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• Evaporative fraction increased by 16–49 % on cloudy days compared with clear days. • Clearness index controlled evaporative fraction through direct and indirect paths. • High NDVI reduced evaporative fraction response to cloudiness. Cloudiness influences gross primary productivity and evapotranspiration in terrestrial ecosystems by increasing diffuse radiation and improving the photosynthesis of shaded leaves. However, the effect of cloudiness on energy partitioning in terrestrial ecosystems is not well understood. Using latent heat (LE) and sensible heat (H) datasets observed by the eddy covariance systems and meteorological variables, the regulation of biophysical factors on energy partitioning under different sky conditions was investigated at 6 plantation ecosystems (2 deciduous broadleaf and 4 coniferous plantations) in eastern China from 2019 to 2021. The averages of evaporative fraction (EF) during the growing season were 0.34±0.06–0.50±0.04. The EF values increased by 16−49% on cloudy days, which was attributed to the lower decrease in LE compared to H. On the half-hourly scale, EF was mainly driven by air temperature (T a), relative extractable soil water (REW) and clearness index (CI) (i.e., a metric to quantify cloudiness). For plantation ecosystems, CI had an indirect effect on EF mainly through net radiation (R n). Moreover, the positive effect of cloudy sky conditions on EF was higher under low T a , vapor pressure deficit (VPD) and REW conditions. The increment of EF on cloudy days was weaker at high normalized difference vegetation index, which may be explained by higher T a and VPD limiting canopy transpiration during the mid-growing season. Cloudiness can also influence energy partitioning by regulating stomatal behavior in plantation ecosystems. These results contribute to understanding the drivers of energy partitioning in response to cloudiness in plantation ecosystems. [ABSTRACT FROM AUTHOR]

• 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]