Dryland biodiversity–productivity relationships remain poorly resolved. Specifically, the environmental conditions governing the shift between complementarity and mass ratio mechanisms remain unclear, limiting the effectiveness of restoration and management strategies. To address this gap, the aim of this study was to investigate the geographical patterns of diversity and biomass production in herbaceous communities along a 2,100-km precipitation gradient in North China. We studied how α- and β-diversity affect community-wide productivity using linear mixed-effects models and piecewise structural equation models, along with rolling-window change-point analyses. In arid regions, biomass productivity was primarily driven by interspecific niche complementarity, where higher functional diversity (FD(α)) enhanced resource-use efficiency. However, in semi-arid regions, productivity was regulated by the mass ratio effect, specifically through the traits of dominant species, including community weighted mean height and specific leaf area, as these species exploited broader resource spectra with increasing water availability. A critical mechanistic shift occurred at a mean annual precipitation (MAP) threshold of ∼168 mm (95% CI: 152–171 mm; p < 0.001). Below this threshold, productivity was driven by diversity-mediated complementarity and stress tolerant strategies. Conversely, as MAP surpassed 168 mm, the system transitioned to mass ratio control, coincident with a shift toward competitive strategies. Overall, our study provides empirical evidence to guide dryland management: prioritising the maintenance of functional diversity in arid communities, while emphasising dominant-trait optimisation (plant height and specific leaf area) in semi-arid communities to maximise aboveground biomass. Contribution of herbaceous plant diversity to community productivity in China's drylands. Unveiling the shift from complementarity effects in arid regions to mass ratio effects in semi-arid regions.This study investigates the transition processes of ecological drivers regulating aboveground biomass (AGB) along a 2,100-kilometre precipitation gradient in northern China. (a) Mechanistic Shift: In arid regions (MAP < ∼168 mm), productivity is primarily driven by the complementarity effect (high functional diversity), enhancing resource use efficiency under stress. A critical threshold occurs at approximately 168 mm, beyond which the mass ratio effect becomes dominant, with productivity increasingly determined by the functional traits (Height, SLA) of dominant species. (b) Strategy Shift: This functional transition corresponds to a shift in community assembly strategies from Stress-tolerant (S) species in water-limited environments to Competitive (C) species in semi-arid regions. • A 168 mm precipitation threshold separates complementarity and mass ratio effects. • Functional diversity enhances productivity in arid regions through niche differentiation. • Dominant trait expression (Plant height, specific leaf area) drives productivity in semi-arid ecosystems.
H et al. (Fri,) studied this question.