Aim: The biogeography of European forests presents many interesting case studies, as inferred from phylogeography, contemporary population genetics, and distribution modelling of saproxylic species. Here, we pose four main hypotheses: (i) the phylogeographic paradigm known for temperate species in Europe is also applicable for saproxylic taxa; (ii) current expansion is occurring only in some genetic lineages; (iii) genetic diversity decreases from east to west, reflecting differences in forest naturalness and management; and (iv) climatic changes will force a northward range shift.Location: Europe.Taxon: The saproxylic beetle, Cucujus cinnaberinus (Scopoli, 1763).Methods: Beetles from 37 locations throughout the species range were genotyped using microsatellites and mtDNA (468 and 237 individuals, respectively). Molecular data were analysed to describe population diversity, structure, and demography. 200 records were used to model distribution based on climate data.Results: mtDNA analyses revealed three clades: the most divergent in Italy and two less distinct in north-western and central - to-southeastern Europe. Microsatellites showed no clear geographic distribution pattern except for a distinct lineage in Italy. The highest genetic diversity was found in southern refugia, in natural forests of Central-Eastern Europe, and in a mixing zone in Pannonia. During the Holocene, the Italian population declined, whereas populations in the rest of Europe showed no clear changes. Distribution models revealed the existence of glacial refugia along the coasts of the Adriatic and Black Seas, and northward dispersal in the Holocene continued in the XXI century.Conclusions: C. cinnaberinus has a complex historical biogeography, discordant with patterns known for temperate or boreal taxa. The current expansion occurs only in populations from mainland Europe, whereas southernmost populations are in decline. Current genetic diversity is shaped by recent forest management, which is the least intensive in Eastern Europe. Climate change will most likely enhance northward dispersal and threaten the survival of the southernmost populations.

Recent progress in the taxonomy of flat bark beetles (Cucujidae), specifically, in the genus Cucujus, has revealed great diversity in subtropical Asia, but the seemingly well-known temperate and boreal taxa need further attention because of their conservation status. Here, we used an integrative approach using morphology, DNA, and species distribution modelling to disentangle phylogenetic relations, verify the number of species, and understand the historical biogeography of Palearctic and Nearctic Cucujus beetles, particularly the C. haematodes species group. Species distinctiveness was supported for C. cinnaberinus, but present-day C. haematodes turned out to be a species complex made up of separate lineages in the western, middle and eastern parts of its Palearctic range. Cucujus muelleri was a member of that complex, being sister to Asian C. haematodes. Moreover, C. haematodes caucasicus was found to be phylogenetically closely related to Italian C. tulliae, and both to be sister to European C. haematodes. North American C. clavipes clavipes and C. c. puniceus resulted to be enough divergent to be considered different species. Interestingly, western American C. puniceus turned out to be closely related to the C. haematodes complex, whereas eastern American C. clavipes constituted a separate lineage, being distantly related to both C. puniceus and C. cinnaberinus. These patterns suggest former trans-continental connections among the ancestors of extant flat bark beetle species. Moreover, a divergent lineage of C. cinnaberinus was found in Calabria, which should be regarded at the very least as a subspecies. The ancestor of C. hameatodes group originated in mid-Miocene, and next, ca. 6.2 Mya, a line leading to C. cinnaberinus had split. Speciation of the American lineages occurred during Pliocene (4.4 Mya for C. clavipes and 3.3 Mya for C. puniceus). Species classified as C. haematodes, C. tulliae and C. muelleri, as well as distinct lineages within C. cinnaberinus split during mid Pleistocene (ca. 1.5 Mya). A comparison of species climatic requirements and their present distribution allowed to identify glacial refugia in south-eastern areas of North America (C. clavipes), south-western areas of North America (C. puniceus), and the Mediterranean and Caspian Sea Basins (European Cucujus species), or south-eastern areas of Asia and the foothills of the central Asian mountains (eastern C. haematodes). Subsequent climatic changes in the Holocene forced these beetles to move their ranges northwards along the coasts of the Pacific (C. puniceus) or Atlantic (C. clavipes), north-eastwards to central, northern, and eastern Europe (C. cinnaberinus and European C. haematodes) or Siberia (Asian C. haematodes). The combined use of molecular, morphological and climatic data allows a comprehensive understanding of the phylogenetic relations and past distributions of Cucujus beetles, highlighting the complexity of C. haematodes species group evolution.

Identification of forest stands with priority for the conservation of biodiversity is of particular importance in landscapes with a long cultural and agricultural history, such as Central Europe. A group of species with a high indicator value for the naturalness of forest ecosystems are saproxylic insects. Some of these species, especially within the order Coleoptera, have been described as primeval forests relicts. Here, we compiled a list of 168 “primeval forest relict species” of saproxylic beetles based on expert knowledge. These species can serve as focal and umbrella species for forest conservation in Central Europe. They were selected because of their dependence on the continuous presence of primeval forest habitat features, such as over-mature trees, high amounts of dead wood, and dead wood diversity, as well as their absence in managed Central European forests. These primeval forest relict species showed a moderately strong clumping pattern within the phylogeny of beetles, as indicated by phylogenetic signal testing using the D-statistic. When we controlled for phylogenetic relatedness, an ordinal linear model revealed that large body size and preference for dead wood and trees of large diameter are the main characteristics of these species. This list of species can be used to identify forest stands of conservation value throughout Central Europe, to prioritize conservation and to raise public awareness for conservation issues related to primeval forests.