## 3 Results
### Impact of Environmental Factors on SIF, Tr, and the SIF-Tr Relationship  
At the half-hour scale, SIF and Tr were more susceptible to the influence of environmental factors (Figure 6). After aggregating the temporal scale from half-hour to daily, the positive correlation between PAR and both SIF and Tr slightly weakened. In contrast, the positive correlation between Ta and both SIF and Tr strengthened for Chinese cork oak and poplar. VPD was positively correlated with both SIF and Tr, and this correlation decreased or became insignificant after temporal aggregation. Another parameter characterizing vegetation moisture availability, SM showed an enhanced positive effect on Tr and SIF after temporal aggregation. To determine the influence of key environmental factors PAR, VPD, SM, and Ta on SIF and Tr, we aggregated the half-hour scale data based on these factors. With the increase in PAR, both SIF and Tr in Chinese cork oak and poplar showed an increasing trend (Figure 7a,e). Chinese cork oak SIF increased more rapidly at high PAR levels, while poplar SIF showed greater enhancement at low PAR. The increasing rate changes in PAR by Tr in Chinese cork oak and poplar were opposite to those of SIF. With the increase in PAR, both SIF and Tr of arborvitae initially rose and then decline, with the SIF threshold appearing slightly earlier (Figure 7a,e). The response of Chinese cork oak's SIF and Tr to Ta showed differences; after reaching the threshold, SIF declines while Tr continued to increase, albeit at a reduced rate. As Ta increases, both SIF and Tr in poplar showed an increasing trend, but the rate of increase changes. SIF and Tr of arborvitae showed a trend of increasing first and then decreasing with the rise in Ta (Figure 7b,f). With the increase in VPD, SIF and Tr in the three plantations showed an initial rise followed by a trend of slowly increasing or decreasing, with the threshold occurring 1-2 kPa (Figure 7c,g). The impact of SM on SIF and Tr varied significantly across different plantations. For Chinese cork oak, SIF and Tr were higher when SM was at an intermediate level. Both SIF and Tr in poplar initially increased with rising SM and then decreased. As SM increases, the SIF and Tr in arborvitae rose (Figure 7d,h). Figure 6: Pearson correlation coefficients between SIF, Tr, and various environmental factors in Chinese cork oak (**a**), poplar (**b**), and arborvitae (**c**) plantations at half-hour and daily scales. Environmental factors include PAR, Ta, SM, VPD, and wind speed (Ws). Blue indicates a positive correlation; red indicates a negative correlation; and a flatter ellipse indicates a stronger correlation. *—significant correlation at \(p<0.05\) and **—significant correlation at \(p<0.01\). As PAR increased, the relationship between SIF and Tr (SIF-Tr) in Chinese cork oak and poplar initially showed a negative correlation, which gradually switched to a positive correlation. A decline in the SIF-Tr relationship occurred when PAR reached 1500 \(\upmu\)mol\(\cdot\)m\({}^{-2}\)\(\cdot\)s\({}^{-1}\) (Figure 8a,b). The SIF-Tr relationship in arborvitae was positively correlated at various levels of PAR, and it showed an increasing trend as PAR increased (Figure 8c). With PAR increasing, the trend of Tr/SIF was essentially opposite to that of the relationship (Figure 8a-c). As Ta raised, the SIF-Tr relationship of Chinese cork oak and poplar showed a trend of increasing initially and then decreasing, with an inflection point at approximately 26 \({}^{\circ}\)C (Figure 8d,e). The SIF-Tr relationship of arborvitae exhibited a decreasing trend (Figure 8f). When 1 kPa \(\leq\) VPD \(<\) 2 kPa, the SIF-Tr relationship was relatively high, while 2 kPa \(\leq\) VPD \(<\) 3 kPa, the SIF-Tr relationship showed a decreasing trend. As the meteorological drought intensified, the SIF\(-\)Tr relationship showed a gradual increase. When VPD \(>\) 4.5 kPa, the SIF-Tr relationship of Chinese cork oak decreased.