Undisturbed forests benefits many specialised species

There are only a few forests in Switzerland that have been completely untouched by humans. These ‘primeval forests’ contain significantly older trees and a much higher proportion of dead wood than commercial forests, where timber is harvested regularly. But is it possible to restore primeval forest-like stands? This refers to forests where natural processes can run their course undisturbed and trees can grow to a very old age. In line with this goal, around 5% of Switzerland's forest area is to be withdrawn from use by 2030 and designated as natural forest reserves. This was agreed in 2001 by the Federal Office for the Environment and the cantonal forestry departments.

Reserves and commercial forests in comparison Between 2015 and 2023, a total of 18 selected natural forest reserves were studied in the same way. Specialised experts (see box) recorded the species diversity of organism groups that are particularly typical of forests but rarely studied: wood-dwelling fungi, beetles and bats. At the same time, characteristics of habitat quality were recorded, in particular the amount of deadwood and characteristics of the tree population. The same measurements were also taken in a neighbouring, as similar as possible forest stand that is still used for forestry purposes (commercial forest). This paired arrangement of study areas with and without abandonment of use allows the effect of abandonment of use to be assessed by directly comparing the results.

Surprisingly clear results

These comparisons show that natural forest reserves perform significantly better than neighbouring commercial forests in terms of key characteristics. The decisive factor for this assessment is the number of species that are specialised in near-natural, old-growth and deadwood-rich forests or are considered at least potentially endangered according to the Red Lists. The number of species in all three groups of organisms studied is higher in natural forest reserves than in commercial forests (see graph). The difference in the average number of species per study area ranges between 20 and 30 per cent, depending on the group and indicator. The differences are most pronounced in the case of fungi. The total number of species is also higher in natural forest reserves.

In addition, it was found that the supply of deadwood in natural forest reserves is two to three times greater than in comparable commercial forests. These differences in deadwood supply may explain at least some of the differences in species diversity, at least in the case of fungi. In the case of beetles, however, the amount of deadwood alone is a poor indicator of species diversity. Rather, the presence of old, still-living trees and the diversity of different types of deadwood seem to play a significant role for beetles. For bats, which are active over large areas, the biotope network with the surrounding forests is important. The larger the area of old-growth stands in the vicinity of a reserve, the more species were found and the higher the measured flight activity.

Commercial forests are also important

With three groups of organisms studied and 18 study areas distributed across the canton, the scope and depth of the study from Aargau is remarkable. It provides clear evidence that, in the long term, refraining from use preserves species communities that are important for nature conservation and that differ from those found in comparable commercial forests. However, the study also showed that commercial forests can be diverse and biodiverse. This is often only the case on a small scale, especially where there are areas rich in old and dead wood. However, the species diversity found there suggests that old-growth islands and biotope trees in commercial forests play an important role in the biotope network of organisms associated with dead wood. Considering the large area covered by commercial forests, the importance of near-natural elements in commercial forests for biodiversity becomes obvious.

A team of experts searched – and found!

— Hintermann & Weber AG was commissioned to develop the study concept, coordinate all work and statistically evaluate the data for the final report. Key technical work was carried out by specialist project partners:

— Fungi: Stefan Blaser and Andrin Gross (WSL Birmensdorf)

— Beetles: Ulrich Bense (D-Mössingen), Adrienne Frei (Zurich)

— Bats: Lucretia Deplazes, Fabio Bontadina, (SWILD), Andres Beck (Bat Conservation Canton Aargau)

As part of this study, the team identified no fewer than 237 species of fungi for the first time in the canton of Aargau! Among the beetle species recorded are six new finds for Switzerland and 99 first finds for the canton of Aargau!