## 4 Discussion
### Effect of Meteorological Factors on WUEs of Larch Forest at Different Canopy Heights
Comparative analysis showed that VPD and RH contributed the most to WUE\({}_{\rm s}\) at different canopy heights, implying that under future climate change conditions, cold-temperate zones with higher Ta and lower RH [69] will cause a gradual increase in VPD at different canopy heights, which may lead to a gradual rise in larch-forest WUE. Effect of Photosynthetic Factors on WUE\({}_{\rm s}\) of Larch Forest at Different Canopy Heights  
In this study, Pn was found to be an important factor influencing the WUE\({}_{\rm s}\) of larch-forest leaves at different canopy heights. Moreover, compared with other photosynthetic factors, the contribution rate of Pn to WUE\({}_{\rm s}\) at different canopy heights is higher, which is consistent with the results of [PERSON] et al. [47] and [PERSON] et al. [70]. It was explained that this is because the changes in plant Pn and Gs directly affect plant WUE, whereas Tr and C\({}_{\rm i}\)/C\({}_{\rm a}\) indirectly affect WUE\({}_{\rm s}\)[71]. In this study, the changes of Pn, Gs, Tr and C\({}_{\rm i}\)/C\({}_{\rm a}\) increased with canopy, and there are significant differences between the canopies. This indicates that layer Gs is not limited by meteorological factors or the carbon absorption rate of each layer. At the same time, the magnitude of Gs, Tr and C\({}_{\rm i}\)/C\({}_{\rm a}\) was less than that of Pn [44], which is the reason why Gs and Tr contributed less than Pn in the results of the random forest analysis. However, the effect of C\({}_{\rm i}\)/C\({}_{\rm a}\) on larch-forest WUE\({}_{\rm s}\) at different canopy heights shown in this study was slightly different from the results of [PERSON] et al. [72] and [PERSON] et al. [73]. The upper-canopy C\({}_{\rm i}\)/C\({}_{\rm a}\) importance was lower than those of the lower and middle canopies. One possible explanation for this might be that atmospheric CO\({}_{2}\) concentrations were in a relatively stable state at different canopy heights [74], with higher C\({}_{\rm i}\)/C\({}_{\rm a}\) in the upper canopy than in the lower and middle canopy, resulting in greater stomatal limitation of carbon gain and reduced intercellular CO\({}_{2}\) molar fraction (C\({}_{\rm i}\)) in the upper canopy than in the lower and middle canopy. Another explanation is that the gradual decrease in C\({}_{\rm i}\) photosynthesis is increasingly restricted by Rubisco enzymes in the canopy, which means that the lower-middle canopy is less restricted by Rubisco enzymes than the upper canopy, resulting in a higher C\({}_{\rm i}\)/C\({}_{\rm a}\) contribution compared with the lower-middle canopy. The above analysis shows that the pattern of changes in photosynthetic factors Pn, Gs, Tr and C\({}_{\rm i}\)/C\({}_{\rm a}\) at different canopy heights is consistent with the pattern of the leaf WUE\({}_{\rm s}\) at different canopy heights. The contribution of Pn to WUE\({}_{\rm s}\) at different canopy heights is the largest, which means that changes in Pn at different canopy heights in the cold-temperate zone significantly affect the WUE of the larch forest. This means that changes in Pn at different canopy heights in the cold-temperate zone significantly affect the WUE of the larch forest, while factors that influence changes in Pn also affect the WUE of the larch forest.