1. Global warming induces new constraints on forest ecosystems and requires forest management adaptation. The reduction in stand density is currently debated as a potential tool to face increasing summer drought risk by improving forest resistance to climate change-induced tree mortality. However, few studies have yet assessed the impacts of this management change on soil biodiversity. 2. We conducted a large-scale, multi-site assessment of the response of soil macro-detritivore assemblages and soil functioning to experimental manipulations of stand density. A total of 33 stands were studied covering a wide gradient of stand density, that is stand basal area from 2·5 to 43·7 m2 ha−1, stand age, that is 18–171 years old, and local abiotic context. 3. We observed contrasting responses as a function of both taxonomic and functional groupings. Exploratory analysis using causal diagrams, that is path analysis, highlights that these changes were mainly related to alterations in understorey vegetation, microclimatic and soil pH conditions. The response of soil macro-detritivore assemblages to stand density manipulation was consistent over the gradient of stand ages. 4. Among the litter-dwelling macro-detritivores, millipede abundance and diversity decreased with stand density reduction, while woodlice and epigeic earthworms were unaffected. Further, a shift in soil-dwelling earthworm community composition was observed in mull stands. Endogeic earthworm abundance showed a sharp increase with stand density reduction, which translated into an increase in soil respiration. In contrast, anecic earthworm abundance decreased and was strongly associated with a decline of the rate of forest floor turnover. 5. Synthesis and applications. Our study provides strong evidence that reductions of stand density will have substantial impacts on soil macro-detritivore assemblages and cascading effects on soil functioning, particularly in mull stands. Managing stand density of oak forests at an intermediate level, that is 25 m2 ha−1, appears to be best to optimize the trade-off between improving forest resistance to climate change and ensuring the conservation of functional diversity to preserve forest ecosystem functioning and stability.