Why do leaves change colour?

In autumn the temperatures drop, and the days grow shorter. This is a signal for trees to cut back on photosynthesis – the process whereby they convert sunlight, carbon dioxide and water into glucose and oxygen. For photosynthesis, trees need the green leaf pigment chlorophyll. Ahead of the bleak and dark days of winter, trees reduce the chlorophyll in their leaves to safeguard all their valuable resources: the leaves send nitrogen, iron, manganese and other vital substances into the heart of the tree. These substances are then stored in the roots, branches and trunk until spring. This process is the reason why the yellow, orange, and red pigments in the leaves are revealed. These are what is known as beta-carotene and xanthophyll pigments. They are always in the leaves but are usually hidden by chlorophyll.

Leaves constantly evaporate water. Once winter arrives, temperatures drop and the air gets dry, trees have a hard time absorbing water through their roots. The trees therefore form a separation layer between the branches and leaf stalks, which corks and protects the trees from water loss. The leaves are not rotten or old; this is rather a natural mechanism so that trees can survive the harsh winter. The trees now await the next gust of wind, when the leaves will fall to the ground. Trees don’t shed their leaves equally quickly. Brown leaves may cling onto beech or yew trees until springtime. The reason for this are cells that clog the waterway and prevent the formation of separation layers.

We often ask ourselves in spring where the vast amounts of leaves have disappeared to. Of course, in the cities, a large portion are taken away. But what happens to the leaves that fall in the forest? Forests don’t need leaf blowers. Instead, they are able to rely on nature, as the leaves provide food for organisms living in the soil. Centipedes, woodlice, springtails, mites and earwigs feast on the leaves, nibbling through them until nothing but the veins remain. This meal passes through the intestines of the soil organisms, resulting in a product that is processed into humus by both fungi and bacteria. In turn, the humus contains a high proportion of minerals that provide the tree with important nutrients.

“Indian summer” is a familiar term, but where does it come from and what does it mean? In English, an “Indian summer” is a spell of warm, dry weather occurring in late autumn. During this kind of spell, the changing colours of the leaves on the trees are particularly intense, brightening up the forests. The origin of the term “Indian summer” is not entirely clear to this day. It probably originated on the East Coast of the United States, where warm weather in autumn is fairly common. Autumn was peak hunting season for Native Americans – the time of the year when they built up food supplies to last them through the cold, dry winter.