Effect of dead wood enrichment in the canopy and on the forest floor on beetle guild composition

In managed forests, the abundance and diversity of saproxylic insect species and beetles in particular has been argued to be primarily limited by the amount of dead wood while the importance of other factors for community composition of this functional group has been largely neglected. It has also been argued that management regimes that lead to increasing amounts of dead wood in forests will increase the abundance and diversity of saproxylic species, but experimental evidence for this assumption is rare. We studied the importance of dead wood and a number of abiotic and biotic factors for community composition of saproxylic species in a range of managed forests in Germany for 2. years and found that the total amount of dead wood in the forests only played a minor role in explaining the composition of the saproxylic beetle community compared to other factors such as precipitation, temperature, tree species and vertical stratum. We then tested for eight forest plots whether dead wood experimentally exposed in the canopy and forest floor is attractive to saproxylic beetles. We found that a high diversity and abundance of beetles of various habitat and feeding guilds, were attracted to fresh dead wood, many more than in control areas without dead wood manipulation. Thus, in managed forests where abundance and diversity of saproxylic beetles is low, enrichment of dead wood resulted in an immediate increase of species and a change of guild composition particularly in the canopy. Our results suggest that the amount of ground dead wood is by itself not a sufficient measure for the local diversity of saproxylic beetles. We conclude that dead wood enrichment in managed forests is an effective strategy to promote saproxylic beetle diversity. Changes in the functional composition are expected to be large enough to influence ecosystem processes such as decomposition of dead wood or the control of herbivores.