## 4 Discussion  
### Effects of Two Tree Species on CO2 and H2O Budget  
Photosynthesis is a physiological process closely related to gas diffusion in plant stomata that plays an important role in regulating terrestrial carbon and water fluxes [57; 58]. The annual accumulation of net ecosystem exchange (NEE) and gross primary productivity (GPP) in the _Larix principis-rupprechtii_ forest ecosystem were higher than those in the _Pinus tabulaeformis_ forest ecosystem (Figure 2B). At the monthly scale, the carbon flux of the two forest ecosystems had the same results (Figure 3A-D), and the seasonal changes of carbon flux were consistent with plant growth dynamics. At the half-hourly scale, the carbon flux of the _L. principis-rupprechtii_ forest ecosystem was greater than that of the _P. tabulaeformis_ forest ecosystem, but NEE and GPP reached their peak values at different times (Figure 3E,G). Previous studies showed that _L. principis-rupprechtii_ had higher photosynthetic capacity than _P. tabulaeformis_[59; 60], which may result in larger GPP in the _L. principis-rupprechtii_ forest ecosystem. Moreover, _L. principis-rupprechtii_ is a deciduous coniferous species, and had no leaves or stomata yet to function during the non-growing season, indicating a low carbon-absorption capacity. Therefore, the differences in photosynthetic capacity and tree species between the two forest ecosystems was the fundamental reason for the differences in GPP [47]. Furthermore, the vegetation carbon uptake and canopy evapotranspiration were related to the leaf area index (LAI) [17], which was also the reason for the differences in carbon and water fluxes between the two forest ecosystems. In addition, the ecosystem respiration (RECO) was the major pathway for plant-fixed carbon returning to the atmosphere [45; 61], and forest ecosystems with larger soil organic \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Forest Ecosystem Types** & **Stepwise Regression Equation** & _F-_ & _P-_ & **TS** & **TA** & **PAR** & **RH** & **VPD** & **SWC** \\  & & **Value** & **Value** & & & & & & \\ \hline \multirow{4}{*}{_Larix_} & WUE = 3.954 \(-\) 0.024 PAR & 8.49 & 0.00 & & & \(-\)0.23 ** & & \\  & uWUE = 1.800 \(-\) 0.188 TS + 0.134 TA & 6.38 & 0.00 & \(-\)0.27 ** & 0.21 ** & & & \\ \cline{1-1}  & LUE = \(-\)0.094 \(+\) 0.003 RH \(-\) 0.002 PAR & 64.59 & 0.00 & & 0.40 ** & \(-\)0.51 ** & 0.49 ** & 0.30 ** \\ \cline{1-1}  & + 0.010 TA + 0.005 SWC & & & & & & & \\ \hline \multirow{4}{*}{_Pinus tabulaeformis_} & WUE = 0.780 \(+\) 0.018 PAR & 22.70 & 0.00 & & & 0.37 ** & & \\ \cline{1-1}  & uWUE = 3.030 \(-\) 0.027 RH + 0.011 PAR & 48.98 & 0.00 & & & 0.20 * & \(-\)0.34 ** & \\ \cline{1-1}  & LUE = 0.186 \(-\) 0.001 PAR & 165.06 & 0.00 & & & \(-\)0.73 ** & & \\ \hline \end{tabular}
\end{table} 
Table 2: Results for the multiple stepwise regression analysis of effects of potential meteorological drivers (TS, TA, PAR, RH, VPD, and SWC) on the water use efficiency (WUE), underlying water use efficiency (uWUE), and light use efficiency (LUE) in two forest ecosystems during growing season. matter have higher RECO, because RECO is dominated by the heterotrophic respiration [45]. In this study, RECO of the _L. principis-rupprechtii_ forest ecosystem was higher than that of the _P. tabualaeformis_ forest ecosystem, accounting for 52.52% and 37.11% of GPP, respectively. Our previous study confirmed that _L. principis-rupprechtii_ forest stand had higher litter carbon input, litter decomposition rate, and thicker litter layer than _P. tabualaeformis_ forest stand [62], which could be considered as the reason for the higher soil heterotrophic respiration of the _L. principis-rupprechtii_ forest [45]. Moreover, the low soil moisture usually inhibits the decomposition of organic matter in arid and semiarid ecosystems [45; 63], which is consistent with the result that SWC of the _P. tabualaeformis_ forest ecosystem was lower than that of the _L. principis-rupprechtii_ forest ecosystem in this study (Figure 2).