Experimental enhancement of structural heterogeneity in forest landscapes promotes multidimensional hoverfly diversity.

The homogenization of temperate forests due to intensive management has led to biodiversity loss at local and landscape scales, threatening species persistence and ecosystem functions. However, experimental evidence on how structural heterogeneity among forest patches influences diversity at the landscape scale is lacking. Here, we test whether enhancing structural heterogeneity, via diverse deadwood enrichment and canopy gap creation treatments (Enhancement of Structural Beta Complexity, ESBC), can increase hoverfly diversity within landscapes, and elucidate the contributions of local (α-) diversity enrichment and turnover (β-diversity) to this effect. We conducted a large-scale forest experiment in 11 regions across Germany. Each region included two districts, representing small forest landscapes. In one district, we implemented patches with ESBC treatments, while in the other, serving as the control, we established patches without ESBC. Hoverflies were sampled in three seasonal intervals and across, in total, 234 forest patches (50 x 50 m) using pan traps. We applied a new integrative meta-analytic framework that incorporates sample completeness to quantify taxonomic, functional, and phylogenetic diversity (TD, FD, PD) using Hill numbers at α, β, and γ scales. All three γ-diversity dimensions - TD, FD, and PD - were significantly higher in structurally heterogeneous forest landscapes than in homogeneous ones. The strongest effects were observed for TD, indicating functional and phylogenetic redundancy among species. Effect sizes declined with increasing order of Hill numbers, suggesting that rare species benefit most from structural heterogeneity. In most regions, γ-diversity gains were driven by increases in α-diversity rather than β-diversity, highlighting the importance of interventions to increase local structural complexity. However, several regions also showed elevated β-diversity, indicating context-dependent effects of spatial heterogeneity. Our results provide the first experimental evidence that enhancing structural heterogeneity at the landscape scale can restore multi-dimensional insect diversity in temperate forests. They underscore the management value of ESBC as a scalable tool to restore biodiversity, increase ecological resilience, and counteract biotic homogenization in production forests.