## 4 Discussion
### The Relationship Between SIF and Tr and Their Environmental Response
The rate of CO\({}_{2}\) diffusion from the environment to the chloroplasts increases with rising temperatures. Photosynthesis involves a series of enzymatic reactions, and the activity of the Rubisco is crucial in the process of CO\({}_{2}\) fixation. Ta can affect the rate of enzymatic reactions [58]. When the temperature increases,the vapor pressure inside the leaf rises more than the vapor pressure of the surrounding air. This difference in vapor pressure between the inside and outside of the leaf enhances the transpiration rate. VPD creates a greater evaporative demand, but an increase in VPD also leads to leaf water loss, which in turn induces stomatal closure. After accounting for the covariation with PAR or Ta, VPD generally has a negative effect on SIF and Tr (Figure S4). When soil moisture is sufficient, plants can fully utilize water for transpiration to regulate leaf temperature and facilitate nutrient transport. SM and VPD collectively influence stomatal conductance and transpiration rate [59]. The increase in Ws can reduce the thickness of the vapor boundary layer around the leaves, thereby decreasing diffusion resistance and increasing the transpiration rate. At the same time, a light Ws can carry away vapor, increasing evaporative demand. However, strong winds can cause guard cells to lose water rapidly, leading to stomatal closure. After analyzing the actual observation results, it was found that when key environmental factors reached a threshold, the trends of SIF and Tr changed [21]. To prevent hydraulic failure, plants employ a range of mechanisms, including both structural and physiological adjustments [60]. When environmental conditions exceed the optimal range for vegetation, the elevated transpiration rate leads to leaf water loss. To balance water potential, plants may close stomata or increase the uptake of soil moisture. In the short term, stomatal closure is the primary mechanism plants use to limit transpiration loss. At noon, under high levels of PAR, Ta, and VPD, vegetation undergoes short-term stress, leading to a decline in both SIF and Tr. The fraction of open PSII reaction centers (qL) gradually decreases with increasing PAR, qL decreases, and the rate of photon absorption declines [61]. During thermal stress, leaves trade off the potential thermal dissipation demand with the need for water saving due to increased evaporation [62]. When water loss exceeds the threshold, the photosystem enhances photorespiration to protect photosynthetic structures. Stomatal responses to environmental factors are related to water availability and are influenced by the water supply and demand balance [63]. SM and VPD collectively determine stomatal conductance by affecting leaf water potential, with plants in dry locations having stomata that are more sensitive to changes in VPD [64]. Due to differences in hydraulic architecture, plants regulate transpiration differently to maintain water balance. Isohydric plants exhibit stricter stomatal control, while anisohydric species have weaker regulation of Tr [65]. The minimum water potential for survival is determined by the vulnerability of the hydraulic structure, and differences in structure account for the variation in water potential regulation among species [66]. Species with lower xylem embolism resistance tend to have more sensitive stomatal control [67]. In diffuse-porous species, the vessel diameters are small and densely distributed in the sapwood, contributing to greater hydraulic. In ring-porous species, the vessel diameters are large and only distributed in the sapwood of recent years, which increases the risk of embolism. Therefore, the Chinese cork oak (ring-porous) experiences a decline in Tr during midday, while poplar (diffuse-porous) exhibits a peak in Tr at noon on sunny days. The response of GPP to PAR confirms that the stomata of Chinese cork oak are more sensitive to stress than poplar [48].