## 3 Results
### Altitudinal Patterns of \(SIF_{yield}\) Changes
#### 3.1.1 Altitudinal Variation in \(SIF_{yield}\)
(**a**) Altitudinal \(SIF_{yield}\) variation on day of year (DOY) 197 (July, the peak of summer growth period); (**b**) \(SIF_{yield}\) changes along altitude in 12 years on DOY 197 (July, the peak of summer growth period). Graph (**a**) is derived from the average of 12-year observations, while (**b**) shows altitudinal effects on \(SIF_{yield}\) and \(SIF\) in each year from 2007–2018. Colors in (**a**) represent the temperature of ambient environment, and colors in (**b**) represent year; shaded areas represent the 95% confidence interval Error bars in (**a**) represent the standard error of \(SIF_{yield}\) at different altitudes. Figure 2: \(SIF_{yield}\) changes from 2007–2018 at different elevations. Results are derived from \(SIF\) observations on (**a**) DOY 197 (July, the peak of summer growth period) and (**b**) DOY 149 (May, representing period at late spring). 3.1.2. Altitude-Dependent \(SIF_{yield}\) Changes at Seasonal Scale. Figure 3 illustrates seasonal changes of \(SIF\) and \(SIF_{yield}\), as well as potentially related ambient environment variables (PAR and air temperature). As shown in Figure 3a, latitudinal variation in \(SIF\) seasonal changes is especially apparent. However, getting rid of the influence of \(APAR\) (which contains the impact of structure and incident light), \(SIF_{yield}\) still exhibits altitudinal differences, particularly in summer (although not as significant as \(SIF\)). This result may indicate the existence of altitudinal variation in physiological properties. \(SIF\) is lower at higher altitudes according to Figure 3a, whereas the altitudinal trend of \(SIF_{yield}\) is the inverse in the middle of the growing season (from approximately DOY 181 to 245), as shown in Figure 3b. Different positions of peaks among these four variables are also found in Figure 3. The peak time of PAR is the earliest, and the others are far behind. Whereas the \(SIF\) peak time is slightly earlier and closer to the PAR peak, the peaks of \(SIF_{yield}\) and temperature are extremely close and become the latest ones. The synchronicity of \(SIF_{yield}\) and temperature dynamics indicates that \(SIF_{yield}\) may be relevant to ambient temperature on the seasonal scale. However, from about DOY 120-200, PAR starts to decrease, but the value of \(SIF_{yield}\) continues to increase at this time, indicating that the change of \(SIF_{yield}\) cannot be explained well by PAR. In contrast, due to the temperature lapse along the altitude gradient, the thermal condition in alpine regions is not ideal, thus when it gets warmer, physiological activities (reflected by \(SIF_{yield}\)) are prone to increase sharply. Figure 3: Seasonal variations in (**a**) \(SIF\), (**b**) \(SIF_{yield}\), (**c**) photosynthetically active radiation (PAR), and (**d**) temperature at different elevations. Lines in each subgraph indicate DOY when variables reach their peaks, and gray rectangles indicate \(SIF_{yield}\) peak time for comparison. The \(SIF\) and \(SIF_{yield}\) peaks are both delayed at high elevations (Figure 3), but the altitudinal effects on their seasonal changes are not always the same. For \(SIF\), it starts to increase earlier and grow faster at lower altitudes in spring; however, for \(SIF_{yield}\), the growth rate shows no significant differences at different altitudes. In addition, \(SIF\) in low altitude regions also declines later than in higher places, as shown in Figure 3a, but \(SIF_{yield}\) declines earlier in low land regions. As temperature can influence \(SIF\) and \(SIF_{yield}\) significantly, their responses to the temperature along the altitude gradient were also investigated (Figure 4). Contrary to the result in Figure 3b showing similar \(SIF_{yield}\) increasing rates with DOY at different altitudes in spring, the increasing rates of \(SIF_{yield}\) with temperature vary along the altitude (Figure 4b).