## 4 Discussion
During the early stage of the growing season, vegetation growth is mainly controlled by temperature. Our results are consistent with those of previous research demonstrating the optimal adaptation of alpine vegetation to its high and cold environment. Nevertheless, the authors of these previous studies also pointed out that temperature played a key role in vegetation growth. They indicated that projected warmer temperatures could increase frost availability at the local scale, having a greater impact on alpine vegetation. It is critical to determine the physiological traits and thresholds affecting alpine vegetation during the frost period. Thefreezing resistance of plants has largely been regarded as reaching its seasonal minimum, without much variation, in summer months. Most studies merely show the potential correlation of vegetation dynamics with climatic variations (temperature and precipitation), thereby neglecting other crucial environmental variables [50]. Our results supported previous analyses, suggesting that temperature-related factors might be the main drivers of vegetation variability in the Third Pole, with less influence of precipitation, radiation, and frost (Figure 10). Due to climate change, the controlling effects of temperature and frost days were both decreasing, whereas those of precipitation and radiation were increasing. Overall, spring, summer, and autumn remain relatively crucial seasons in climate change studies within alpine ecosystems [15]. In contrast, no increase in the freezing resistance of the alpine vegetation was expected to occur at the end of summer, especially as it was undergoing senescence. Owing to climate change, the controlling effects of temperature and frost days were both decreasing, while those of precipitation and radiation were amplifying. Some people demonstrated a declining sensitivity to precipitation and an increasing sensitivity to temperature, which was not supported by our results [52]. Given the complexity of climate variation drivers, research on the sensitivity of alpine vegetation to climate change on the Third Pole needs to be integrated with a combination of factors. Overall, the long-term probability of frost might adjust the sensitivity of alpine vegetation to diverse environmental cues, thereby minimizing the exposure of its sensitive tissue to frost. Understanding how this change in sensitivity to environmental factors will affect the alpine vegetation growth response will be important for understanding regional drivers in future changes. Changes in SIF exhibited greater magnitude compared to NDVI, implying vegetation photosynthesis is more sensitive to climate change than greenness, potentially due to variations in light use efficiency [53]. In this study, we found that SIF was more sensitive to frost events than the NDVI. As a direct proxy of photosynthesis, SIF demonstrated greater susceptibility to environmental stress compared to the traditional greenness-based NDVI [27]. SIF can serve as a good indicator in monitoring and assessing the impacts of extreme climatic events such as heatwaves, droughs, and frost on vegetation growth [54]. Some people found that satellite-based SIF observations exhibited a clear response to heat and water stress in vegetation from 2009 to 2010 across southern China, in contrast to the less pronounced response observed in the NDVI [19]. Others also found that SIF showed a significant reduction and an earlier response in winter wheat in northwestern India, whereas the NDVI and EVI could not capture the thermal stress effect [18]. Some scholars demonstrated that SIF could detect early-stage drought, while the NDVI could detect drought lasting over a long-time scale [20]. In line with most previous studies, heat and moisture stress tended to manifest earlier or in a more pronounced way in SIF than in the NDVI. Furthermore, they observed supplementary responses of different vegetation types [55].