## 4 Discussion
### Interpretation of Patterns Observed at Different Scales  
As highlighted by [PERSON] [1], although they are under the umbrella of altitude gradient, vertical variation patterns of vegetation described from different perspectives and in different regions are usually controlled by diverse drivers. Altitude-dependent \(SIF_{yield}\) patterns observed at different scales in this study also reflect different issues and involve various latent factors. Therefore, discrepancies in these \(SIF_{yield}\) variation patterns do not necessarily represent a contradiction. \(SIF_{yield}\) patterns discovered on the interannual scale are probably close to analysis results based on controlled experiments. Due to the similarity of seasonal cycle phases on the same DOY across years, the influence of growth stages, incident solar energy, and other corresponding environmental variables driven by seasonal cycles is diminished on the interannual scale. In this case, the contribution of environmental changes (temperature and PAR) to \(SIF_{yield}\) variation is enhanced and accentuated, and the results obtained from this perspective are relatively independent of some common factors in seasonal cycles. Therefore, interannual patterns of \(SIF_{yield}\) might reflect the response of photosynthesis-related physiological activity of plants in a given place. The stable negative correlation between \(SIF_{yield}\) and PAR on different DOYs discovered on the interannual scale (Figure 5a) is consistent with previous ground observations showing a negative impact of PAR on \(SIF_{yield}\)[73]. Figure 9: Schematic diagrams showing that temperature sensitivity makes high \(SIF_{yield}\) at high altitudes possible. (**a**) Altitudinal patterns of \(SIF_{yield}\) when there is no altitudinal variation of temperature sensitivity. (**b**) Possible altitudinal patterns of \(SIF_{yield}\) when accounting for altitudinal variation of temperature sensitivity. A, B, and C represent regions from high to low altitude, corresponding to temperature variation from low to high. In comparison with the pattern above, \(SIF_{yield}\) variation on the seasonal scale reflects the mixed effects of seasonal variables on photosynthesis-related vegetation activity. The formation of this phenomenon involves entangled covariations of different variables. Thus, it is reasonable that they would be different from interannual results. Peaks of \(SIF_{yield}\) at different altitudes occur in summer, which is also the peak time of vegetation activity. The \(SIF_{yield}\) at all altitudes increases with rising temperature at this scale, which is consistent with the increased fluorescence yield and non-photosynthetic quenching (NPQ) discovered at a high-elevation evergreen forest in summer [74]. To summarize, patterns in this study actually indicate the different responses of vegetation to environmental changes along the altitude at seasonal and interannual scales. The altitudinal variation in \(SIF_{yield}\) may result from natural selection at the ecosystem scale. It involves the adaptation of plants to long-lasting environmental stresses in their habitats and actually reflects the long-term response of plants to environmental changes [1]. Actually, photosynthetic capacity is considered to be one of the ways the adaptation mechanism and survival strategies work for alpine plants on the Tibetan Plateau [75]. Change on the seasonal scale may be the result of long-term cyclically changing environmental conditions involving the influence of seasonal cycles. The variation in seasonal patterns at different altitudes may be related to phenological issues, indicating the adaptation of plants to the harsh environmental condition and the short growing season at high altitudes [15]. In contrast, the patterns on the interannual scale reflect the shortest-term response of \(SIF_{yield}\) to environmental changes in the absence of plant adaptation, which is similar to the condition of global changes when there is not enough time for plants to develop and adjust to the changing climate.