## 4 Discussion
### Relationship Between the Leaf Area Index and Vegetation Transpiration  
Vegetation transpiration is directly related to canopy conductance, which is positively correlated with the leaf area index, indicating that the leaf area index directly affects transpiration [71; 72]. To further understand the importance of plant-water interactions, we compared the relationship between the leaf area index and transpiration in both the dynamic andodynamic experiments (Figure 10). We observed a significant positive correlation between the leaf area index and transpiration at monthly scales, with \(R^{2}\) values greater than 0.80 (\(p<0.05\)). Similar results have been reported in previous studies, although this relationship is constrained by a threshold in the leaf area index [72; 73; 74]. When the leaf area index exceeds this threshold, transpiration may either stabilize or decrease. Moreover, the threshold value varies across different vegetation types and regions, influenced by many factors such as soil moisture, nutrients, light, and climatic conditions [72; 73; 74]. In the nododynamic experiment, a wide range of transpiration values corresponded to the same leaf area index at the monthly scale. However, transpiration negatively correlated with the leaf area index in the nododynamic experiment at the annual scale. In contrast, the relationship between the leaf area index and transpiration in the dynamic experiment more closely reflected the actual conditions. Specifically, when the simulated leaf area index was below 4.0 at the monthly scale, transpiration increased alongside the leaf area index.  
Figure 9: The amount of (**a**–**d**) root water uptake in different soil layers during the growing season and (**e**) annual transpiration for parameter a schemes. Parameter \(a\) in Equation (10) determines carbon allocation to different soil layers in the root zone.  
Plant-water interactions significantly influence transpiration at both timescales, as evidenced by the varying ratios of dynamic to nobynamic (Figure 10). A linear relationship between transpiration and the leaf area index was evident at both timescales, consistent with previous studies [74,75]. However, the deviation from linearity was more pronounced at the annual scale when the leaf area index was either greater or smaller in the dynamic experiment, indicating that transpiration was lower in the dynamic experiment. We also observed some deviation from a 1:1 relationship at the monthly scale, particularly the dynamic-to-nodynamic ratio of transpiration near 1.0 (Figure 10d). This deviation may be attributed to soil water limitations [75].  
Previous studies have also revealed that vegetation dynamics significantly influence hydrological processes [55,76]. For instance, vegetation dynamics in the improved CLM4.0 produced higher leaf area index and evapotranspiration but lower runoff and root zone soil water in the Wuding river basin [55]. Similarly, CLM5.0 produced a high leaf area index in summer and autumn across most of the Three Rivers Source Region of the Yellow River [76]. The UA version of Noah-MP outperformed the multi-model ensemble mean in capturing interannual variations and long-term trends in the leaf area index in drylands [77]. Although several land surface models include vegetation dynamics, models such as ORCHIDEE and LPJ-GUESS often simulate unrealistic hydrological behavior in dryland ecosystems because of their use of simplified bucket-type hydrological schemes. When a discretized soil hydrology scheme was implemented in the ORCHIDEE model, the simulation of daily and monthly evapotranspiration in semi-arid regions improved; however, the transpiration-to-evapotranspiration ratio remained lower than observed values [78].  
Figure 10: Relationship (**a,b**) between the simulated leaf area index and transpiration and (**c,d**) between the ratio of the leaf area index and the ratio of transpiration at both the annual and monthly scales. \"\" means significance at 95% level.