What is natural forest in a nutshell

How much water does the forest need?

One of these factors is the leaf mass. The more leaves a tree has, the larger its leaf surface, over which water evaporates. A spruce, for example, needs up to 3 liters of water per square meter on a beautiful summer day. Calculated over the year it is 350 to 700 liters. A beech tree evaporates only about 300 to 600 liters per square meter in the same period. The difference is due to biological factors, because deciduous trees can only transpire as long as they have leaves - usually from April to November. "Evergreen conifers, on the other hand, evaporate not inconsiderable amounts of water even in the winter months when the weather is good," says Stephan Raspe.

The trees have to replace every drop of water that evaporates in the crown with water from the ground. In addition, it develops a fabulous suction power in its water veins and fine roots. With a pulling force of 15 to 20 bar, they pull the moisture out of the soil. Any house vacuum cleaner would look pale in comparison.

The water cycle in the forest

The leaf mass of a forest also influences how much rainwater can actually penetrate to the root system. When a rain shower falls over a forest area, the drops first wet the leaves, twigs and branches of the tree canopy. The denser the canopy, the more precipitation it can hold. If the shower is only brief, this water then evaporates again without ever touching the forest floor. If, on the other hand, the rain continues, the crowns can no longer carry the amount of rain after a certain time. The water then drips onto the forest floor or runs down the trunk in small rivulets. A small part of the precipitation also evaporates on the forest floor. The remaining water - often less than half of the original amount of precipitation - seeps into the ground and fills the ground water reservoir from which the trees use their roots.

“You can imagine the forest floor like a sponge with many pores,” says Stephan Raspe. “Depending on the nature of the soil, the pores of this sponge vary in size. The finer the pores, the more the water is bound. But not everyone gives their water back to plants and trees in the end. A certain part of the water, the so-called dead water, is so firmly bound in the ground that even the enormous suction power of the trees is no longer sufficient to pull it out. "