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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]

• GPP and ER of needleleaf plantations were more than those of broadleaf plantations. • NEP of broadleaf stands was consistent with that of needleleaf stands. • Annual NEP, GPP and ER decreased with latitude. • NEP was regulated by MAT and LAI, and GPP was correlated with AP, LAI and MAT. Plantation has large carbon sequestration potential, and it plays an important role in mitigating global warming. However, the responses of carbon fluxes to biophysical factors across plantation sites are still not clear. We synthesized carbon flux data measured by the eddy covariance method over the plantations at the global scale to explore the shifts of carbon exchange with latitude, discuss the link of carbon fluxes with biophysical variables, and compare the difference in carbon sequestration between needleleaf and broadleaf plantations. Annual net ecosystem production (NEP), gross primary production (GPP) and ecosystem respiration (ER) across all plantations were 353±27, 1762±60 and 1385±46 g C m−2 yr−1, respectively. The mean annual NEP of needleleaf biomes was similar to that of broadleaf biomes. GPP and ER were 48% and 64% larger at needleleaf forests than at broadleaf forests. Annual NEP, GPP and ER decreased with the increase of latitude significantly. At the half-hourly scale, the increase of diffuse radiation enhanced carbon assimilation of needleleaf and broadleaf biomes remarkably. On the annual scale, both GPP and NEP were sensitive to mean annual temperature (MAT) and leaf area index (LAI). Annual precipitation was the dominant factor regulating the variability of GPP and ER, and it explained 34% and 47% of the variation in GPP and ER, respectively. The extension in growing season length (GSL) and rising soil water content (SWC) enhanced ecosystem photosynthesis and respiration of planted forests. However, the effect of GSL and SWC on net carbon uptake was not remarkable. These results highlight the importance of biophysical factors in regulating carbon dynamics of the plantations, and contribute to understanding of the differences in carbon sequestration between needleleaf and broadleaf biomes. [ABSTRACT FROM AUTHOR]

Croplands play a vital role in regional carbon budgets. We hypothesized that biophysical factors would be important for soil respiration in a wheat-maize rotation cropping system. Soil CO 2 efflux was measured using the closed chamber method, and net CO 2 exchange between the cropland and the atmosphere obtained by the eddy covariance technique in a winter wheat-summer maize double cropping system over four years (Oct 2002–Oct 2006). In addition to soil temperature, soil respiration was controlled by leaf area index and soil moisture in the wheat field and soil moisture in the maize field. Temperature sensitivity ( Q 10 ) of soil respiration was 2.2 in the wheat and maize growing seasons. In the wheat field, the Q 10 value during the sowing–returning green period (4.9) was more than that during the returning green–ripening period (2.0). On a monthly time scale, soil respiration was controlled by gross primary productivity in the wheat field, indicating that soil respiration was coupled with ecosystem photosynthesis. Annual soil respiration was 825 ± 73g C m −2 in the wheat–maize rotation system in 2003–2006. Over a 4–year average, soil respiration was 355 ± 50 g C m −2 in the wheat growing season and 470 ± 67 g C m −2 in the maize growing season, which accounted for 43% and 57% of the annual value respectively. At an annual time scale, soil respiration contributed to 72% of ecosystem respiration in the winter wheat–summer maize double cropping system. [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]

• iWUE of the young stand was higher than that of the old stand. • Trees at the more precipitation site had the stronger resistance to drought. • SPEI had a significant effect on iWUE at the young stand. As one of the main afforestation species, Pinus tabulaeformis is an important carbon sink in China. We developed tree-ring chronologies of Pinus tabulaeformis at five plantation sites (Lingkong Mountain (LKM), Fujia (FJ), Heishui (HS), Caijiachuan (CJC) and Jiufeng (JF)), and measured the δ13C values of tree rings using the stable carbon isotope method to calculate intrinsic water use efficiency (iWUE). The responses of net primary productivity (NPP) and iWUE to climatic variables were explored. The annual variation in both NPP and iWUE had increasing trends, with lower iWUE increasing rate at LKM site in comparison with those at other 4 sites. iWUE of the young stand was higher than that of the old stand. Annual patterns of the δ13C values showed significant decreasing trends in LKM and CJC but a rising trend in HS. The growth of Pinus tabulaeformis was more sensitive to temperatures, and temperatures exerted positive effects on NPP and iWUE. At the old stands, both NPP and iWUE had closer relationships with temperatures. On the annual scale, precipitation had little effect on NPP and iWUE. Annual mean standardized precipitation evaporation index had a significant influence on iWUE at the young stand. Resistance was positively correlated with recovery, resilience and iWUE, but negatively correlated with NPP. Pinus tabulaeformis at CJC site had the stronger resistance to drought events in comparison with that at LKM and FJ sites. This study provides insight into the responses of Pinus tabulaeformis productivity and water use to climate change, and helps evaluate carbon sink and water-carbon trade-off of plantations. [ABSTRACT FROM AUTHOR]

• NIRv and SIF were more effective at estimating phenological information than NDVI and EVI. • SOS and EOS played important roles in determining seasonal variabilities of GPP. • The variation of annual GPP could be captured by GSL GPP × GPP max , GSL NIRv × NIRv max , and GSL SIF × SIF max effectively. Phenology plays an important role in affecting carbon sequestration in terrestrial ecosystems in the context of climate change. Remote sensing techniques have been widely used to investigate land surface phenology and the effects of phenology on ecosystem production at regional and global scales. Recently, the near-infrared reflectance of vegetation (NIRv) and solar-induced chlorophyll fluorescence (SIF) have been shown to be more promising metrics of gross primary production (GPP) than the normalized difference vegetation index (NDVI) and the enhanced vegetation index (EVI). However, there is a lack of comparison in the performance of these techniques for deriving phenological metrics. In this study, we explored the consistency in phenological metrics derived from both remote sensing approaches (NDVI, EVI, NIRv, and SIF) and flux tower GPP at six plantations (two broadleaf forests (BF) and four coniferous forests (CF)) in eastern China over the period 2006–2020. The vegetation indices (NDVI, EVI, NIRv) were derived from MODIS data, and SIF was based on the global, OCO-2 based SIF product (GOSIF). We further evaluated the effects of spring and autumn phenology on GPP. The results showed that the flux tower GPP was effectively tracked by NDVI, EVI, NIRv, and SIF (P < 0.001). Meanwhile, the phenological metrics derived from EVI, NIRv, and SIF, including the start of growing season (SOS), the end of growing season (EOS), and the length of growing season (GSL), had significant relationships with those derived from GPP at the six plantations (P < 0.05). NIRv and SIF were more effective at estimating phenological information than NDVI and EVI. In addition, the root mean squared deviation (RMSD) values between the GPP- and NIRv-retrieved phenological dates were less than those derived from NDVI, EVI, and SIF at the BF sites. However, the differences among RMSD values of NDVI, EVI, NIRv, and SIF were not significant at the CF sites. The linear regression analysis showed that the advance of SOS GPP (i.e., SOS derived from GPP) significantly increased GPP (R2=0.29, P < 0.05) over the period from March to April, and the delay of EOS GPP (i.e., EOS derived from GPP) remarkably enhanced GPP (R2=0.61, P < 0.001) over the period from September to October at the BF sites. The relationship of EOS GPP with GPP (R2=0.90, P < 0.05) over the period September-October was strong at the CF sites. In addition, the variations of annual GPP could be captured by GSL GPP × GPP max , GSL NIRv × NIRv max , and GSL SIF × SIF max effectively across the BF and CF sites. These findings can help us understand the potential ability of NIRv and SIF in estimating phenological metrics and in revealing the effects of vegetation phenology on the carbon cycle. [ABSTRACT FROM AUTHOR]

• Diffuse fraction (k d) was important factor for affecting light use efficiency (LUE). • LUE increased linearly with increasing k d , with a slope of about 0.326 g C mol−1. • Gross primary production was maximized when k d was about 0.53. • The Boland model was the best for modeling of daily k d in the North China Plain. The increase of diffuse radiation fraction (k d) has been reported to significantly impact light use efficiency (LUE) and carbon uptake in terrestrial ecosystems. The impact of k d on LUE should be considered in crop models to accurately evaluate the effect of radiation changes on crop production. However, the magnitude of the k d effect is difficult to quantify because of the complicated interacting relationships among all of the meteorological parameters, as well as the changing effects for various ecosystem types and research sites. Eight site-years of flux data and two years of diffuse radiation data from two field ecosystems in the North China Plain were used to (1) compare the performance of five k d models, (2) explore the impacts of environmental factors on LUE and gross primary production (GPP) of winter wheat (Triticum aestivum L.), and (3) quantify the relationships between k d and both LUE and GPP of winter wheat. Comparison results showed that the k d model developed by Boland et al. performed the best of the five models evaluated. This model was chosen to calculate k d in this research. Path analysis show that k d was the main factor affecting LUE of winter wheat, explaining up to 55% of the variability in LUE. The relationship between k d and LUE was significantly linear (slope of about 0.326 g C mol−1). GPP initially increased and then decreased with increasing k d. A moderate radiation condition (k d = 0.53) was favorable for increasing GPP. The effect of k d on LUE should be added in the LUE module of APSIM-Nwheat to improve simulation accuracy. The results of this study highlight the importance of k d in correctly modeling LUE for winter wheat with a crop model and provide quantitative relationships between these two parameters. These relationships will be helpful in improving crop model simulation accuracy under changed climate conditions. [ABSTRACT FROM AUTHOR]