Diversity patterns of Coleoptera and saproxylic communities in unmanaged forests of Mediterranean mountains

Beetle communities are excellent indicators to monitor biodiversity in forest environments. The most common indicators monitored for assessing the effectiveness of the sustainable forest management measures are deadwood amount, forest structure, and microhabitat traits. In Mediterranean mountain forests, however, the relationships between stand-level attributes and species diversity indicators have been still poorly studied. We have investigated the patterns of species diversity of beetle communities in relation to stand structural traits in two unmanaged forests located in central Apennines (Italy). The two forest stands are characterized by different tree species composition and management history. The experimental area extends over approximately 300 ha, within which 50 circular plots of 530 m2 were sampled for each studied forest. Coleoptera were collected using window flight traps and emergence traps on decaying deadwood. We evaluated i) the differences in species diversity (richness, Shannon diversity and Simpson diversity) between the two forests and ii) the beetle Shannon diversity and composition in relation to selected forest variables, with a particular focus on saproxylic species. Principal Component Analysis followed by rarefaction analysis were applied to examine differences in species diversity. Within each forest stand, a Boosted Regression Tree model and a Non-Metric Multidimensional Scaling were performed to relate the species Shannon diversity and community assemblages to the forest structural attributes. Results revealed how these forest structural attributes have a fundamental role in influencing the diversity of Coleoptera and their assemblages. Forest vertical heterogeneity, snag volume and tree dominance were the most important variables for saproxylic assemblages in these Mediterranean mountain forests, while the amount of coarse woody debris positively affected the overall beetle diversity and that of saproxylic species. At stand level, differences in species diversity were mainly driven by tree composition, probably indicating that food resources, determined by tree species, are of primary importance in determining species assemblages and beetle diversity in these forests; while at plot level, we found that the variables, which drove species composition and beetle diversity, varied between the two forests. This study provides the basis for the integration of independent biodiversity indicators in sustainable solutions for the conservation of unmanaged forests in Mediterranean mountain systems.