## 3 Results  
We integrated the PAR data from the Himawari 8 meteorological satellite with ERA5-land meteorological data to calculate the diurnal variations in GPP for the fir forest in Huitong from 2016 to 2022. Our results were compared with those obtained from the flux tower measurements. The fitting performance over the seven-year period was satisfactory, with an R\({}^{2}\) value of 0.605 for the satellite data and 0.631 for the flux tower data, indicating that geostationary satellite data can be effectively utilized to identify the midday depression in plant photosynthesis (Figure 2). Between 2016 and 2022, seasonal trends in GPP (measured every half-hour), temperature, and precipitation exhibited similar patterns across the years (Figure 3). GPP values typically reached their annual nadir in January, then increased gradually, reaching their zenith in the months of July through September, after which they began to decrease (Figure 3a). The maximum annual GPP was recorded in 2017. Temperatures reached their zenith during June to August each year, with heatwaves predominantly occurring in this period (Figure 3b). The fir forest in Huitong experienced its highest precipitation during June to July, with a notable decrease during the winter (Figure 3c). The average daily temperatures during heat, drought, as well as compound drought and heat events were 22.6 \({}^{\circ}\)C, 17.5 \({}^{\circ}\)C, and 23.0 \({}^{\circ}\)C, respectively (Figure 3d). The average daily precipitation during heat events was 3.0 mm, followed by 0.8 mm during drought, and the least at 0.5 mm during compound drought and heat events (Figure 3e). Figure 4 shows the number of midday depression occurrences at the Huitong fir forest site across different years, months, and seasons. The year 2022 had the highest number of occurrences with 126, followed by 2020 with 119, and 2016 with 113 times (Figure 4a). The year with the fewest occurrences was 2019, with 89 times. From 2018 to 2022, there  
was an overall increasing trend in the annual count of midday depression. The number of occurrences exhibited a unimodal pattern, with the fewest times in January, a gradual increase to a peak in August, and a subsequent decline (Figure 4b). Notably, the midday depression was most frequent during July, August, September, and October, each with more than 10 occurrences. The fir forest in this region experienced a higher incidence of midday depression in the summer and autumn, with 48 and 34 occurrences, respectively, and fewer events in the spring and winter seasons. Figure 3: Observed variables at the Huitong fir forest site from 2016 to 2022. (**a**) GPP value, (**b**) air temperature, (**c**) precipitation. (**d**) Average daily temperature and (**e**) precipitation during climatic events. In (**d**,**e**), 1 represents average daily temperature or precipitation during heat events, 2 indicates drought events, 3 indicates compound events, and Ave. indicates overall climatic event averages. Figure 2: Comparison of GPP estimates with measured values of the flux tower between 2016 and 2022. The number of available samples was 14633. Each sample represents a half-hourly observation. (**a**) Prediction using observed climate data from the flux tower. The black dashed line is the data fit line. The red solid line is the 45\({}^{\circ}\) tangent. The color intensity of the right bar represents the density of data points. Unit for vertical coordinates is g C m\({}^{-2}\) h\({}^{-1}\). (**b**) Residual distribution, RMSE, and MAE corresponding to (**a**). (**c**) Prediction using the satellite data. (**d**) Residual distribution, RMSE, and MAE corresponding to (**c**). The mean annual occurrences of midday depression at Huitong fir forest from 2016 to 2022 demonstrated a significant positive correlation with the mean annual occurrences of heat, drought, and compound heat and drought events (Figure 5). Specifically, the number of midday depression occurrences in photosynthesis increased with the frequency of heat events (R\({}^{2}\) = 0.92, \(p\) = 0. 004; Figure 5a).