Global patterns and trends of vegetation water use efficiency inferred from solar-induced chlorophyll fluorescence from 2001 to 2020

Abstract

This study introduces a novel satellite-based framework that integrates solar-induced chlorophyll fluorescence (SIF) with plant physiological theory to simultaneously estimate gross primary productivity (GPP) and transpiration (T_r). This integrated approach enables global assessment of vegetation water use efficiency (WUE_T = GPP/T_r) and ecosystem water use efficiency (WUE = GPP/evapotranspiration) from 2001 to 2020. Site-scale validation using flux tower data shows strong agreement for WUE (R² = 0.615) and WUE_T (R² = 0.573). Globally, WUE_T increased from 2001 to 2010 due to CO₂ fertilization but stabilized from 2011 to 2020 as rising vapor pressure deficit (VPD) offset physiological gains. In contrast, WUE continued to rise driven by increased T_r/ET linked to vegetation greening (R² = 0.776 with LAI). Attribution analysis shows that CO₂ and VPD together explained over 70 % of interannual variation in WUE_T. Future projections using CMIP6 models indicate that WUE_T will closely follow changes in the CO₂-to-VPD ratio (C_a/VPD), while WUE is expected to increase through 2100 under high-emission scenarios (SSP370 and SSP585), supported by continued CO₂ rise and LAI-driven increases in T_r/ET. These results highlight the utility of SIF for large-scale carbon–water coupling assessment and underscore the need to jointly consider atmospheric dryness and vegetation structure in evaluating ecosystem responses to climate change.