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Beat Wermelinger

Forschungsanstalt WSL

Swiss Federal Research Institute WSL
Forest Entomology
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Article

Author(s): Adrienne Frei
Editorial office: WSL, Switzerland
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Sunlight and Deadwood – a paradise for wood living beetles

Over the last three decades there have been intensive discussions about leaving deadwood in forests. It is undisputed that more deadwood needs to be left in forests if the variety of species is to be increased. New habitats for many species including rare ones are being created by additional thinning out.

Ameisenbuntkaefer
Fig. 1 - Checkered-beetles (Thanasimus formicarius) feed on bark beetles.
Photo: Beat Fecker (WSL)
 

Loose forests shelter a large number of wood living beetles. This group of insects play an important role in the functioning of the ecosystem: wood living beetles contribute greatly to the wood decomposition process, create new nesting facilities for various bird species by their incessant wood eating and are an important source of food for birds, bats and lizards.

What are wood living beetles?

All beetles which at some time in their life span are dependent on wood substrate are classed as wood living beetles (saproxylics). To this group belong the bark beetles which decompose wood directly, as well as the checkered-beetle (Thanasimus formicarius, Fig. 1), which feeds on bark beetles as well as other species which eat wood decomposing fungi.

In central Europe 8,000 species have up to present been identified, including 1,340 saproxylic species. In Switzerland one fifth of the 6,400 known beetles live on or in wood. Half of the saproxylic beetles are on the red list in Germany. In Switzerland a red list of saproxylic beetles is being compiled. The fact that so many wood living beetles are considered endangered suggests that necessary structures and habitats are greatly endangered.

Deadwood – a scarce commodity

Schrot-Zangenbock
Fig. 2 - Blackspotted pliers support beetle (Rhagium mordax)
Photo: Beat Wermerlinger (WSL)
 

Deadwood is an important structural element in our forests. It provides a food resource for many organisms. It is assumed that above a density of 30m³/ha of deadwood the largest possible amount of saproxylic species can exist with a stable population. In forests in the Swiss Midlands a density of only 4.9m³/ha of deadwood was measured in the last forest survey.

Not only does the amount of deadwood play a role in the biodiversity, but also the location, position (standing or lying) and the level of decomposition. In the first two years a dead beech tree provides ideal development conditions for the blackspotted pliers support beetle (Rhagium mordax, Fig. 2). The same beech tree is only an optimal useful resource for the lesser stag beetle (Dorcus parallelopipedus, Fig. 3) years later, when the decomposition process is further advanced.

The number of species of saproxylic beetles varies depending on the tree species. Oak trees are considered to be the trees most "rich in species". They accommodate around 650 wood living beetle species, whilst the beech tree "only" has approximately 240 different beetles living on it and the spruce a mere 60.

Beetles love light

Balkenschröter
Fig.3 - The lesser stag beetle (Dorcus parallelopipedus) belongs to the stag beetle family.
Photo: Beat Fecker (WSL)
 

An abundance of dead wood is not enough to achieve a varied beetle fauna. Light and sunshine play just as an important role in the whole system. A study in the Arlesheim Forest (near Basel) showed that sufficient dead wood or blossom alone did not lead to an increase in diversity. However, when dead wood and blossom were combined the number of species from the red list doubled.

The explanation for this is simple: many longhorn and jewel beetles eat their way as larvae through deadwood. After they have reached adulthood pollen and nectar are at the top of their menus. Many beetles also have a preference for a particular colour of flower. For example the jewel beetle Anthaxia salicis loves yellow buttercups.

Encouraging saproxylic beetles

To provide an optimal habitat for wood living beetles experts recommend the following:

  • Increase the percentage of deadwood in the form of roots, branches and lying or standing trunks. An amount of 30m³/ha is desirable.
  • Keep enough standing deadwood and perhaps even increase the amount through girdling.
  • Lying deadwood should, whenever possible, be deposited/stored in sunny or half shady places. If the wood is stacked up it should not be covered with plastic tarpaulins, which enclose the wood, because such covers become beetle traps. Corrugated iron or planks are better.
  • Every three to five years "new" deadwood should be added so that various levels of decomposition are available.
  • Thinning of stands: this increases the variety of blossom at the herbaceous level. This variety can be prolonged by the periodic cutting of young growth.

  • Translation: Dawn Meister (Stallikon)

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