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• Divide 192 months into 4 groups by PCA and CA based on 4 meteorological factors. • Any individual meteorological factor is far from adequate to determine C fluxes. • Effective accumulated temperature plays a vital role in regulating C fluxes. • Precipitation is the limiting factor in the transitional phase of growing period. • Short-term seasonal drought has significant negative impacts on C fluxes. Influences of meteorological factors on the carbon fluxes of a forest ecosystem can vary with time scales. While many studies were made on daily to annual variations of measured carbon flux of forest ecosystems, few were conducted on the variations of observed carbon fluxes over longer (decadal or more) time scale. This study quantified the influences of meteorological variables on the measured carbon fluxes in a Quercus variabilis plantation from 2006 to 2021 using principal component analysis (PCA) and cluster analysis (CA). PCA was used to identify the most important meteorological variables. They are monthly mean daily maximum global radiation (Rg), monthly effective accumulated temperature (Tac), monthly mean daily maximum vapor pressure deficit (VPD), and monthly precipitation (PP). CA was used to group 192 observed months into four groups based on those four meteorological variables and their effects on the observed carbon fluxes. Structural equation modeling (SEM) was then used to quantify the relative importance and direct or indirect effects of those four meteorological factors on carbon fluxes at the interannual timescale in each of the four groups. The results showed that air temperature had dominant influence on net ecosystem productivity (NEP) and ecosystem respiration (Re) with its relative importance measured by standardized coefficients (SC) of 0.59 and 0.97, respectively. PP directly impacted gross primary production (GPP) with an SC of 0.40 and indirectly influenced NEP and Re. The importance of a given meteorological factor on carbon fluxes and the dominant meteorological factors varied among the four groups. Tac mainly influenced Re with a SC of 0.79 in Group 1 (the nongrowing season). The direct effects of PP on NEP and GPP and indirect effects on Re were most prominent in Group 2 (the transition stage between the nongrowing season and the growing season). Tac and PP had greater influences than other two variables in Group 3 (the transitional stage of early growth and vigorous growth and the transitional stage of steady growth and dormant period). NEP increased with Tac, while excessive rainy weather accompanied by a decrease in temperature and radiation reduced Re and GPP in Group 4 (the wet season). The short-term seasonal droughts with warm air temperatures but little precipitation (mainly in June) led to significant reductions in both GPP and NEP of the Quercus variabilis plantation ecosystem in the warm-temperate continental monsoon climate zone. Our results can be used to guide managing forest plantations in temperate region in China to maximize their carbon sequestration. [ABSTRACT FROM AUTHOR]

• CWSI is first examined to detect water status of ecosystem in the humid conditions. • The CWSI dTe fail to monitor water status of cork oak plantation. • The work discussed the reason for the failure in establishment of NWSB. • The CWSI dTt could monitor water status in the plantation. • The optimum monitoring time was 13:00-14:00. The crop water stress index based on canopy–air temperature differences (CWSI dT) is an appropriate index for monitoring the water status of croplands but is rarely used to evaluate the water status of forest ecosystems. In this study, we selected a cork oak plantation in the lithoid mountain area of northern China as the research object and observed the canopy temperature (T c), actual evapotranspiration (ET a), and meteorological factors of the plantation continuously and synchronously during the period of canopy closure (May–September) in 2020 and 2021. Taking the crop water stress index based on the ratio of ET a to potential evapotranspiration (CWSI et) as a standard, the applicability of CWSI dT for detecting water status in the plantation was evaluated. The results showed that, under well-watered conditions, the canopy–air temperature differences (dT) and the canopy surface transpiration rate were both controlled by the net radiation received by the canopy surface (R nc), and the warming effect of R nc masked the cooling effect of latent heat on dT , resulting in a failure establishment of the non-water-stressed baseline in the plantation. Therefore, the empirical CWSI dT (CWSI dTe) is not suitable for evaluating the water status in this plantation. However, the theoretical CWSI dT (CWSI dTt), based on the energy balance and Penman–Monteith equation, captured daily variations in the water status in our plantation and showed a significant linear correlation with the measured relative available water content in the root zone (R2 = 0.59, P < 0.01) and CWSI et (R2 = 0.75, P < 0.01). The CWSI dTt showed an upward trend during the day, and midday (13:00−14:00) was the optimal time to measure dT for detecting the daily water status of the plantation. The CWSI dTt provides an effective, sensitive, and non-contact method of assessing daily water status changes in the cork oak plantation. [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]