## 5 Discussion  
### Relative Importance of Environmental Factors  
Figure 11 highlights the relative importance of key environmental factors in the GPP prediction model on the Tibetan Plateau. Among these, longwave radiation (lard), shortwave radiation (srad), the specific humidity (shum), and speed (wind) are identified as the five most influential factors. Longwave radiation, in particular, stands out, contributing 25.16% to the overall importance, highlighting its crucial role in regulating the temperature and water evaporation processes [31]. Interestingly, [PERSON] et al. [31] found that shortwave radiation is the most significant environmental variable affecting the global GPP, due to its essential function in driving photosynthesis by providing solar energy at the top of the atmosphere. The discrepancy between the Tibetan Plateau and the global situation could be attributed to the fact that shortwave radiation is not the primary factor influencing the GPP on the Tibetan Plateau. Supporting this, [PERSON] et al. [47] noted that, among seven light use efficiency (LUE) models on the Tibetan Plateau, only two suggested that radiation was as influential as water and the temperature in determining the GPP, with the remaining models considering either the temperature or water as the dominant factors. Additionally, Figure 11 reveals that srad (15.60%), shum (13.51%), and wind (11.02%) are also significant contributors, as they collectively influence the temperature and precipitation, which are closely associated with longwave radiation.  
The specific humidity, which represents the moisture content of the air, serves as a composite indicator influenced by both the temperature and precipitation. It plays a crucial role in GPP by regulating plant transpiration. Typically, lower specific humidity can increase the transpiration rates; however, when the humidity levels fall too low, plants may close their stomata to conserve water, resulting in reduced photosynthesis [48]. Additionally, as shown in Figure 11, CO\({}_{2}\) contributes 6.27% to the overall importance, significantly impacting the GPP on the Tibetan Plateau, which is consistent with the findings of [PERSON] et al. [49]. The rising atmospheric CO\({}_{2}\) concentration enhances the efficiency of light energy and water use by vegetation, thereby promoting plant growth in this region [50]. In the alpine grassland ecosystems of the Tibetan Plateau, studies on the CO\({}_{2}\) flux dynamics and plant-limiting factors have shown that the interplay between water and the thermal conditions is a key determinant of GPP. In humid regions, increasing temperatures can boost photosynthesis and carbon fixation in plants. Conversely, in arid regions, higher temperatures may exacerbate soil moisture evaporation, intensifying drought stress on plants and consequently reducing ecosystem productivity. Overall, despite the fact that elevated CO\({}_{2}\) levels generally have a positive effect on GPP in the Tibetan Plateau, this effect is moderated by various environmental factors, including the hydrological conditions, temperature, and atmospheric dryness. In short, these factors collectively and interactively shape the trends in vegetation productivity within this unique and sensitive ecosystem.