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• WUE increased by 4.45–21.49% under cloudy conditions compared with clear conditions. • DF interacted with VPD, PAR and g c to regulate WUE at evergreen coniferous forests. • The regulation of DF on WUE was less in the dry year than that in normal year. Ecosystem water use efficiency (WUE) is an indicator characterizing carbon-water coupling of the terrestrial ecosystems. The increasing diffuse radiation enhances carbon assimilation. However, the impact of diffuse radiation interacting with biophysical factors on WUE is still not well-understood. Based on carbon and water vapor fluxes measured by the eddy covariance systems during 2019–2020, we explored the effects of biophysical factors on WUE under different sky conditions at 6 plantation ecosystems (2 deciduous broad-leaved and 4 coniferous plantations) in eastern China. WUE on cloudy days was 4.5–21.5% higher than that on clear days at 6 sites, which was attributed to the increase in ecosystem carbon gain and reduction in water loss. Diffuse fraction (DF) exerted significantly positive direct effects on WUE at JZ, MQ, JY and SHB sites (p < 0.05), with the path coefficients of 0.30–0.58. However, it had positive indirect effects on WUE at HS 1 and HS 2 sites by altering photosynthetically active radiation (PAR), vapor pressure deficit (VPD) and canopy conductance (g c). WUE was primarily regulated by VPD and g c , and the path coefficients of the direct effect were -0.34−-0.73 and -0.33−-0.73, respectively. The indirect effect of VPD on WUE through stomatal behavior was remarkable. Compared with the normal year, the regulation of DF on WUE was less in the dry year. For subtropical evergreen coniferous and warm-temperate deciduous broad-leaved plantations, WUE of the dry year was higher than that of the normal year. Warm-temperate deciduous broad-leaved plantations avoided drought by reducing g c to remain conservative water-use strategy, whereas the subtropical evergreen coniferous plantation kept high g c to obtain lower WUE in the dry year. These findings will contribute to understand the influence of diffuse radiation on the coupling between carbon and water vapor fluxes. [ABSTRACT FROM AUTHOR]