Lichen on Trees: Harmful or Harmless?


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Botanically, lichens are not plants, but a collective of fungi and algae. They populate the bark of many trees, but also rocks, rocks and barren sandy soil. The two organisms form a cohabitation, a so-called symbiosis, the two sides of the advantage: the fungus can absorb water and minerals from the soil and its environment, but do not operate without chlorophyll photosynthesis. The algae, on the other hand, are able to produce sugars through photosynthesis, but due to the lack of roots they can not reach important sources such as water and minerals. The fungus also forms the body of the lichen (Thallus), whose color spectrum ranges from white to yellow, orange, brown, green to gray. It also offers the algae protection against dehydration and mechanical damage.

Lichens are one of the longest living organisms on earth and can live for several hundred years, in some cases even several thousand years. However, they grow very slowly and have a hard time countering overgrowth with competing plants such as mosses. For some forest animals, they are an important source of protein rich in protein.

Subdivision of lichen species

Around 25,000 species of lichens in various forms are known worldwide, of which 2,000 are found in Europe. Depending on the growth form, these species are divided into three groups: leaf and foliage lichen, crust lichen and shrub lichen. The leaf lichens form a flat shape and lie loosely on the ground. Crusted lichens grow together tightly with the ground, shrub lichen have a shrubby form with fine ramifications.
Lichens colonize extreme habitats such as mountains, deserts, bogs or heathlands. In the garden they grow on stones, on walls and on roof tiles as well as on trees. Lichens are most commonly found here at base-rich tree bark. Deciduous trees such as poplar, ash and apple tree are the most populated.

Lichens on trees

Although lichens are often perceived as pests, they are not harmful to the infested trees. These are not parasites that divert important nutrients from the pathways of the bark - they only use the substratum as a habitat to grow. The symbiotic union allows lichens to meet their own needs and does not need to deprive the plant of nutrients or minerals. Also, the growth of the bark is not hindered by lichen, since it is formed in the subjacent tissue, the so-called cambium. Since lichens do not penetrate into the tree, they have no influence on the bark growth.

Duckweed on wood

Elkhorn lichen forms many small branches and thus belongs to the shrub lichen

One reason for the suspicion of lichens as supposed tree pests is that the organisms often settle on woody plants that are very old or otherwise no longer vital - a classic confusion of cause and effect. The preference of the organisms for weakened trees stems from the fact that these woody plants put less energy into the production of defensive substances, which make a bark normally unappealing by its low pH value. This favors the colonization of the bark with Aufsitzer organisms such as lichens and algae. However, there are also many lichen species that feel well on vital trees, so lichen is not always an indication of a bad condition of the infested tree. The lichens vegetation even has advantages, because the living beings protect the populated areas from other fungi and bacteria. For that reason, they should not be removed either. An exception relates to the care of older fruit trees: The loose bark with moss and lichens vegetation is removed because it offers hibernation pests such as the codling and tree lice shelter opportunities.

Pointer plants for clean air

Since lichens have no root anchored in the soil and absorb thus water and nutrients from the air, they are dependent on a good air quality. They do not have an excretory system and therefore they are very sensitive to pollutants. Thus, the organisms are important indicators of air pollutants and heavy metals. In large cities, for example, lichens are seldom found, as there is a higher level of air pollution and the air is also drier than in rural areas. Respiratory diseases also occur more frequently in places where no lichen grows. Thus, living things also indicate the health value of the air for humans. So there are plenty of reasons to protect lichens instead of fighting them lightly.

Video Board: Lichens are harmless.

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