Enhanced forest structural heterogeneity increases functional β-diversity but reduces α- and γ-diversity in soil nematodes

Most production forests are managed primarily for timber production, leading to homogenous forests at landscape scale and reduced biodiversity. To explore whether silviculturally enhanced forest structural heterogeneity can promote biodiversity at landscape scale, we conducted a large-scale experiment in eight German forests. We manipulated structural β complexity, i.e., the heterogeneity of structural elements between forest patches, by experimentally introducing variation in canopy cover and different types of deadwood across 156 patches of 50 × 50 m each, to investigate its effects on biodiversity. Here we analyzed the response of soil nematode communities to heterogenization by assessing taxonomic and functional diversity across patch (α-diversity), site (γ-diversity), and between-patch (β-diversity) scales using Hill–Chao numbers as diversity indices. Additionally, we tested whether environmental variables correlate with nematode diversity responses. Our results show that functional diversity is more responsive than taxonomic diversity. Increases in β-diversity of common and dominant functional groups occurred simultaneously with declines in α- and γ-diversity. This pattern indicates that local community dissimilarity can rise without an increase in overall landscape-level richness and suggests a shift toward more specialized nematode communities in response to the interventions. Moreover, we found that certain site-specific conditions, such as soil texture and understory plant biomass, correlated with these effects. Overall, our findings reveal complex, scale-dependent responses of nematode diversity to changes in aboveground forest structure and highlight the need to further investigate the context dependence of forest biodiversity management to provide informed recommendations. This study represents an important first step toward understanding how to increase soil β-diversity through enhanced forest structural heterogeneity at management-relevant (i.e., landscape level) spatial scales. • Enhanced structural heterogeneity in forests affects soil nematode diversity • Effects vary across diversity facets, scales, and diversity orders • More functional diversity metrics were changed than taxonomic ones • β-diversity of common and dominant taxa/functional groups increased • β-diversity can increase while α- and γ-diversity decrease • Site-specific environmental conditions link to diversity changes