## 3 Results
### Effects of Heatwaves on Vegetation  
To compare the differences in the FD trends, the FD trends in the Third Pole in both the 2001-2009 and 2010-2018 periods were examined in Figure 5. For the period of 2001-2009, the percentage of pixels with decreasing trends in SPR_FDs, SUM_FDs, FAL_FDs, and GS_FDs accounted for 12.70%, 11.60%, 14.33%, and 16.32%, respectively. The percentages of pixels with increasing trends in SPR_FDs, SUM_FDs, FAL_FDs, and GS_FDs accounted for 1.39%, 1.82%, 4.32%, and 3.23%, respectively. For the period of 2010-2018, the percentage of pixels with decreasing trends in SPR_FDs, SUM_FDs, FAL_FDs, and GS_FDs accounted for 7.71%, 5.01%, 4.02%, and 5.51%, respectively. The percentages of pixels with increasing trends in SPR_FDs, SUM_FDs, FAL_FDs, and GS_FDs accounted for 4.73%, 7.48%, 14.92%, and 13.98%, respectively. Both the FAL_FDs and GS_FDs from 2001 to 2018 had similar spatial distributions of pixels with increasing or decreasing trends, while the SPR_FDs (2010 to 2018) had a larger area of pixels with increasing trends than that of pixels with decreasing trends. The SUM_FDs trends were different from those of SPR_FDs and FAL_FDs. Moreover, the pixels with no significant decreases (\(p>0.05\)) in FD trends accounted for the largest and most widespread area during the entire period in the Third Pole. Geographically, the SPR_FDs and SUM_FDs showed significant increasing trends (2001-2009) and no significant decreasing trends (2010-2018) at the junction of the Qinghai Plateau, Western Plateau, and Tibetan Plateau. The FAL_FDs displayed increasing trends in the western part of the Qinghai Plateau and the central Third Pole and decreasing trends on the eastern plateau (2010-2018). The spatial patterns of FAL_FDs and GS_FDs trends coincided greatly in the Third Pole. Overall, the FD trends exhibited greater heterogeneity between the two periods and different seasonal cycles. Figure 5: Spatial distribution of frost day trends in the Third Pole from 2001 to 2018: (**a**,**e**) the trend of SPR_FDs; (**b**,**f**) the trend of SUM_FDs; (**c**,**g**) the trend of FAL_FDs; (**d**,**h**) the trend of GS_FDs. The Mann–Kendall trend test was separately performed for the periods of 2001–2009 and 2010–2018. Spatiotemporal Patterns and Trends of Seasonal GOSIF and NDVI Data from 2001 to 2018 on the Third Pole  
The GOSIF and NDVI patterns were also calculated in the same way as the FD was calculated in Figure 6. The temporal trends in the GOSIF and NDVI values at the growing season and seasonal scales in the Third Pole during periods of different lengths starting between 2000 and 2018 are shown in Figure 7. In particular, the GOSIF trend values ranged from \(-0.002\) to \(0.002\), and most of them were less than 0, which meant that the GOSIF at the growing season and seasonal scales in different study periods showed no statistically increasing or decreasing trend (Figure 7a-d). The NDVI trend values ranged from \(-0.004\) to \(0.004\), and most of them were greater than 0, which meant that the NDVI at the growing season and seasonal scales in different study periods showed an insignificant increasing trend (Figure 7e-h). Generally, the GOSIF and NDVI trends showed obvious differences with the change in season due to the periodicity of the vegetation. The GOSIF trends for the later columns (starting from 2003 to 2010) showed a decline, and a relatively large number of periods passed the significance test. The NDVI trends in each of the different study periods showed an increasing trend. Generally, the GOSIF and NDVI trends at the growing season and seasonal scales showed an increasing or decreasing trend from 2001 to 2018, which was most likely due to the different internal mechanisms between the NDVI and GOSIF. Figure 6: Spatial distribution of the average GOSIF and NDVI values in the Third Pole for 2001–2018: (**a–h**) The seasonal GOSIF and NDVI values over the 2001–2009 period, respectively; (**i–p**) the seasonal GOSIF and NDVI values over the period of 2010–2018, respectively.