Energy Partitioning is Linked to Cloudiness in Plantation Forests in Eastern China

Cloudiness influences gross primary productivity and evapotranspiration in terrestrial ecosystems by increasing diffuse fraction (DF) and improving the photosynthesis of shaded leaves. However, the effect of cloudiness on energy partitioning of 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 cloudy sky conditions was investigated at 6 plantation ecosystems (2 deciduous broadleaf and 4 coniferous plantations) in eastern China from 2019 to 2021. The average seasonal evaporative fraction (EF) (0.37–0.52) during the growing season was primarily controlled by precipitation and vegetation growth. Compared with coniferous plantations, net radiation (Rn) allocated more LE to deciduous broadleaf plantations. The EF values increased by 11−50% on cloudy days, which was attributed to the less decrease in LE than H. On the half-hourly scale, EF was mainly driven by relative extractable soil water (REW), the direct effect was 0.41−0.63. Except for HS1 and HS2 sites, DF exerted a positive direct effect on EF at other sites, with a standardized regression coefficient of 0.05–0.47. DF had an indirect effect on EF through Rn at JZ, JY, HS1 and HS2 sites. Moreover, the positive effects of cloudy sky conditions on Priestley-Taylor coefficient (α) and EF were more sensitive under low temperature (Ta), vapor pressure deficit (VPD) and REW conditions. The increment of EF on cloudy days significantly decreased with the increasing normalized difference vegetation index (NDVI), which may be explained by higher Ta and VPD during the mid-growing season. Cloudiness can also influence energy partitioning by regulating stomatal behavior in plantation ecosystems. These results contribute to understanding the driving mechanisms for energy partitioning in response to cloudiness in coniferous and deciduous broad-leaved plantations.