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1.  Cloud cover affects carbon exchange between biota and the atmosphere. Recent studies have demonstrated that an increase in the diffuse radiation fraction enhances the photosynthetic efficiency of canopies. Although the exact mechanism behind this effect is not clear, a more even distribution of light among leaves across the vertical profile of the canopy is considered to be the most important cause of this difference. 2.  To test this hypothesis, the net ecosystem production (NEP) of a Norway spruce forest (30‐year‐old) was measured under cloudy and sunny skies by the eddy covariance method. In parallel, measurements of the diurnal courses of gas exchange and chlorophyll fluorescence parameters were made in the upper sun (5th whorl; 1‐year‐old needles), middle (8th and 10th whorl; 1‐ and 2‐year‐old needles) and lower shade (15th whorl; >2‐year‐old needles) shoots. 3.  The higher diffuse radiation fraction during cloudy days resulted in significantly higher ecosystem carbon uptake than at corresponding incident photosynthetic photon flux density on sunny days. Our shoot‐level data show that shoots from deep within the canopy contribute substantially to the overall carbon balance during cloudy days. But, although shade‐adapted shoots had a markedly positive carbon balance over a 24‐h period on cloudy days, their performance was impaired on sunny days contributing only a marginal or even negative carbon balance from the middle and shaded parts of the canopy. The uppermost sun shoots contributed 78% of the total carbon assimilated during a sunny day, but only 43% during a cloudy day. 4.  In addition, afternoon depression of canopy NEP and CO 2 assimilation rates of the uppermost shoots (5th and 8th whorl) occurred in response to irradiance on sunny days, characterized by significant decreases in CO 2 uptake and apparent quantum yield; however, this depression did not occur under cloudy conditions. Stomatal and non‐stomatal regulations of carbon assimilation in the afternoon are discussed.

Photosynthesis drives terrestrial carbon uptake, yet its diurnal dynamics remain poorly resolved due to the sparse availability of flux towers and the coarse spatial resolution of current satellite observations. Solar-induced chlorophyll fluorescence (SIF) provides a direct proxy of carbon uptake, but the existing global monthly mean diurnal total canopy SIF product is limited to 0.5° resolution. We developed a random forest-based downscaling framework to generate a global monthly mean hourly SIF dataset (SIFtotal_01) at 0.1° resolution for 2000–2022. When validated against eddy-covariance-based gross primary productivity (GPP) data, SIFtotal_01 showed a strong correlation (R2 = 0.81) and reduced root mean square error when compared with SIFtotal (2.89→2.8 mW m−2 nm−1), providing notable gains in broadleaved forests (R2: 0.80→0.88 with a root mean square error of 2.32→1.81 mW m−2 nm−1). The SIFtotal_01 dataset revealed a distinct double-peak in the SIFtotal_01–GPP slope, reflecting widespread afternoon depression of photosynthesis, with normalized slopes declining from 1.03 in the morning to 0.98 in the afternoon. Soil moisture modulated this depression pattern, as the afternoon–morning SIFtotal_01 difference increased from 0.02 to 0.10 mW m−2 nm−1 across dry to wet years. Under water stress, SIF yield was more sensitive than absorbed photosynthetic active radiation (APAR), with a doubling of the afternoon–morning SIF yield difference (0.5→1.1 10−3 nm−1), while the afternoon–morning APAR difference showed a smaller change (−300→−180 kJ m−2). This study improves the potential for bridging observational gaps and constraining models offer valuable insights for fundamental and applied research in the analysis of ecosystem productivity, climate-carbon feedbacks, and vegetation stress.

Monitoring drought stress is crucial for estimating productivity and assessing the health status of forest ecosystems under global climate change. Solar-induced chlorophyll fluorescence (SIF) is mechanistically coupled to photosynthesis and has advantages over greenness-based vegetation indices in detecting drought. In recent years, SIF has commonly been used in monitoring drought stress in crop ecosystems. However, the response of tower-based SIF to drought stress in forest ecosystems remains unclear. In this study, we investigated the potential of tower-based SIF to monitor drought, which was quantified using the plant water stress index (PWSI) in a Chinese cork oak plantation. The results show the negative effect of drought on SIF, and afternoon depression of SIF emission under drought stress was observed. Canopy SIF (F) exhibited a nonlinear relationship with PWSI, while the quantum yield of SIF (ΦF) exhibited a significant linear relationship with PWSI at 687 nm and 760 nm (ΦF687: R2 = 0.90; ΦF760: R2 = 0.85). Incident radiation (PAR) and canopy structure affected the response of SIF to drought stress, with PAR as the main factor causing the nonlinear relationship between F and PWSI. Afternoon depression was described as the afternoon/morning ratio (AMR). AMRF and AMRΦF exhibited a negative linear response to PWSI. AMRF was less affected than F by PAR and canopy structures, and AMRΦF was more physiologically representative than ΦF. Moreover, AMRΦF was sensitive to VPD and REW, and it might be a good indicator of drought. Red SIF was more sensitive to drought than far-red SIF, as the R2 of PWSI with AMRΦF687 (R2 = 0.89) was higher than that with AMRΦF687 (R2 = 0.84). These results highlight the potential of tower-based SIF, especially red SIF, for drought monitoring in a plantation, and consideration of the physiological diurnal variation in SIF under drought stress is crucial for improving the accuracy of drought stress monitoring.

1. Cloud cover affects carbon exchange between biota and the atmosphere. Recent studies have demonstrated that an increase in the diffuse radiation fraction enhances the photosynthetic efficiency of canopies. Although the exact mechanism behind this effect is not clear, a more even distribution of light among leaves across the vertical profile of the canopy is considered to be the most important cause of this difference. 2. To test this hypothesis, the net ecosystem production (NEP) of a Norway spruce forest (30-year-old) was measured under cloudy and sunny skies by the eddy covariance method. In parallel, measurements of the diurnal courses of gas exchange and chlorophyll fluorescence parameters were made in the upper sun (5th whorl; 1-year-old needles), middle (8th and 10th whorl; 1-and 2-year-old needles) and lower shade (15th whorl; > 2-year-old needles) shoots. 3. The higher diffuse radiation fraction during cloudy days resulted in significantly higher ecosystem carbon uptake than at corresponding incident photosynthetic photon flux density on sunny days. Our shoot-level data show that shoots from deep within the canopy contribute substantially to the overall carbon balance during cloudy days. But, although shade-adapted shoots had a markedly positive carbon balance over a 24-h period on cloudy days, their performance was impaired on sunny days contributing only a marginal or even negative carbon balance from the middle and shaded parts of the canopy. The uppermost sun shoots contributed 78% of the total carbon assimilated during a sunny day, but only 43% during a cloudy day. 4. In addition, afternoon depression of canopy NEP and CO 2 assimilation rates of the uppermost shoots (5th and 8th whorl) occurred in response to irradiance on sunny days, characterized by significant decreases in CO 2 uptake and apparent quantum yield; however, this depression did not occur under cloudy conditions. Stomatal and non-stomatal regulations of carbon assimilation in the afternoon are discussed.

Background The worldwide prevalence of type 2 diabetes mellitus in adults is experiencing a rapid increase. This study aimed to identify the factors affecting the survival of prediabetic patients using a comparison of the Cox proportional hazards model (CPH) and the Random survival forest (RSF). Method This prospective cohort study was performed on 746 prediabetics in southwest Iran. The demographic, lifestyle, and clinical data of the participants were recorded. The CPH and RSF models were used to determine the patients’ survival. Furthermore, the concordance index (C-index) and time-dependent receiver operating characteristic (ROC) curve were employed to compare the performance of the Cox proportional hazards (CPH) model and the random survival forest (RSF) model. Results The 5-year cumulative T2DM incidence was 12.73%. Based on the results of the CPH model, NAFLD (HR = 1.74, 95% CI: 1.06, 2.85), FBS (HR = 1.008, 95% CI: 1.005, 1.012) and increased abdominal fat (HR = 1.02, 95% CI: 1.01, 1.04) were directly associated with diabetes occurrence in prediabetic patients. The RSF model suggests that factors including FBS, waist circumference, depression, NAFLD, afternoon sleep, and female gender are the most important variables that predict diabetes. The C-index indicated that the RSF model has a higher percentage of agreement than the CPH model, and in the weighted Brier Score index, the RSF model had less error than the Kaplan-Meier and CPH model. Conclusion Our findings show that the incidence of diabetes was alarmingly high in Iran. The results suggested that several demographic and clinical factors are associated with diabetes occurrence in prediabetic patients. The high-risk population needs special measures for screening and care programs.