Why Trees Lose Their Leaves In Winter

In autumn, the leaves of deciduous trees turn colorful and fall to the ground. What exactly is happening and why are the trees bare in winter?

We all know the sight of deciduous trees in autumn and winter: As summer comes to an end, the first leaves turn color. Soon the foliage of the trees glows in a wide variety of colors. The leaves of poplars and maples turn yellow, those of the red oak bright red – before they finally fall down. The trees spend the winter completely bare before new leaves sprout in the spring. But why does this actually happen?

Ready for winter

By shedding their leaves, trees prepare for winter. They “notice” in the fall that the days are getting shorter and the nights are getting longer. The trees begin to produce certain messenger substances, so-called hormones. These substances signal inside the tree that the green pigment chlorophyll is being drawn from the leaves. The tree stores the chlorophyll in the branches, trunk and roots.

This is also the reason for the coloration of the leaves: in summer, the green chlorophyll covers the other pigments. When it is missing, the yellow and red pigments, for example, become visible. The leaves only turn brown when they have died.

Inside the trees, even more is happening: important nutrients are shifted from the leaves to the trunk, roots and branches. The trees’ entire metabolism slows down and prepares for winter.

Without foliage, the trees save water

The fact that the leaves eventually fall off completely protects the deciduous trees from drying out in winter. This is because large amounts of water evaporate from them. In winter, however, the trees cannot absorb water as well as in summer because the ground often freezes. With leaves, however, they would still release water – and gradually dry out.

Conifers don’t go bare in the fall because their needles evaporate less water. They are surrounded by a thick wax layer and a firm skin. An exception is the larch: it sheds its needles in the fall.

Dropping the leaves also helps the trees stay healthy: With the leaves, they also get rid of the pollutants that have accumulated in them.

However, if the leaves fall from the trees while they are still green or already in summer, this is usually a bad sign. Causes can be heat waves, excessively moist soil or even the pollutants from too many car exhaust fumes.

By the way, the fallen leaves are a great fertilizer. Earthworms, snails, woodlice, beetles, spiders and millipedes break it down before bacteria and fungi convert it into nutrient-rich soil. And: piles of leaves offer many animals such as hedgehogs a resting place for the winter!