## 4 Discussion
### Effect of Meteorological Factors on WUEs of Larch Forest at Different Canopy Heights  
Our study showed that VPD was the main factor affecting the WUE\({}_{\rm s}\) of leaves in a larch forest at different canopy heights, which was similar to the results of [PERSON] et al. [56] and [PERSON] et al. [57]. This can be explained by the fact that the fewer short-term changes in LAI, the relative stability of the ratio of transpiration (Tr) to ET [46] and changes in VPD directly affect changes in Gs that in turn affect WUE\({}_{\rm s}\)[23]. When VPD increases with the canopy height, Gs and Tr also increased, which causes lower water potential in plant leaf cells and stems. In contrast to the restrictions on carbon absorption rates [47], VPD has a much greater impact on Gs. In this case, the effect of VPD on leaf WUE\({}_{\rm s}\) at different canopy heights plays a dominant role. However, slightly different from the results of [PERSON] et al. [13] and [PERSON] et al. [58], VPD ranked first among the factors affecting WUE\({}_{\rm s}\) in this study, which may be explained by the fact that greater variation in temperature and humidity at different canopy heights of larch forests of the zonal area caused a more direct effect of VPD on Gs and Pn than on other factors [36]. Therefore, VPD is the dominant factor affecting WUE\({}_{\rm s}\) in larch forests. RH was found to be a secondary factor affecting leaf WUE\({}_{\rm s}\) at different canopy heights of larch forests, which is in accord with the results of [PERSON] et al. [59] and [PERSON] et al. [60]. This is because RH is the main determinant of atmospheric water potential and has a significantly negative correlation with VPD, and thus changes in RH directly affect changes in VPD at different heights. RH also strongly determined leaf Gs [61]. On the one hand, high RH leads to a decrease in atmospheric evapotranspiration capacity; on the other hand, low RH can increase Gs [62] that rises when the indirect positive effect exceeds the negative effect. In this study, RH performance at different heights decreased with the increase in canopy height, and RH was contrary to the law of change in VPD and Gs, which also indicates that the change in RH at different canopy heights of larch forests led to changes in VPD and Gs, thus affecting the changes in WUE\({}_{\rm s}\) at different canopy heights. The contribution of Ta to WUE\({}_{\rm s}\) at different canopy heights of a larch forest washigher in the middle and lower than in the upper canopy, which is similar to the results of [PERSON] et al. [63]. This is because Ta can directly affect Pn and Gs, and thus affect WUE\({}_{\rm s}\)[64; 65]. Ta was significantly different across canopy heights, possibly with this difference [66] resulting in an inconsistent contribution of Ta to the WUE of different layers. Since changes in the light conditions at different canopy heights lead to the significant variations of light effect on chlorophyll, phototaxis and photosynthesis carboxylase activity of each layer, the composition of plant stable carbon isotopes varies significantly accordingly, and thus affects WUE\({}_{\rm s}\)[67]. It was shown in a previous study that Pn and Gs increase with PAR within a certain range [68]. In this study, the upper canopy PAR is significantly higher than the middle and lower canopy. Simultaneously, the upper layer Pn and Gs both were significantly higher than in the middle and lower layer, which may be the reason why the contribution of PAR is higher in the upper canopy than in the middle and lower canopy. Our study suggests that the patterns of VPD, Ta and Ra at different canopy heights are consistent with that of the leaf WUE\({}_{\rm s}\) at different canopy heights, while RH is the opposite.