It's hard to imagine today, but around 7,000 years ago the whole of Central Europe was almost completely covered by mixed forests. At this time, people ate almost exclusively roots, mushrooms, fruits or animals that they caught while hunting. Only gradually did they tame sheep, pigs and cattle and switch to agriculture. Hunters and gatherers became farmers, and the first forests were cleared for fields and settlements.
As early as the end of the 13th century, wood threatened to become scarce in Germany for the first time - by 1500 the forest had already been reduced to almost its current size. Large areas were cleared for the timber needs of the growing cities and for new pastures for cattle. Industrialization in the 19th century further increased the predicament of the forests - the increased release of substances caused massive damage to the trees.
The deteriorating condition of the forests and the associated shrinking wood supplies forced people to farm more sustainably. There was a steady conversion of mixed forests into coniferous monocultures. The spruce tree in particular was ideal for building and produced three times as much timber as a beech tree in the same time. In addition, pure conifers grow very well in Germany - so the population could be built up quickly. However, the monocultures soon showed their ecological instability. Pest infestations and wind throws continue to cause massive damage to them today.
The resulting destruction of the forest not only influenced wood production, but also the original purposes of a mixed forest - from regulating the water balance to binding the greenhouse gas carbon dioxide to filtering out pollutants from the air. Many forest areas can no longer fulfill these tasks today. Due to monocultures, air pollutants and groundwater pollution have increased enormously in recent decades. Last but not least, the "forest dieback" that has been observed since the 1980s - the increasing death of trees in forest areas as a result of increasing environmental pollution - shows the urgency of converting the forests back into species-rich mixed crops.
The Federal Forest Inventory defines mixed forests as forests in which trees from at least two botanical genera occur, each having at least ten percent of the area. This means that beech forests with oaks or spruce forests with firs are also mixed forests. However, mixtures of botanical species of the same genus, such as English oak and sessile oak, are not mixed forests. When distinguishing between deciduous forest and coniferous forest, deciduous forest is considered mixed with a ten percent share of conifers, and the same applies vice versa.
Above all, species mixtures make forests more resilient. Thanks to their complementary crown and root systems, trees in mixed forests are often better supplied with light, water and soil nutrients. This makes them more resistant to environmental challenges such as drought or pest infestation.
Mixed forests also offer a variety of ecological niches for numerous animal and plant species and can therefore accommodate a wide variety of species. As Prof. Hans Pretzsch, researcher and head of the Chair of Forest Growth Science at the Technical University of Munich, explains in a press release, mixed forests are very valuable as a diverse habitat.
They are significantly more productive than forest monocultures. This is confirmed by an international overview study in which Pretzsch was a co-author. The study appeared in Science magazine in October 2016 and examines the relationship between tree species diversity in forests and their growth performance, i.e. their wood productivity. To this end, 126 case studies from around 60 areas in five continents were evaluated on the influence of mixed forests on productivity and the results were compared with data on monocultures. The results show that there is a positive relationship between tree species diversity and biomass growth. Biomass refers to all organic substances of plant or animal origin that are used as energy sources. Broadly defined, biomass is all the organic matter produced or produced by plants, animals and humans.
Forests with a high level of tree species diversity grow significantly better and more productively worldwide than monoculture forests and also have a high level of resilience. Monocultures, on the other hand, are now suffering greatly from the effects of the climate crisis, as climate change significantly influences the productivity and interaction of tree species in the forest. According to the researchers, mixed forests, on the other hand, support climate protection through increased storage of carbon.
Mixed stands will become increasingly important worldwide in view of climate change and the increasing ecological, economic and social demands on forests, says Pretzsch.
From an ecological point of view, mixed forests have a number of advantages. They are resistant to insects or fungi as well as weather conditions such as wind, snow and fire. Due to the intensive soil rooting, they not only have great stability, but can also make optimal use of the soil's nutrient and water supply. The pronounced crown space also contributes to the resilience and protection of the forest. Last but not least, the mixed cultures lead to favorable forms of humus - the form of humus itself is a main ecological feature of the landscape and an integrative indication of the ecological conditions of the site. It forms under the influence of the site climate, moisture conditions, vegetation, nutrient status and biological activity in the soil. Humus refers to the entirety of the finely decomposed organic matter in a soil - to put it simply, material that once lived in or on the soil and then died. Humus in the soil makes heavy soil looser due to its crumbly structure, so that plants can root in it better. In addition, the roots are better ventilated. Last but not least, humus can store nutrients and thus make them more available to trees and plants.
Mixed forests also bring direct benefits for us humans. They are more attractive for outdoor activities such as walks or forest bathing, provide better drinking water and provide effective protection against natural hazards, as their intensive and deep root penetration mitigates or even completely prevents landslides and other erosion processes. This is of immense advantage, especially in Alpine regions. They also provide habitat for game.
In addition to ecological and social advantages, mixed forests also bring economic benefits. As the above-mentioned study from 2016 shows, higher economic returns can be achieved with mixed forests than with wood from monocultures. More stable stocks can better buffer direct and indirect climate damage. Trees in mixed forests grow significantly faster and also offer a wider range of wood species, resulting in lower price fluctuations in the wood market.
But how can local monocultures be converted back into mixed forests? The procedure differs depending on the age of the tree population and the initial situation in general. On the one hand, a mixed forest can be created through natural regeneration. New crops are created by falling or flying seeds. Another option is pre-cultivation, where shade-tolerant tree species are planted under the existing canopy. Young plants can also be introduced in groups or planted in missing areas. Removing older trees partially creates light and space for offspring. However, the wild population should be taken into account in all planting projects. In order for reforestation to be successful, young trees must be protected from browsing by wild animals - for example with a forestry fence.
Sources: Technical University of Munich, Natur.de, The Royal Society Publishing, Federal Forest Inventory, Bayerischer Rundfunk, Spektrum, Researchgate
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