## 3 Results  
### WUE\({}_{\rm s}\) at Different Canopy Heights of Larch Forest  
As shown in Table 2, the daily differences in atmospheric \(\rm{\delta^{13}C}\), leaf soluble sugar \(\rm{\delta^{13}C}\) and WUE\({}_{\rm s}\) were small over the 3-day period. The variation of WUE\({}_{\rm s}\) at different canopy heights of larch forest ranged from 3.89 to 4.34 mmol-mol\({}^{-1}\), with WUE\({}_{\rm s}\) increasing with canopy height. The atmospheric \(\rm{\delta^{13}C}\), leaf soluble sugar \(\rm{\delta^{13}C}\) and WUE\({}_{\rm s}\) in the upper canopy were significantly higher than those in the middle and lower canopy (\(p<0.01\)), while the difference between the middle and lower canopy was not significant (\(p>0.05\)). The variation of atmospheric \(\rm{\delta^{13}C}\) and leaf soluble sugar \(\rm{\delta^{13}C}\) at different heights of larch forest ranged from \(-8.97\) to \(-9.26\) and \(-29.07\) to \(-29.95\), respectively. Both atmospheric \(\rm{\delta^{13}C}\) and leaf soluble sugar \(\rm{\delta^{13}C}\) showed the same ranking as WUE\({}_{\rm s}\): upper canopy \(>\) middle canopy textgreater lower canopy, and that the upper canopy leaf soluble sugar \(\rm{\delta^{13}C}\) was significantly higher than those in the middle canopy and lower canopy (\(p<0.01\)). However, the difference between the middle canopy and the lower canopy was not significant (\(p>0.05\)). highly significantly positively correlated with upper and middle PAR (\(p<0.01\)) and significantly positively correlated with lower PAR (\(p<0.05\)). This suggests that the WUE\({}_{\text{s}}\) of larch forest at different canopy heights is largely influenced by meteorological factors at different canopy heights, especially VPD and RH. Effect of Photosynthetic Parameters on WUE\({}_{\text{s}}\) at Different Canopy Heights of Larch-Forest Canopy  
As shown in Figure 4, the variation of photosynthetic parameters Pn, Tr, Gs and C\({}_{\text{i}}\)/C\({}_{\text{a}}\) at different canopy heights of larch forest showed that the upper canopy \(>\) the middle canopy \(>\) the lower canopy. Pn varied between 6.21 to 8.71 \(\upmu\)mol CO\({}_{2}\)m\({}^{-2}\)s\({}^{-1}\) and showed a highly significant (\(p<0.01\)) difference with the upper canopy and the middle and lower canopy, while the difference between the middle and lower canopy was not significant (\(p>0.05\)). Tr ranged from 1.01 to 1.38 mmol H\({}_{2}\)Om\({}^{-2}\)s\({}^{-1}\), showing nonsignificant differences between the upper and middle canopy. Gs ranged from 0.07 to 0.17 mol H\({}_{2}\)Om\({}^{-2}\)s\({}^{-1}\), showing significantly higher differences in the upper canopy than in the middle and lower canopy (\(p<0.05\)). C\({}_{\text{i}}\)/C\({}_{\text{a}}\) ranged from 0.63 to 0.75, but the intercanopy differences were not significant (\(p>0.05\)). Table 4 shows the correlation between WUE\({}_{\text{s}}\) and meteorological factors in different canopy layers of a larch forest. WUE\({}_{\text{s}}\) were highly significantly positively correlated with Pn (\(p<0.01\)) at the different canopies; the upper canopy and middle canopy WUE\({}_{\text{s}}\) were significantly positively correlated with Tr (\(p<0.05\)), while the lower canopy WUE\({}_{\text{s}}\) were not significantly correlated with Tr; different canopy WUE\({}_{\text{s}}\) were highly significantly positively  
Figure 4: (**a**) Net photosynthetic rate (Pn), (**b**) transpiration rate (Tr), (**c**) stomatal conductivity (Gs) and (**d**) intercellular and atmospheric CO\({}_{2}\) concentration ratio (C\({}_{\text{i}}\)/C\({}_{\text{a}}\)) change at the different canopy heights of the larch forest. Notes: Capital letters represent significance at different canopies at 0.01 level.