## 4 Discussion
### Spatio-Temporal Distribution Characteristics of GPP over the Qinghai-Tibet Plateau  
A 500 m spatial resolution, monthly interval GPP dataset for the Qinghai-Tibet Plateau was generated in this study based on the improved GPP estimation model. The spatial distribution of GPP across the Qinghai-Tibet Plateau was investigated along latitudinal and longitudinal gradients (Figure 5a). Remarkably high GPP values were observed in the upper southeast region, characterized by a humid climate and extensive forest coverage (Figure 1A). In contrast, the northwest region exhibited lower GPP values, consistent with the patterns observed in the eco-geographical zoning (Figure 1B). The GPP variation in altitude was mainly caused by the temperature heterogeneity in the Qinghai-Tibet Plateau. The region exhibited a discernible pattern of increasing temperatures towards the east and decreasing temperatures towards the west, following the topographical gradients. Additionally, precipitation decreased from southeast to northwest, while solar radiation gradually diminished from southwest to northeast on the plateau. Higher terrains receive more intense solar radiation. Consequently, the spatial distribution of GPP on the Qinghai-Tibet Plateau was a combination of multiple factors. The long-term time series of monthly evolution of GPP (Figure 5b) showed that there was a clear pattern of seasonal characteristics on the Qinghai-Tibet Plateau, with the mean GPP ranging from 80 to 100 g C/m\({}^{2}\)/mon during the growing season and a minimum of 20 g C/m\({}^{2}\)/mon at other periods of the year (spring and winter). This extended timescale variation in GPP is closely associated with changes in meteorological and environmental factors in the area. A distinct seasonal pattern of GPP was evident throughout the year, as depicted in Figure 6, in which the temporal variation was mainly driven by climatic factors, including precipitation, temperature and solar radiation. A warm temperature promotes plant growth in cold regions [6], so in alpine regions, vegetation productivity gradually increases with the beginning of spring, which is the main reason for the GPP increase on the Qinghai-Tibet Plateau. With global warming, the growing season has been significantly lengthened in the past decades, and studies have highlighted that this prolonged growing season can further stimulate vegetation growth, consequently boosting the annual GPP [38]. The growing season is generally from April to September on the Qinghai-Tibet Plateau. Notably, even in October, the GPP remains high, approximately at 523.06 g C/m\({}^{2}\)/year, which indicates that the extended growing season plays a pivotal role in enhancing vegetation productivity in this region. Furthermore, it has been demonstrated that GPP fluctuations in the sub-arid region are closely influenced by drought conditions [39]. Given that a substantial portion of the Qinghai-Tibet Plateau falls within arid and semi-arid regions, moisture conditions play a vital role in regulating regional GPP. Particularly, increased soil moisture resulting from augmented precipitation and the melting of glaciers and permafrost during the summer season stimulates GPP enhancement. The variability of solar radiation will directly affect the potential electron transfer rate and thus regulate the rate of RuBP regeneration-limited photosynthesis and affect GPP. Solar radiation exhibits seasonal variations, with higher levels during summer and reduced levels during winter. Consequently, this seasonal fluctuation in solar radiation contributes to a pronounced increase in GPP on the Qinghai-Tibet Plateau during summer compared to other seasons (Figure 6). The trend in inter-annual GPP changes from 2003 to 2018 is depicted in Figure 7a. It was evident that the regions exhibiting an increasing trend (15.8%) are more extensive than those displaying a decreasing trend (4.7%).