Optimized Land-Water Management Expands Afforestation Potential Under Land and Human Water-Use Constraints.

Afforestation is a low-cost and high-potential climate change mitigation strategy, but its feasibility remains highly uncertain because most existing assessments focus primarily on biophysical suitability and land resource constraints, while largely overlooking the critical limitation imposed by human water demand. Here, we developed an integrated framework combining machine learning, statistical modelling, the Biome-BGCMuSo model, and multi-scenario analysis to quantify China's afforestation potential under coupled land and water constraints and explore how optimized land-water management can enhance afforestation potential and associated carbon sink. We found human water demand reduced China's feasible afforestation potential from 78.13 to 40.57 Mha, with climate change causing a further 15.23%-15.91% decline. Notably, optimized land management and water-saving measures could individually increase feasible afforestation area by 6.04-7.05 Mha and 5.89-6.97 Mha, respectively. When combined, these measures would further increase afforestation potential by 14.95 and 13.66 Mha under SSP-245 and SSP-585, respectively, generating an additional carbon sink of 109.64 ± 12.08 and 125.92 ± 13.40 Tg C yr- 1 by 2060. Our findings reveal that ignoring human water demand substantially overestimates afforestation potential and identify practical pathways to expand feasible afforestation potential while minimizing land-use conflicts and safeguarding water security under future climate change.