Experiments with dead wood reveal the importance of dead branches in the canopy for saproxylic beetle conservation

Vertical gradients of habitats are a typical characteristic of forest ecosystems. Sun-exposed dead wood in the upper canopy, for instance, provides a habitat for saproxylic beetles distinct from that in the more shaded dead wood below the canopy. Canopy research, however, is challenging due to both the limited accessibility and potential confounding effects of temperature on trapping probability when activity traps are used. We studied saproxylic beetle assemblages along a complete vertical gradient without bias caused by temperature effects on activity. Using crane-like constructions attached to the top of large Silver Fir trees (Abies alba), we exposed bundles of freshly cut branches of European Beech (Fagus sylvatica), Silver Fir and Norway Spruce (Picea abies) in three different vertical strata (upper canopy, mid-canopy and near the ground). The bundles in the upper canopy were fully exposed to the sun and the bundles in the mid-canopy and near the ground were in the shade. We allowed beetles to colonize the bundles of branches for one growing period and then reared beetles from each bundle over three years. The species composition of saproxylic beetle assemblages differed between bundles in the upper canopy and near the ground; bundles in the mid-canopy had an intermediate assemblage composition. The abundance of saproxylic beetles was higher near the ground than in the upper canopy, whereas the number of species showed the opposite pattern. Overlapping confidence intervals of sample-based rarefaction and extrapolation curves for species diversity indicate that estimated gamma diversity per stratum is similar across the three strata. Our results support earlier studies that revealed the importance of habitat heterogeneity as a driver of the biodiversity of taxa associated with dead wood. As we controlled for wood diameter and tree species diversity, our study suggests that the microclimatic variability within dead wood - and thus habitat heterogeneity for saproxylic beetles - is higher in the upper canopy than near the ground. For biodiversity conservation in forests, our results support a strategy of enhancing the number of trees with microhabitats, particularly those with dead branches in the upper tree crown. Dead branches and standing dead trees should only be removed, e.g. for safety reasons, if no other option is available.