The impact of tree mortality and post‐disturbance management on insect diversity in temperate forests: Insights from a replicated experiment

In recent years, climate change has caused an unprecedented wave of tree mortality across Europe, reshaping forest canopy cover and deadwood structures. Nevertheless, the effects of tree mortality on species‐rich groups, such as insects are still unclear, mainly because few replicated disturbance experiments exist and most studies focus on small taxonomic subsets of the species‐rich class of insects, both of which prevent generalization. To disentangle the effects of canopy cover and deadwood on total above‐ground insect diversity collected with Malaise traps, we experimentally applied different disturbance treatments in 11 commercial forests throughout Germany. We quantified taxonomic (TD) and phylogenetic diversity (PD) among 22,889 operational taxonomic units from 362 insect families along Hill numbers (q = 0, 1, 2) using metabarcoding, and standardized diversities by sample coverage as an objective measure of sample completeness. Our results showed a systematically lower sample coverage in canopy gaps compared with closed canopy forests, highlighting that the true diversity in open forest habitats is underestimated in bulk samples if sample coverage is not considered. Controlled for sample coverage, TD increased by up to 47% in canopy gaps with deadwood, compared with controls under closed canopy and without deadwood. The increase in TD was accompanied by a similar increase in PD only in rare species (q = 0), underlining the gain in evolutionary diverse communities of rare species, but redundancy in dominant species (q = 2). Synthesis and applications. Our experiment provides strong evidence for the importance of clustered tree mortality for comprehensive insect diversity in temperate forests and highlights the potential of forest management to enhance insect diversity in homogeneous commercial forests, particularly through the creation of canopy gaps and the retention of deadwood. Both mimic natural dynamics and can support rare species along with the ecosystem functions they may provide.