Spatial patterns of saproxylic beetles in a relic silver fir forest (Central Italy), relationships with forest structure and biodiversity indicators

The conservation of biological diversity is one of the main goals for managing forests in an ecologically sustainable way. Presence and abundance of microhabitats, such as tree cavities or bark pockets, can be conveniently used as indicators to evaluate the effectiveness of sustainable forest management measures. In Mediterranean forest ecosystems, the relationships between stand-structure attributes and species-diversity indicators are still poorly studied. We described the structural attributes, deadwood characteristics and microhabitat occurrence in a silver fir forest of Central Apennines (Italy), which has not been submitted to silvicultural interventions for several decades. We assessed linkages between these characteristics and the abundance, distribution and diversity of saproxylic beetle fauna. A systematic aligned sampling method was conduced on 240 ha, examining 50 plots of 530 m2 each. Saproxylic beetles were sampled using window flight traps and emergence traps in relation to abundance and species richness at the plot level, but also on decaying deadwood. The heterogeneity in types and frequencies of microhabitats, and the link between structural attributes associated with stand complexity and saproxylic species, were also analysed. With the aim of describing the complex saproxylic ecological network, beetle species were classified according to the type of interactions with wood and other insects fauna, but also in relation to the trophic guilds. Linear regressions were conducted for highlighting metric variability and relationships between parameters, while geostatistical analyses were used to describe the spatial variability of structural features and the spatial pattern of beetle distributions. Results of linear regression and geostatistical analysis showed how the saproxylic beetle community is influenced by the amount, type and stage of decay of deadwood, but also by the forest structural complexity and the occurrence of microhabitats. Gap dynamics and natural disturbances had effects on deadwood amounts and microhabitat abundance, which was significantly higher than in managed and structurally simplified forest stands. Most of the entomological variables (namely, Families, Species, Total individuals, Saproxylic individuals, Staphylinidae, Elateridae, Nothodes parvulus, Curculionidae, Ernoporus fagi, Phyllobius emery) were clumped, highlighting the existence of aggregation areas in the sampled forest. In several cases the insect distribution was linked to the spatial pattern of forest attributes, particularly deadwood components.