Tree cover gains dominate vegetation greening and incremental differences between productivity and biomass in China’s drylands

Trees play a vital role in structuring processes of dryland ecosystems, and China’s drylands have experienced significant vegetation greening in recent decades because of large-scale forestation. However, the contributions of tree cover (TC) expansion to increases in vegetation greenness (leaf area index, LAI), productivity (gross primary production, GPP), and biomass (vegetation optical depth, VOD), along with their incremental differences, remain unclear. This study indicated that the China’s drylands revealed a significant TC increase (2.3% ± 0.3% decade⁻¹, p < 0.05) from 2001 to 2018, whereas non-tree vegetation cover (NTC, i.e., shrubs, grasses, and crops) exhibited a nonlinear shift—rising before 2010 but declining afterward. Forestation-driven TC expansion accounted for more than 75% of LAI increase throughout the study period, as well as over 70% of GPP and VOD increases pre-2010; however, TC expansion contributed to 42.6% of increase in GPP but little to VOD post-2010. Furthermore, rising GPP/LAI ratios coupled with declining VOD/LAI ratios indicated vegetation carbon sequestration enhanced but moisture content reduced per unit leaf area, and TC gains explained over half of the observed divergence between productivity enhancement and biomass accumulation. The results highlight the leading role of tree restoration in the greening of China’s drylands and the subsequent increased incremental differences between productivity and biomass, characterized by “trading water for carbon” at the leaf and canopy scales. The findings underscore the critical need to monitor both biomass distribution and moisture dynamics within the vertical structure of dryland ecosystems, particularly given the carbon–water imbalance driven by large-scale forestation efforts.