Deadwood enrichment in European forests – Which tree species should be used to promote saproxylic beetle diversity?

Modification of natural ecosystems has threatened biodiversity worldwide, with forests suffering especially. Strategies aimed at mitigating such loss in forests often include enrichment of deadwood, a critical resource for many decomposer species. However, it remains unclear how deadwood can best be enriched to most effectively promote the diversity of saproxylic species. In this study, we investigated saproxylic beetle diversity in experimentally exposed deadwood logs of 13 different tree species across 30 forests in three regions of Germany. We tested whether gamma-diversity differs between tree species and whether the alpha-diversity within an individual log depended on whether logs were placed in unmanaged beech forests, managed beech forests, or managed conifer forests. We found significant differences in gamma- and alpha-diversity of saproxylic beetles among tree species, but the ranking of tree species differed between regions, suggesting differences in regional beetle species pools. Randomization tests aiming to identify how many and which deadwood logs would need to be exposed to best conserve saproxylic beetle diversity, showed that the overall diversity of beetles increased with the number of tree species exposed, due to turnover of beetle species between tree species. However, some species (e.g. Carpinus) and species combinations (e.g. Carpinus-Picea) reached exceptionally high beetle diversity. Alpha-diversity was higher in conifer than in beech forests, but did not differ between managed and unmanaged beech forests. Canopy cover above logs and average stand temperature strongly influenced alpha-diversity, suggesting that environmental conditions that may be affected by management act as habitat filters for species assemblages. We conclude that deadwood enrichment strategies would be most effective when combining particular tree species that support highest diversity.