Sun exposure as a key factor influencing saproxylic beetle communities

The ability to locate and colonize ephemeral deadwood resources is crucial to saproxylic beetle assemblages. Saproxylic beetles locate suitable substrates mainly through visual cues and via olfactory cues emitted by deadwood, other insects and, wood-decaying fungi. For the conservation of saproxylic beetles, it is essential to understand which abiotic factors and biotic interactions most significantly influence their habitat requirements when locating suitable substrates. In a field experiment, in sunny and shaded plots, we exposed 400 bundles of freshly cut deadwood, each consisting of three logs with a combination of different tree species and treatments (i.e., fungi inoculation), mimicking biotic interactions. We sampled arriving saproxylic beetles via sticky traps directly applied on the deadwood substrate to evaluate the effect of sun exposure and the biotic interactions on colonizing saproxylic beetles. We found higher species numbers and an abundance of saproxylic beetles under shaded than sunny conditions, but there was no effect on the standardized number of species (diversity). However, we observed a shift in species diversity from sunny to shaded conditions from early to late season. Beetle assemblages differ between shaded and sun-exposed deadwood. Treatments (fungi inoculation, sterilization, and burning) did not affect saproxylic beetle colonization.  Our results suggest that colonization of freshly cut deadwood by saproxylic beetles is mainly driven by sun exposure rather than biotic interactions, despite assumed close associations between beetles and fungi. To protect the full spectrum of saproxylic beetle species, we recommend managing both sunny and shaded areas, as each provides unique habitats that support different species compositions.