## 4 Discussion  
### Possible Altitudinal Increase in \(Sif_{yield}\) Observed in Summer  
The increasing trend of \(SIF_{yield}\) along the elevation on summer days can be explained by the altitude-dependent relationship between \(SIF_{yield}\) and environmental variables such as temperature. In addition to the different relationships between \(SIF_{yield}\) and air temperature at different altitudes, Figure 4b also shows the altitudinal variation in \(SIF_{yield}\) on DOY 205 and 213. From the seasonal results, we can infer that the higher temperature sensitivity at high altitudes makes the altitudinal increase in \(SIF_{yield}\) possible on DOY 205 and 213. If the temperature sensitivity at different altitudes stays the same, only the result that \(SIF_{yield}\) decreases with altitude (increases with ambient temperature) can be obtained. The schematic diagrams in Figure 9 show that the difference in sensitivity at different altitudes makes the high \(SIF_{yield}\) phenomenon possible at high altitudes. If the altitudinal variation in temperature sensitivity of \(SIF_{yield}\) (or its sensitivity to other environmental elements) is not taken into account, as shown in Figure 9a, then it is only possible to conclude that \(SIF_{yield}\) is lower at high altitudes, and we cannot obtain results consistent with satellite observations. Therefore, even if \(SIF_{yield}\) is found to be higher at higher elevation, it does not necessarily mean that the correlation between temperature and \(SIF_{yield}\) is negative for given site observations (Figure 9b). The varied vegetation along the altitude gradient may also play an important role in the formation of altitudinal \(SIF_{yield}\) patterns. Numerous studies report special physiological responses and photosynthetic behaviors of alpine plants, especially in tropical and subtropical regions [17; 62]. Previous studies also reported relatively high activity of photosynthesis and its related traits at high altitudes. Ground observation in the eastern Qinghai-Tibetan Plateau found an increase in the maximum carboxylation rate (Vcmax) and maximal electron transport rate (Jmax) along the elevation gradient, indicating higher levels of carbon assimilation and function to cope with the harsh conditions and a shorter growing season for plants at higher elevations [15]. Actually, the increase in Vcmax and drawdown of carbon dioxide (CO2) with increased altitude have been consistently observed for many years and proved by numerous studies [2; 4; 5; 63; 64; 65; 66; 67]. In addition, equal or even higher photosynthetic rates were also measured [68], and various hypotheses have been proposed to explain this puzzling phenomenon, mainly including theories focusing on low temperatures [5; 66; 69] and low pressure [70; 71]. Optimality principles such as the least-cost hypothesis and coordination hypothesis were also employed in a theoretical model in 2017 [2]. This model successfully simulated increasing Vcmax and decreasing ambient CO2 partial pressure with rising altitude from 0-3000 m. Although a consensus on the mechanism of this phenomenon has not been reached, all of the above findings confirm  
Figure 8: (**a**) Spatial distribution of seasonal temperature sensitivity of \(SIF_{yield}\): (**b**) spatial distribution of tau statistic in Mann–Kendall test. Dots indicate significance of regression or Mann–Kendall test (\(p\)-value \(<\) 0.05). (**c**) Elevations of study region (data source: STRM-DEM); (**d**) spatial distribution of vegetation subtypes of Tibetan meadows (data source: vegetation map provided by the Institute of Botany, Chinese Academy of Sciences). Only regions recognized as meadows are displayed in these maps. that there are changes in the properties of the plants themselves along the altitude. These results also demonstrate that there are occasions when photosynthetic capacity increases at higher altitudes, implying there may also be enhancement of other photosynthesis-related traits, such as \(SIF_{yield}\), along the altitude gradient.