Windstorms make room for biodiversity: New opportunities for endangered insects

Even if it pains the foresters to see it: a windstorm sweeping through the forest is a blessing for insect diversity. Fallen trees and the subsequent vegetation provide plenty of nutrition and habitat for many insects and other invertebrates, which is why their diversity increases considerably in the first few years after a storm. This was shown by a large-scale study spanning 20 years, that was conducted by the Swiss Federal Institute for Forest, Snow, and Landscape Research WSL.

Previous to the winter storms Vivian (1990) and Lothar (1999), foresters would clear storm-damaged timber from the forests to, among other reasons, minimise the risk of bark beetle damage, harvest the fallen timber, or restore order. The research team wanted to find out whether leaving fresh deadwood at the windthrow site, thereby leaving it unsalvaged, would make a difference to the insects or not. So, it set up different insect traps (see box) on a total of 16 windthrow plots created by Vivian and Lothar as well as unaffected control plots in comparable forest sites. In some windthrow plots, the fallen deadwood had been salvaged, while others remained unsalvaged. In total, the researchers collected more than 500,000 individuals of over 1600 arthropod species, which include insects, spiders, and other arthropods.

Beat Wermelinger, lead author of the study published in the Journal of Applied Ecology, explains: ‘The data collection was a huge, joint effort and resulted in a globally unique data set. Never before has data on arthropods been collected at regular intervals for 20 years after an ecological disturbance. Whatever the weather – in heat, snowfall, or rain – a field team was visiting the sampling plots every week to empty the insect traps. They then sorted them in the laboratory and insect specialists identified them.’

More endangered species on unsalvaged plots

What the researchers had not expected: The number of species was comparable on both salvaged and unsalvaged windthrow sites. ‘That surprised us,’ says Wermelinger.

However, the species composition differed; for instance, certain deadwood-dependent specialists were found almost exclusively in unsalvaged windthrow areas. One out of ten of the collected species were only found on salvaged storm plots and a fifth were retrieved exclusively from unsalvaged storm sites. These included the endangered longhorn beetle Rhagium sycophanta and the rare mason bee Hoplitis villosa.

Until 20 years after the storms, the research team found significantly more endangered insects on unsalvaged windthrow plots than on salvaged ones. An important conclusion of this study is therefore that deadwood created by storms is an indispensable resource for many endangered insect species, especially deadwood-dependent species.

However, this does not mean that storm areas should no longer be salvaged at all. In spruce forests, which are intended to protect against natural hazards in the mountains, the risk of bark beetle mass propagation must always be taken into account. In deciduous or mixed forests, on the other hand, the researchers recommend a mosaic of salvaged and unsalvaged windthrow areas as well as intact forest areas, as this best supports biodiversity.