## 4 Discussion
Our results indicated that heat events had the strongest correlation with midday depression compared to other climatic events. Heat stress is the primary cause of the suppression of plant photosynthesis at midday [34]. When temperatures exceed the optimal temperature for photosynthesis, they inhibit the activity of photosynthetic enzymes [35], including Rubisco [36], a key enzyme in photosynthesis. Heat stress can also damage components of the photosynthetic electron transport chain, affecting the conversion and utilization of light energy [37]. Furthermore, heat stress intensifies the process of photorespiration, a metabolic pathway that occurs under high oxygen and low CO\({}_{2}\) concentrations, consuming some of the energy and fixed carbon produced by photosynthesis [38]. Lastly, heat stress also increases the evaporation of internal water in plants, thereby reducing their water content [39]. To maintain water balance, plants close their stomata, further leading to the midday depression of photosynthesis [40]. Drought events had the weakest relationship with midday depression compared to other climatic events. Drought does not directly damage the biochemical and photochemical systems of photosynthesis, but primarily downregulates photosynthesis by increasing the diffusive resistance to CO\({}_{2}\) entry into chloroplasts [37]. Under drought conditions, guard cells on leaves shrink due to the loss of turgor pressure and close the openings of stomata to reduce water evaporation [41]. This response, while effectively reducing water loss, also limits the entry of CO\({}_{2}\), leading to a decrease in the rate of photosynthesis, and affects the carbon fixation and growth of the plant [42; 43]. In addition, stomatal closure may also activate a series of signal transduction pathways within the plant, such as the accumulation of abscisic acid, further strengthening the stomatal closure response to adapt to the water-deficient environment [44]. Midday depression in subtropical fir trees is most likely to occur during compound drought and heat events. The combination of drought and heat exacerbates their individual impact on plants [45]. Firstly, superimposed water stress leads to a more severe water shortage in plants [46; 47]. Drought reduces the moisture in soil and the atmosphere, limiting the water absorption of plants, while heat stress increases plant transpiration, further accelerating the loss of water [48]. This dual effect poses a significant challenge for plants in obtaining water and maintaining the water balance. Secondly, the superimposed temperature stress damages the cellular structure and function of plants, reducing the efficiency of photosynthesis. When drought and heat occur simultaneously, plant heat tolerance is reduced, resulting in higher susceptibility to heat damage, which further affects the energy balance within plants [49]. In addition, the complexity of physiological regulation increases the difficulty for plants to adapt to environmental stress. Faced with the dual pressures of drought and heat, plants need to regulate multiple physiological processes such as water absorption, transpiration, and photosynthesis simultaneously, which not only increases the complexity of regulation but also raises the difficulty of maintaining physiological balance [15; 50]. Therefore, compared to single drought or heat events, plants are more likely to adopt midday depression as an adaptive strategy during compound drought and heat events to reduce water transpiration, lower energy consumption, and protect themselves from heat damage, thereby increasing their chances of survival in extreme environments [51]. The current GPP model lacks comprehensive factors, such as soil nutrients and tree species, and does not account for the effects of latent and sensible heat. This oversight can lead to inaccuracies in reflecting carbon flux dynamics, particularly in capturing rapid, instantaneous changes.