Slash harvesting does not undermine beetle diversity on small clear-cuts containing sufficient legacies

Logging residues (slash) constitute an increasingly important source of renewable energy in forested countries, but their intensive extraction can reduce biodiversity. Important research issues include the mitigation potential of reduced logging and slash extraction levels, and their feasibility in production forestry. We performed a comparative study on beetle assemblages in relation to slash extraction practices in Estonia, where silviculture retains higher deadwood amounts than in intensive forestry systems, slash extraction level is ca. 50 %, and final fellings retain at least 5 % of the standing stock. We sampled beetles using flight-interception traps on six pairs of retention cut sites (one conventional and one slash-harvested; average size ca. 2 ha; in three forest site-types). The material comprised 11,948 beetle specimens identified to species level (500 species). Species of conservation concern occurred regularly in all sites, while a total of 18 putative pest species comprised only ca. 3 % of individuals in both treatments. There were no clear influences of slash extraction on species richness, abundance or assemblage composition. The main gradients in the assemblage composition were trapping season and site type; the latter co-varying with tree-species composition and deadwood volumes. We conclude that slash harvesting that retains (depending on forest type) 5–25 m3 ha−1 of coarse deadwood and 10–35 m3 ha−1 of fine deadwood neither reduces beetle diversity (at least of highly mobile species in the short term) nor affects pest outbreaks in our study system. To sustain beetle diversity, sufficient deadwood legacies should be supplemented with live-tree retention for a continuous deadwood supply during post-harvest succession.