<p>China is at the forefront of global greening, with afforestation in karst-dominated areas making major contributions. These regions are characterized by inherently low soil volume due to hydrogeological properties of carbonate bedrocks that promote soil erosion, together with nutrient limitations despite generally adequate nutrient concentrations in plant-accessible soil. However, the limitations of low soil volume on nutrient storage and vegetation growth potential were majorly discussed on a local scale, lacking a comprehensive regional evaluation in karst-dominated areas. To investigate the relationships among vegetation growth potential, soil volume, and nutrient concentrations, we compiled a dataset of more than 500 ground survey records across climate conditions and vegetation types in the karst regions in Southwest China. XGBoost model and network analysis were applied to reveal the variations in the correlations. Our results showed that the contribution of soil volume outweighed climate factors and soil nutrient concentrations, accounting for up to ∼70% of the spatial variations in vegetation growth potential. A 57.5-cm threshold of soil thickness was specified for the impact of reduced soil volume, where the limiting factors of vegetation shifted from soil nutrient concentrations to soil volume. The climate correlations were also strengthened under low soil volume conditions. Our work identifies limited soil volume as the principal constraint on karst vegetation growth potential, thereby highlighting it as a paramount challenge hindering the sustainability of greening efforts amid the progressive expansion of soil erosion and rocky desertification.</p>