## 4 Discussion  
We used high-frequency data from geostationary orbit satellites to capture the daily fluctuations in GPP, which serve as indicators of the diurnal patterns of photosynthesis in subtropical Chinese fir forest ecosystems. By comparing this satellite-based estimation method with field measurements from flux towers, its accuracy has been validated [25]. This level of consistency is crucial for subsequent analyses because it ensures that the diurnal variation patterns attributed to the midday depression phenomenon are not artifacts of the estimation process, but rather, reflections of actual physiological processes occurring within the forest stand [26]. Our results suggest that using ten minutes resolution geostationary satellite data to estimate GPP and identify midday depression would lead to a better understanding of the diurnal variation of photosynthetic rates and their response to different climatic events at regional and continental scales [20]. Compared to the most commonly used methods that estimate photosynthetic rates based on ecological models, our methodological framework can provide explanations based on stomatal physiology. Subsequently, we identified the midday depression of photosynthesis in the Huitong fir forest from 2016 to 2022 and found significant changes in the annual occurrence of this phenomenon. The highest number of midday depression occurrences was recorded in 2022 with 126, followed by 119 in 2020, and 113 in 2016. The intensification of midday depression in forest photosynthesis in 2022 may be related to the unprecedented high temperatures experienced during the summer of that year. The average temperature in the region during this summer reached 29.1 \({}^{\circ}\)C, with an average of 58.2 days of temperatures exceeding 35 \({}^{\circ}\)C, ranking first in historical temperature records since 1961 [27]. High temperatures directly affect the stomatal conductance and photosynthetic capacity of plants, leading to a reduction in carbon assimilation at midday. The drought conditions throughout 2020 also exacerbated the scarcity of water resources, with the region receiving only 852.5 mm of precipitation from April to December, which was 22.9% less than the average precipitation for this period, further putting pressure on plants and possibly contributing to the increase in observed midday depression [28; 29]. Similarly, the drought in autumn of 2016 may also be the main factor inducing midday depression in fir trees [30]. These drought events intensified the scarcity of water resources, bringing additional stress to plants, and may have contributed to the observed increases in midday depression. From July to October each year, the occurrence of midday depression in the Huitong fir forest showed a clear peak. In the summer, increased solar radiation and higher temperatures may have led to an increase in transpiration rates [31]. If soil moisture is insufficient to meet the water needs of plants, this could trigger midday depression as a protective mechanism to prevent excessive water loss and potential damage to photosynthetic organs [32]. In contrast, the number of midday depression occurrences was lower during the colder seasons of spring and winter. The lower temperatures during these periods led to reduced transpiration and evaporation rates, which may have alleviated some of the water  
Figure 6: Probability of midday depression being concurrently triggered by climatic events between 2016 and 2022. HT event, D event, and C event represent heat events, drought events, and composite events (compound drought and heat), respectively. stress that causes stomata to close at midday. Additionally, the physiological activity of plants may be inherently lower during the colder months, which could also be a reason for the observed decrease in the number of midday depression occurrences in trees [33]. We further explored the relationship between heat, drought, and their combined events with the occurrence of midday depression from 2016 to 2022.