chlorophyll is a dominant green pigment that masks the rest of the colour pigments found in leaves. These include the carotenoids which give us the orange and yellows, the same pigment that colours daffodil flowers and bananas, and anthocyanins which begin to develop towards the end of summer, giving us the reds and purples, which we see in strawberries and cranberries.
Towards the end of summer and into autumn, the many veins of a leaf that are used to transport liquid to and from the leaves begin to shut down and chlorophyll production decreases. This allows the carotenoids and anthocyanins that are normally masked by the chlorophyll to show through and give us the colours that we know as autumn colour.
So why colour up? We believe for two reasons, the first being that the red anthocyanins protect the leaves from harmful rays of the Sun during cold weather and at the same time act as a warning signal for insect pests to avoid, while the tree sheds its leaves.
Tony Kirkham, Head of Arboretum, Royal Botanic Gardens Kew