Why do leaves fall from trees

Autumn leaves: why leaves turn color in autumn

It doesn't get greener in autumn. Instead, yellow and red: Deciduous trees color their leaves until they finally shed them. Why do the leaves change color in autumn? We clarify these questions

It's about life and death. Like every autumn. When oaks blush and the maple shines yellow. When a breath of wind will soon be enough to blow the leaves off the branch. A horse chestnut throws up to 25 kilograms of leaves to the ground. A birch tree even more: 28 kilograms.

Until the branches stick out bare and bare into the autumn gray sky. This is the only way that deciduous trees will survive the next few months. You need to take precautions ...

How do trees know autumn is coming?

Mainly because the temperatures are dropping and the days are getting shorter. Less light means: The tree slows down photosynthesis - the process in which it converts the carbon dioxide in the air and water into glucose and oxygen.

This is possible thanks to the green pigment in the leaves, called chlorophyll. And especially in spring and summer, when the sun sends a lot of energy-rich light towards the earth and there is enough water in the ground.

Then why are the leaves discolored?

The tree breaks down the chlorophyll and stores it in the roots, branches and trunk for the "offspring" until spring. The effect: Now the yellow, red and orange pigments come to light.

These pigments are found in the leaves, but have so far been hidden by chlorophyll: They have the complicated names carotenoids and xanthophylls. In addition, the tree now also produces anthocyanins - which make the leaves glow red when they are old. Until they finally sail to the ground.

How do the trees shed their leaves?

Ultimately, the trees cut the "aqueduct" when they pull the chlorophyll and other valuable nutrients from the leaves. They form a separating tissue between the branch and the petiole that corks. If a gust of wind rushes into the tree, the leaves fall off, at least in most species.

Beeches and oaks, on the other hand, often have brown, dried-up leaves in their branches well into spring. Instead of a separating tissue, they let cells grow that clog their waterways. Then a real storm has to tear the branches for the tree to defoliate.

What is it all for?

Quite simply: This is how deciduous trees ensure their survival. If they would also bear leaves in winter, they would dry up sooner or later. Because trees evaporate a large part of the water that the roots take up through their leaves.

In the cold season, the roots can draw less and less water from the earth. If the leaves were to "squander" this bit too, the tree would no longer have a chance. So he creates a lot of "waste".

What happens to the foliage on the ground?

Unlike in cities, leaves are found food in the forest - for millipedes, woodlice, springtails, mites and earwigs. The tiny ones go about it, nibble holes in them, bigger and bigger, until nothing remains but a fine leaf skeleton. Earthworms pull the remains into the depths, crush them and smuggle them through their intestines.

What the ground workers then squeeze out of the end of the worm, fungi and bacteria in the soil decompose into humus. Over months and years, the "garbage army" recycles the leaves into new soil on which trees grow that shed their leaves year after year.

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