## 4 Discussion
### Influence of Climatic Factors on CUE Dynamics  
The relationship between climatic factors and CUE has been the subject of extensive study, which has revealed complex interactions that influence GPP and NPP across a range of scales [48; 49; 50]. The results of our study demonstrate that temperature and precipitation exert a pronounced influence on CUE in the Three Rivers Source region.  
The impact of temperature fluctuations on CUE is contingent upon the thermal context of the region in question [51]. In warmer climates, an increase in temperature results in elevated respiratory costs, which, in turn, lead to a reduction in productivity and storage efficiency. This phenomenon is reflected in a decline in CUE as temperatures rises [52; 53]. Conversely, in regions with lower temperatures, the energy required to maintain living tissues is reduced, resulting in decreased respiratory costs and elevated CUE [17]. It is interesting to note that our data indicated a significant reduction in autotrophic respiration even in colder environments with temperature increases [54; 55]. This suggested a more complex response that could have resulted in an extension of the growing season and an increase in GPP, thereby enhancing CUE [56; 57]. Our findings indicate that altitude exerts a pivotal influence on CUE. We observed that at higher altitudes, where temperatures are typically lower, there is an increase in CUE values [51; 58]. This finding is significant in the context of global warming, where alpine regions may become critical carbon sinks due to their higher CUE.  
Furthermore, precipitation exerts a significant influence on CUE. It was observed that there was an inverse relationship between CUE and precipitation levels in the Three Rivers Source region. This finding was consistent with the results of global analyses, which demonstrated a decline in CUE with increasing precipitation, particularly in areas with initially low precipitation rates [51; 58; 59; 25]. As precipitation increases, several factors, such as soil oxygen availability, root respiration, and cloud cover dynamics, contribute to this decline in CUE. In regions with cooler and drier climates, the combined impact of temperature and precipitation on CUE is more significant, underscoring the pivotal role of precipitation in shaping CUE dynamics [22].  
Evapotranspiration, which encompasses both soil evaporation and plant transpiration, emerges as a pivotal determinant influencing CUE [60; 61; 62; 63; 64]. The results demonstrated a positive correlation between augmented evapotranspiration rates and augmented gross ecosystem productivity. This lends support to the notion that elevated transpiration rates facilitate enhanced carbon dioxide uptake by vegetation. This relationship is mediated by stomatal activities [27; 28], whereby increased transpiration results in larger and longer stomatal openings, thereby facilitating greater carbon sequestration [65]. As a direct consequence, a significant rise in CUE was observed in conjunction with rising evapotranspiration levels. This finding lends further support to the hypothesis that there is a positive feedback mechanism between transpiration and productivity.  
In conclusion, our study emphasizes the complex relationship between climatic variables and CUE in the Three Rivers Source region, demonstrating the substantial influence of temperature, precipitation, and evapotranspiration. The findings of our study offer significant insights into the influence of regional climate change on carbon dynamics, emphasizing the pivotal role of evapotranspiration in shaping alterations in CUE.