Seasonal divergence in GPP- and ET-pathway regulation of ecosystem water use efficiency across vegetation types in water-limited ecosystems

Study region The Loess Plateau, China, encompassing grassland, cropland, shrubland and forest ecosystems. Study focus Ecosystem water-use efficiency (eWUE) reflects carbon–water coupling, yet how environmental effects are transmitted through the carbon (GPP) and water (ET) fluxes remains poorly resolved in water-limited regions. We addressed this across the Loess Plateau (2001–2019) at sub-kilometer resolution, combining a spatial random forest, partial-derivative decomposition and conditional probability ratio (CPR) analysis to resolve vegetation-specific controls, their GPP- and ET-mediated pathways and extreme responses. New hydrological insights for the region eWUE was highest in autumn (1.39 gC·m⁻²·mm⁻¹) and lowest and largely stable in spring (1.08 gC·m⁻²·mm⁻¹; 77.34% of pixels with non-significant trend). eWUE extremes were primarily associated with VPD and DSR in spring and summer but shrublands in summer (CPR = 2.14–3.86). Pathway attribution was driver-specific rather than a uniform seasonal switch. Environmental effects were mostly transmitted through ET ( f ET = 0.55–0.94) in spring. In summer, VPD shifted to GPP-mediation in all vegetation types ( f GPP = 0.56–0.85), whereas AW remained ET-mediated ( f ET = 0.52–0.68). DSR and PRE diverged by vegetation type, GPP-mediated in herbaceous but ET-mediated in woody ecosystems. AW switched to GPP-mediation only in autumn ( f GPP = 0.55–0.90). Our findings provide a mechanistic basis for predicting eWUE responses to climate variability.