Foliage discolouration is a sign that the trees are preparing for the cold season. The shorter days and cooler temperatures trigger an ageing process in the leaves whereby nutrients and chlorophyll – the green pigment responsible for photosynthesis in leaves – are broken down into their chemical components and stored in the trunk, branches, and roots.
Where do the autumn leaf colours come from?
After the green colour has disappeared from the leaves, colour pigments emerge that were hidden by the chlorophyll in spring and summer. Although these are also broken down in the autumn, this does not happen as quickly as with chlorophyll. The red pigment (anthocyanin) is produced in the leaves in autumn to protect the dormancy processes from overly intense sunlight. The brown colour only appears as the leaves begin to die.
A cork-like tissue forms between the leaf and the twig in autumn. The next strong gust of wind causes the leaves to fall to the ground, where they continue to decompose.
Each tree species has its own characteristic autumn hues. For example, larch, birch, lime and maple glow golden yellow. Beech trees display yellow, orange and brown-red hues. Reds are not as common on Switzerland's trees as they are, for example, in the oak and maple forests of the USA and Canada, famed for their spectacular displays of autumn colour. However, the intense reds do appear here on ornamental shrubs like the guelder rose, red dogwood and many others.