## 4 Discussion  
### Impacts of Phenology and Photosynthetic Capacity on Seasonal GPP  
Previous studies found most of the QPT experienced increasing trends in vegetation productivity with the advancing of SOS, delaying of EOS and prolonging of LOS for the alpine grasslands [22; 66]. In this study, advanced SOS and EOS and prolonged LOS were found in the SRYR from 2001 to 2019, which was consistent with the results from previous studies [22; 67; 68] in the QPT. Moreover, POS also showed a significantly advanced trend, which was seldom considered in the previous studies in the QTP. To clearly understand the impacts of phenology and photosynthetic capacity on seasonal GPP, we divided the study period into two periods: Period 1 (2001-2009) and Period 2 (2010-2019). Comparisons of the average daily GPP processes over growing season and the average values of vegetation phenology for Period 1 and Period 2 are shown in Figure 9a. Compared to Period 1, the average SOS, POS and EOS during Period 2 advanced 2.81 d, 3.47 d and 0.85 d, respectively, and the average LOS was extended 1.96 d. The average \(\mathrm{GPP_{spr}}\), \(\mathrm{GPP_{sum}}\), \(\mathrm{GPP_{ann}}\) and \(\mathrm{GPP_{max}}\) increased, respectively, 15.52%, 7.71%, 5.59% and 11.85%, while the average \(\mathrm{GPP_{aut}}\) decreased 6.29%. The earlier SOS and POS might result in a greater consumption of soil-water and nutrients from plants in the spring and summer and limitations of soil-water and nutrients in the autumn. The alpine ecosystems in the SRYR exhibited an extended GPP curve in the growing season, an advanced peak in the growing season, and increased maximum carbon uptake capacity. A conceptual figure indicating the influences of the changes of vegetation phenology and \(\mathrm{GPP_{max}}\) on annual accumulative GPP is shown in Figure 9b. Changes in hydrothermal factors induce seasonal variation in the physiological activities of the vegetation, and hence contribute to the seasonal variability of vegetation productivity [69]. In this study, it was found that an advanced SOS prolonged the length of photosynthetic activity in spring in the SRYR. Moreover, an earlier spring phenology might result in greater leaf area, enhancing light interception and photosynthesis and thereby increased spring GPP [70]. Previous studies reported that an earlier spring phenology resulted in a longer growing season and strengthened productivity [21; 23]. However, an earlier spring phenology could also result in soil moisture deficits by increasing water consumption [46; 71], leading to suppressed vegetation activities in summer and autumn due to aggravated water stress for the water-limited grassland ecosystems [66; 71]. Figure 8: Pearson correlation coefficients of seasonal and annual GPP with phenology factors and \(\mathrm{GPP_{max}}\) (**a**) and those of \(\mathrm{GPP_{spr}/GPP_{ann}}\), \(\mathrm{GPP_{sum}/GPP_{ann}}\), \(\mathrm{GPP_{aut}/GPP_{ann}}\) and \(\mathrm{GPP_{max}}\) with phenology factors (**b**). The asterisk on the bar chart indicates the level of significance. Single asterisk (**”) indicates statistically significant correlation coefficients with \(p<0.05\) and double asterisks (**”\(\times\)**) indicate \(p<0.01\). In this study, we found an earlier POS and higher GPPmax in the SRYR, both of which had positive effects on the GPPsum and GPPmin. Previous studies found that POS is an important vegetation phenology for the carbon cycle of terrestrial ecosystems [27], and an advance of POS could result in productivity changes [72]. Researchers found an earlier POS shifted towards spring in the Northern Hemisphere, caused by climate warming, resulted in increased vegetation productivity [44; 73]. We found the spatial patterns of the regions with significantly increased GPPmax (Figure 6b) were similar to those with significantly advanced POS (Figure 4d) in the SRYR. Moreover, both POS and GPPmax had significant correlations to summer GPP (Figure 8a).