## 4 Discussion  
For the first time, a comprehensive study was conducted on a regional scale to investigate the effects of frost on alpine vegetation at different timescales (growing season and seasonal scales) by combining a daily meteorological forcing dataset with a remotely sensed phenology dataset. The results of our study indicated a reduction in the number of frost days during the growing seasons in the Third Pole, indicating reduced exposure of alpine vegetation to freezing events due to climate warming. A recent study examined the spatial-temporal changes in extreme temperature events over Tibet for the period of 1961-2010 and also found a significant reduction in the number of frost days (days with daily minimum temperature \(<\) 0 \({}^{\circ}\)C per year) [45]. The findings of some studies were inconsistent with our results; they calculated the number of frost days over the whole year, neglecting the influence of different seasons. Given the relatively short growing season on the Tibetan Plateau, an annual calculation of frost days would overlook seasonal variations. Some researchers divided the growing season into spring and autumn and examined the period spanning 1982-2012 in the Northern Hemisphere. Their findings also indicated a substantial reduction in the number of frost days on the Tibetan Plateau [46], aligning with our own results. Other studies mentioned that spring frost events emerged as the main driving factors influencing the spring phenology of cold-climate vegetation [47]. One study reported contrasting trends in late spring frost occurrences between 1959 and 2017: a decrease in North America and an increase in Europe and Asia [8]. Another study also analyzed spring frost on the southeastern Tibetan Plateau using in situ observational phenological data. Their study revealed that spring frost exerted control over elevational gradients on the spring phenology [35]. Overall, there are very few studies investigating frost events at various timescales on the Third Pole. Variations in seasonal parameters within the frost calculation algorithm can yield markedly distinct estimates of the number of frost days. Therefore, we still lack a comprehensive understanding of broad-scale spatial and temporal patterns in frost events. Using satellite-based NDVI and SIF products [27; 48], we provided a comprehensive assessment of the spatial-temporal variations and trends in vegetation greenness and photosynthesis in the Third Pole region from 2001 to 2018. Our findings revealed a clear east-to-west gradient of decreasing NDVI and SIF values, consistent with the climatic characteristics of the Tibetan Plateau (Figure 6). While both the NDVI and SIF values showed slight increases from 2001 to 2018 across the Third Pole, the seasonal SIF patterns slightly deviated from those of the NDVI (Figure 7), aligning with most previous findings [49; 50]. However, in spring, the SIF exhibited a heterogeneous spatial distribution without a clear spatial pattern. It was previously unknown whether the SIF dataset repeated the spatial pattern found in the NDVI dataset despite the same extraction algorithms applied to these two datasets [51]. Both the NDVI and SIF values contributed to differences in the mean changes in the SIF and NDVI during each stage of the growing season. Consequently, the spatiotemporal variations and trends in the SIF and NDVI in each growing season are the basis for studying the response of alpine vegetation to frost events. It is still crucial to study frost events, assess actual plant damage, and characterize the response of alpine vegetation to frost events during each season of the growing season on the Third Pole. Currently, evaluating the impact of frost on alpine ecosystems has become a hot topic. Plant responses to frost vary across plant-specific growth stages [13]. However, in the Third Pole, where the frost intensity is higher, the earlier dormancy release triggered by the higher air temperature in May increases the exposure of meristems to frost events.