From forest stand decline to salvage logging: Cascading impacts on saproxylic beetle diversity

Understanding the cascading effects of stand decline on saproxylic communities, via the induced changes in deadwood and tree‐related microhabitats (TreMs), is fundamental for optimizing the management of disturbed forests. We postulated that these cascading effects would vary along decline gradients—from stand decline (proportion of declining trees) to mortality (proportion of dead trees) and subsequent salvage logging—mediated by the shifts in habitat conditions. This study was conducted across three representative European forest contexts: fir forests in the Pyrenees and oak forests in the Loire Valley affected by drought disturbance, and Norway spruce forests in Bavaria affected by windstorm and bark beetle outbreak. We assessed how the taxonomic and functional α‐diversity of saproxylic beetles responded to variations in the diversity and density of deadwood and TreMs, using structural equation modelling. Our analysis confirmed an overall positive influence of decline processes on saproxylic beetles in conifer‐dominated forests. TreM and deadwood diversity played a central role in structuring saproxylic beetle communities in stand decline contexts. The increase in TreM heterogeneity associated with stand decline or mortality enhanced saproxylic abundances, with exposed wood and trunk injuries, and deadwood‐associated TreMs identified as particularly influential. Snags and large deadwood elements, especially in spruce forests, and deadwood diversity further contributed to sustaining high levels of beetle abundance. Conversely, salvage logging exerted detrimental effects, especially on the saproxylophagous guild, primarily through reductions in TreM heterogeneity, and the depletion of deadwood‐associated and exposed wood and trunk injury types of TreMs. Given the pronounced context dependency of the processes driving these cascading community dynamics, and considering the increasing frequency, severity and spatial extent of forest disturbances and global stand decline, it is imperative to integrate this complexity into management and conservation frameworks. Addressing these mechanisms with greater precision will be critical for maintaining biodiversity within rapidly changing forest ecosystems.