Rising atmospheric CO2 is widely expected to enhance plant biomass, yet the mechanisms that partition this effect between physiological regulation and structural adjustment remain poorly understood. Here we use a process-based ecohydrological model to disentangle these pathways across the Yellow River Basin (YRB). We show that biomass increases nonlinearly with rising CO2 in YRB, approximating a logarithmic response, but is dominated by stomatal conductance-mediated regulation rather than structural change. When averaging across the whole YRB, CO2-driven biomass gains rise from 3.2 g C m(-2) at 400 ppm to 62.8 g C m(-2) at 1100 ppm, with stomatal regulation accounting for >91% of the increase and structural contributions inferred from leaf area index remaining small (<9%) across all scenarios. Spatially, the strongest biomass responses to CO2 emerge in the southeastern basin and the Hetao region, whereas muted responses occur over the Mu Us Sand Land and along the northeastern margin of the Tibetan Plateau; this pattern is largely explained by the stomatal conductance pathway, with irregular spatial heterogeneity in the structural effect. Responses also differ among ecosystem types, peaking in croplands, followed by forests, grasslands and deserts, and stomatal effects exceed structural contributions in all ecosystems. These results indicate that CO2-enhanced biomass and carbon uptake in the Yellow River Basin are governed chiefly by stomatal regulation, highlighting the need for region-specific carbon-climate assessments that explicitly represent physiological processes alongside ecosystem heterogeneity.