## 4 Discussion
### Driving Factors of NPP  
Abiotic factors associated with temperature and precipitation are important factors affecting vegetation growth [43]. Numerous studies have revealed a significant positive correlation between temperature, precipitation, and NPP [20; 44]. However, we found that NPP was only positively correlated with precipitation in no more than 50% of the grasslands in this study, but a positive correlation between temperature and NPP was found in more than 80% of the TP grasslands, implying that higher temperatures promote vegetation vigor, whereas increased precipitation inhibits vegetation growth at most sites on the plateau. This was because the areas with a negative correlation between precipitation and NPP were mainly located in the southeastern and northeastern TP (Figure 4), which are dominated by alpine meadows with abundant vegetation communities, sufficient precipitation, and high requirements for light conditions [45; 46]. These regions also experienced the largest increase in precipitation (Figure S5), which inevitably led to a decrease in effective solar radiation [47; 48], and consequently to a negative correlation between precipitation and NPP [49]. The negative correlation between temperature and NPP mainly occurred in the intermountain basins around Qinghai Lake, the suburban mountains of Lhasa, and the mountain valleys of the Brahmaputra River, where no significant changes in precipitation but significant changes in temperature were observed (Figures S2 and S5). An increase in temperature may limit vegetation growth due to water stress, while a decrease in temperature inhibits plant photosynthesis [50; 51]. Our study suggested that climatic factors were not the main contributors to NPP variations in TP grasslands, with an \(R^{2}\) of temperature, precipitation, and their interactions explaining NPP greater than 0.5 in less than 2% of the TP grasslands. Factors such as effective solar radiation, vegetation community characteristics, internal self-rhythms, and soil physicochemical properties may also affect grassland NPP [18]. In particular, we found a phenomenon rarely reported in previous studies, that the explanation of temperature, precipitation, and their interactions for NPP variation gradually decreased with increasing elevation (Figure 7). This may be due to the following reasons: (1) Vegetation at higher elevations is dominated by alpine steppes, and they tend to be more efficient with water use and able to maintain growth under arid conditions, which further weakens the aptness of precipitation in the interpretation of NPP in these areas [15; 16]. (2) Temperature gradually decreases to near or below the physiological limit of vegetation growth with increasing elevation, and the explanatory capacity of temperature for NPP naturally weakens [6]. (3) Vegetation at higher elevation usually possesses strong adaptability to survive and reproduce under harsh climatic conditions. It relies more on its own physiological adaptive mechanisms to withstand harsh environments than on changes in temperature and precipitation alone. Such adaptability makes vegetation growth less dependent on a single climatic factor and thus reduces the explanatory power of temperature and precipitation on NPP [1; 52]. (4) Vegetation growth is affected by a combination of environmental factors. At high elevations, climate factors such as wind speed, radiation, evaporation, etc., apart from temperature and precipitation, are highly variable and may also exert important influences on vegetation growth. Besides, soils in the alpine steppes are more infertile, and the lack of nutrients limits the positive response of alpine steppes to temperature and precipitation changes [31; 39]. Compared with precipitation, temperature is the dominant influence on vegetation growth on the TP and mainly occurs at higher elevations, which is consistent with the findings of previous studies [53; 54]. We further explored the relationship between the sensitivity of grassland NPP and elevation, temperature, and precipitation.