Storms, like Vivian, Lothar or Kyrill, are the most frequent natural disruptive factors in European forests and a driving force of forest dynamics. Windthrows alter habitats by opening up previously shaded sites, thereby changing the local climate and vegetation. Furthermore, storms produce large quantities of dead wood, on which about a quarter of all forest-dwelling organisms depend.
Researchers of the Swiss Federal Research Institute WSL investigated insect biodiversity in three forest areas destroyed by the storm Lothar in 1999: a beech forest in Sarmenstorf (canton of Aargau), a spruce forest in Messen (canton of Solothurn) and a mixed forest in Habsburg (canton of Aargau). Over two summers there, they used flight interception and pitfall traps (fig. 1) to collect insects and then identified the caught species back in their laboratory. Control areas were surveyed in undamaged neighbouring woodland.
Fig. 1 - Using flight interception and pitfall traps, the researchers followed the development of insect fauna in areas hit by storm Lothar and in the neighbouring forest. Photo: Beat Wermelinger (WSL)
Usually, forest owners salvage stem wood after storms, to sell it. However, this affects natural forest development, for example because it removes the dead wood vital for insects. So the researchers determined insect diversity within the three aforementioned windthrow sites, both in areas cleared of uprooted trees and in unsalvaged forest. Because timber harvesting always leaves behind tree stumps and many thin branches, even cleared areas of forest can contain a considerable amount of dead wood.
| Sarmenstorf AG | Messen SO | Habsburg AG |
| Altitude (a.s.l.) | 600 m | 550 m | 450 m |
| Forest type | Beech | Spruce | Mixed (50% conifers, 50% broadleaves) |
| Windthrow area | Unsalvaged: 2.3 ha Salvaged: 5 ha | Unsalvaged: 3.5 ha Salvaged: 1.2 ha | Unsalvaged: 30 ha Salvaged: 13 ha |
| Dead wood, ø>1cm, (m3/ha) | Forest: 36.7 Unsalvaged: 324.6 Salvaged: 148.9 | Forest: 36.5 Unsalvaged: 274.8 Salvaged: 49.5 | Forest: 22.7 Unsalvaged: 643.4 Salvaged: 79.0 |
More exclusive windthrow species
Windthrows are inhabited by both forest insects and open-land species. The researchers found that this not only increases the absolute insect total, but also the number of species. On average, windthrows were found to be home to twice as many species as intact forests (fig. 2 and 3) and to nearly four times as many bee, wasp and bug species.
"In addition, windthrows attract many endangered beetle species, especially those that depend on dead wood“, explains Beat Wermelinger, a forest entomologist at the WSL and lead author of the study. "By contrast, intact forests tend to be home to fewer exclusive insects", he says. For example, ground and bark beetles are often found there, with 72% of bark beetles belonging to a non-native species: the black timber bark beetle.
Fig. 2 - This longhorn beetle (Rhagium sycophanta) is an endangered dead-wood species that can benefit from the large quantities of timber in windthrows. Photo: Beat Wermelinger (WSL)
Fig. 3 - The number of insect and spider species (N) on unsalvaged and salvaged windthrows and in intact forest. Intact forest contains the fewest species. Moreover, just a small number of exclusive insects and spiders inhabit the forest, i.e. species that only occur there.
Structural diversity boosts biodiversity
There was hardly any difference between salvaged and unsalvaged windthrows in terms of species diversity: only the number of spider species – which are not insects - was higher in salvaged areas. However, there was a clear difference regarding species composition, with less than two-thirds of the species found occurring simultaneously on both types of site. The reason for this is that although salvaging timber removes habitats for wood-dwelling insects, it creates new microhabitats, for bees or wasps, for example (fig. 4 and 5).
"Mosaic forest management lays excellent foundations for high species diversity", concludes Wermelinger based on the research findings now published in the science journal Forest Ecology and Management. Mosaic management entails leaving both salvaged and unsalvaged areas after major storms, deliberately with a view to maintaining and promoting forest biodiversity.
Fig. 4 - An unsalvaged windthrow in Sarmenstorf. The insects that particularly benefit here are those which develop in dead fallen beechwood. Photo: Christoph Angst (WSL)
Fig. 5 - Although salvaging reduces the quantity of dead wood, it also creates additional microhabitats, giving other insects a place to live. For example, the majority of wild bees nest on the ground. Photo: Beat Wermelinger (WSL)
The impact can only be assessed in the long term
It may seem surprising that biodiversity in unsalvaged windthrows is not appreciably higher. "The probable reason for this is that plenty of dead wood remains in place even after stem wood has been removed" the insect expert explains. "Unlike in Scandinavia, for example, where only about 10 m3 of wood per hectare remains after windthrown timber has been salvaged, on Swiss windthrow sites some 70 m3 still remain". On average, one hectare of Swiss forest contains in excess of 24 m3 of dead wood.
That said, forests and salvaged windthrows are virtually devoid of thick dead tree trunks. Yet many dead-wood beetle species are dependent on such large-sized wood, because the rotten trunks provide them with stable and sufficiently humid long-term habitats. "So we will probably only be able to assess the actual impacts of salvaging windthrown timber a few decades from now", says Wermelinger.
This post is based on a press release issued by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL).
