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Natural forest reserves: High species diversity of insects and fungi thanks to deadwood

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When a tree dies, it comes alive. What looks dead from the outside is a valuable habitat for hundreds of species of insect, fungus and bacteria. Some of these organisms, such as the larvae of Longhorn beetles or the rare Least stag beetle (Sinodendron cylindricum), use deadwood just as a place to develop. Others tear down the supporting walls of their homes by breaking down lignin or cellulose. Insects thus not only promote the decomposition of deadwood, but also recycle nutrients.


Deadwood can be found in every forest, but it is especially frequent in the 724 natural forest reserves of Switzerland. There nature is in command. Humans do not intervene. Such forests are generally regarded as species-rich, but are these protected areas really more diverse than forests that people maintain and exploit? So far, hardly any reliable data on this topic exists in Switzerland. This is why researchers from WSL and the School of Agricultural, Forest and Food Sciences HAFL in Bern began looking into this question in 2017, focusing on deadwood colonised by fungi and insects as an example.

In a four-year project, they have so far investigated four beech forest reserves and compared them with managed beech forests. In each forest, they concentrated on 11 plots, each with two 500-m2 sub-areas containing at least one large piece of a standing or lying dead stem or rootstock. On each sample area, they collected fungal fruiting bodies and took samples from the decomposing wood to identify the fungal flora through DNA analysis. They also set up an insect trap, which they emptied six times a year, next to the largest piece of deadwood.


Rare species trapped

The beech forests in the Natural Forest Reserve Josenwald near Walenstadt in Canton St. Gallen contain almost a hundred cubic metres of deadwood per hectare and are prime examples of habitats with a wealth of deadwood. Although not all the insects caught there have yet been identified, insect expert Beat Wermelinger has already reported unusual findings: “We found a very rare Flat bug, which has only been observed twice in Switzerland.” Rare insects, such as the Stag beetle and a Rosalia longicorn, have also both been spotted.

In the beech forest reserves Sihlwald ZH, Combe Biosse NE, Tariche JU and Josenwald SG, the mycologist Stefan Blaser has so far identified 304 species of fungi, compared with only 267 in the nearby managed beech forests: “We found six highly endangered and 23 endangered species in the reserves, but only half as many in the commercial forest. This finding indicates that the living conditions for deadwood fungi are better in natural forest reserves than in commercial forests.”

The researchers are keen to find out what they will discover during the next two years when they explore species diversity in eight more forests. “The data so far shows that forest reserves containing a lot of deadwood provide an important refuge for species diversity,” says Stefan Blaser happily. (Reinhard Lässig, Diagonal 1/19)