The taxonomic and functional stratification of saproxylic beetle communities in temperate oak forests is altered by forest decline

Canopy strata in temperate forests remain poorly studied, limiting our understanding of their associated biota. Yet, this biota is immediately surrounded by open-air conditions and may therefore be more exposed to external abiotic factors than biota of lower strata, while living in the forest strata most affected by climate-change-induced dieback. We investigated how saproxylic beetle taxonomic and functional diversity vary along a vertical gradient and respond to forest dieback. During a single insect activity season, we inventoried deadwood and tree-related microhabitats and sampled mobile adult saproxylic beetles in three vertical strata — the understory, lower canopy and upper canopy, using multi-funnel Lindgren traps. Our twenty plots spanned a gradient of dieback intensity in two oak forests of the Loire Basin (France). The saproxylic beetle communities differed among the three vertical strata in both taxonomic and functional composition. All the strata contributed similarly to alpha diversity, while the upper canopy was functionally distinct and hosted the highest beetle abundance. The vertical stratification varied among trophic and microhabitat guilds. Forest dieback was followed by homogenisation of the communities along the vertical gradient. These results highlight the key contribution of canopy strata – especially the upper canopy – to saproxylic beetle diversity, and show that dieback can erode vertical structuring of these communities.