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Context: Forest ecosystems worldwide are facing increasing drought-induced dieback, causing mortality patches across the landscape at multiple scales. This increases the supply of biological legacies and differentially affects forest insect communities.Objectives: We analysed the relative effects of local- and landscape-level dieback on local saproxylic beetle assemblages. We assessed how classic concepts in spatial ecology (e.g. habitat-amount and habitat-patch hypotheses) are involved in relationships between multi-scale spatial patterns of available resources and local communities.Methods: We sampled saproxylic beetle assemblages in commercial fir forests in the French highlands. Through automatic aerial mapping, we used dead tree crowns to assess dieback levels at several nested spatial scales. We analysed beetle taxonomic, phylogenetic and functional diversity related to differing levels of multi-scale dieback.Results: In line with the habitat-amount hypothesis, taxonomic and functional diversity, but not phylogenetic diversity, of beetle assemblages significantly benefitted from forest dieback, at both local and landscape scales. Very few single or interaction effects were detected in the multiplicative models combining local and landscape variables, though a significant positive effect of landscape-scale dieback on the abundance of cavity- and fungus-dwelling species was consistent with a spill-over effect. Increased landscape-scale dieback also caused a functional specialisation of beetle assemblages, favouring those related to large-diameter, well-decayed deadwood.Conclusions: Increasing tree mortality under benign neglect provides conservation benefits by heterogenising the forest landscape and enhancing deadwood habitats. Legacy retention practices could take advantage of unharvested, declining forest stands to promote species richness and functional diversity within conventionally managed forest landscapes.

National audience

International audience; Forest diebacks are likely to increase in response to climate change, with increased frequency and intensity of droughts. In line with climate change scenarios, ecoclimatic modelling predicts a decrease in the range of silver fir, a drought-sensitive species, in its southern limit in the French Pyrenees. Diebacks are expected to induce a pulse of resources potentially favorable to certain forest species, e.g. tree-related microhabitats (TreM) for saproxylic species. The impact of forest dieback on biodiversity has nonetheless been poorly studied. As part of the international Climtree project, we set up a balanced sampling design of 56 plots crossing the intensity of local silver fir dieback and the salvage logging of weakened or dead trees. Detailed stand structure metrics and insect communities sampled by Malaise traps (insect MOTUs) or flight-interception traps (saproxylic beetles) have been measured. The structure of fir stands was affected by the level of decline, and to a lesser extent by salvage logging. We indeed observed a slight increase in CWD and in some TreM-bearing trees (crown deadwood, annual polypores , trunk rot holes) with dieback intensity, and a slight decrease in some TreM-bearing trees (crown deadwood, annual polypores) in salvaged compared with unharvested plots. However, these stand changes did not strongly affect local insect assemblages. Guilds of TreM-associated insects did not increase in abundance or richness with dieback-induced increase in resources. In conclusion, forest changes through dieback result in habitat and resource changes with still hard-to-predict impacts at the stand scale on a major reservoir of biodiversity.