The tree and its roots

About the structure of a tree

A tree is anchored in the ground by its roots. From this grows the trunk, which divides into branches and twigs in the crown. From the branches grow the leaves. Each part of the tree performs tasks that allow it to stand up to 120 meters high (!), grow and reproduce. The bark encloses the trunk and branches. The bark is divided into the outermost, protective layer – the bark – and the inner layer, the „bast“ (phloem).

Energy production – chlorophyll

The tree obtains its energy through photosynthesis. In summary, this could be described as follows: water evaporates on the leaves, sunlight and CO² enter the leaf. With the chlorophyll  the tree produces carbon hydrate compounds, its energy. As a „waste product“ the tree emits oxygen. The production of nutrients with the help of sunlight is probably the most important chemical reaction on earth, because plant substances form the basis of (almost) all life forms.

 

The sap flow

But where does the water that the tree needs to evaporate come from? The tree absorbs it with its roots (along with nutrient salts dissolved in the water) and pumps it to the leaves in the area of the young, outer wood (sapwood or xylem). The capillary effect („fine tube effect“), osmosis, and chains of water molecules trailed by evaporation all play a role. But how this really works is not yet fully understood.

The energy produced (carbohydrates) is dissolved in water. This tree sap is carried to all parts of the tree in the bast or phloem, the inner part of the bark. The bark with the bast and the sapwood are separated by a thin layer of growth tissue, the cambium.

 

In some tree species, heartwood can be seen darker colored in the middle, in some it does not differ in color. It is the oldest wood of the tree and gives strength. In it no more tree sap is conducted, the tree stores resins, oils, tannins and also waste materials here. The annual rings can also be seen.

Freshly felled silver maple: on the very outside, the relatively thin bark. The reddish bast (phloem) and the light sapwood with the delicate annual rings can be easily recognized. More noticeable than the annual rings are the saw marks.

Roots

The roots give the tree support in the ground. Often the roots spread out further than the crown of a tree (up to twice as wide). Each root ends in countless relatively hard root tips. These „bore“ into the soil as they grow. Just behind the tip are fine hairs. With them, the tree absorbs water and nutrients dissolved in it. These hairs grow quickly, but only last a few days before they die and new hairs grow back. The root itself becomes thicker as it grows, wedging itself well into the soil.

Tree species have different root types. Spruce trees, which occur naturally in high mountain areas, form shallow root plates. This allows them to thrive on steep slopes only thinly covered with humus. Mountain pines spread their roots up to 10 m above scree slopes and thus secure dangerous avalanche slopes. Beeches with their heart roots anchor themselves deep in the ground and can defy a storm. The larch and the oak have particularly deep roots, they are also supports for other species in storms.  Due to the different root types, there is a „height gradation“ of the trees in the soil, the precincts are divided up, so to speak. Overall, in a mixed forest with different root types, a greater soil thickness is strengthened and revitalized by roots.

Fungal partner

The best roots, the finest humus are of no use to a tree. To absorb nutrients, it needs „fungal partners“ and other helpers. Each root tip keeps a „fungus partner“. This fungus penetrates into the finest cavities in the soil ahead, creates there a slimy milieu, in which the tree nutrients are dissolved out of the humus. The root then slides into this prepared slime cavity. As a „reward,“ the fungus receives carbohydrates from the roots, which the tree produces by photosynthesis. Since fungi do not have chlorophyll on their own , they depend on plant partners. Trees can give up to one third of their energy to their fungal partners. The „services“ of the fungus are diverse. Unfavorable substances such as heavy metals are pre-filtered, as are some pathogens. And last but not least, fungi offer connection to the „Wood wide web“, see below.

As in human partnerships, mushroom-tree partnerships are not always equally favorable for both partners, some try to benefit from the other without reciprocation. The association of tree roots and fungi is called mycorrhizae, from the Greek fungus (mykes) and root (rhiza).

In addition to fungi, the tree uses bacteria and viruses that work with the fungi. They dissolve the nutrients from the humus. In a teaspoonful of soil (one cubic centimeter) there are about one million bacteria and 10 million viruses. The fine root filaments of the fungi (hyphae) can make up several kilometers in such a teaspoon of forest soil.

Wood wide web

The roots of neighboring trees touch each other, with them the trees can exchange information (chemically and electrically (!), but only at a speed of about one centimeter per second – brave researchers even speak of „brain-like structures“ in the root tips). More effective is the communication by means of the fine fungal threads (hyphae). A single fungus can spread over several square kilometers in the course of centuries and thus network entire forests. What is exchanged here, and how, is at best only rudimentarily understood. (For example, it is assumed that wind-pollinated trees in particular „communicate“ about the time of flowering).

In addition, trees also exchange scents that they release into the surrounding air. For example, about predators, which are countered by the release of bitter substances, so that the tree no longer tastes good to the predator. The secreted molecules could be described as the „vocabulary of plant language“, about 100,000 are known – but not their meaning. Recently, more and more research has been done to show that breathing in the molecule-enriched forest air has positive effects on human health. Keywords here are „forest bathing“ or „forestness“( forest+ wellness). Read more in „essential oils„.

Source: : Jake Page u. A. „Der Planet Erde. Wälder“, Time Life Books, 1984.
Erwin Thoma „Die geheime Sprache der Bäume“, Fischer 2017. Peter Wohlleben „Das geheime Leben der Bäume“, Ludwig 2015. Detlev Arens „Der deutsche Wald“, Vemag 2010.