Forest management, canopy cover and geographical distance affect saproxylic beetle communities of small-diameter beech deadwood

Abstract Beetles (Coleoptera) are the most diverse and species-rich insect order in saproxylic communities, and play a key role in forest dynamics. To prioritize conservation measures, a clear understanding is needed of what habitat features are the most important for beetles that depend on deadwood. Here, we investigate (i) how the assemblages of saproxylic beetles emerging from small-diameter beech deadwood (Fagus sylvatica L.) are influenced by several environmental factors and type of forest management, and (ii) if geographical distance of the investigated sites influences the similarity of these saproxylic beetle assemblages. We visited 10 high beech forest stands along a latitudinal gradient from northern to southern Italy, and collected 200 logs of 50 cm length, 12 ± 2 cm diameter, and Hunter’s decay class 2, from deadwood lying on the forest floor. Logs were stored in ex-situ emergence traps, in common garden conditions, from April to September, and individuals emerging from the deadwood were collected weekly. A total of 2571 beetles were collected, including several species which were either threatened, firstly recorded or endemic. We found that, compared to even-aged stands, forests managed to increase structural heterogeneity (uneven-aged high forests, and former coppice stands converted to high forest), with closed canopy, were colonized by the richest and most abundant saproxylic beetle assemblages. Additionally, we observed distance decay of similarity between sites, indicating geographical structure in community composition. Our findings have important implications for biodiversity conservation in beech forests: (1) small-diameter deadwood hosts a rich biodiversity of Coleoptera and should therefore be left on the ground; (2) forest management practices should aim at preserving or restoring structural heterogeneity and closed canopies; (3) these conservation measures should be carried out along the entire area of occurrence of beech forests, due to turnover in species composition with geographical distance.