## 4 Results
### Spatiotemporal Variation in Grassland AGB during 1982-2018 in the TRHR  
Figure 7 shows the overall spatial patterns of the annual average grassland AGB in the TRHR during 1982-2018. The annual average AGB decreases from the southeast to the northwest. A total of 3.2% of the total area in the grassland of the TRHR with a high AGB distribution (120-180 g C m\({}^{-2}\)) is mainly located in the far east region of the study area (Zeku, Henan and Jiuzhi). Figure 7 clearly shows a low AGB distribution (0-60 g C m\({}^{-2}\)), which occupies approximately 72.6% of the northern, western, and central areas of the TRHR (Qumalai, Maduo, Zhiduo, Tanggulashan, Zaduo and Chengduo). The southern and eastern areas of the TRHR (Yushu, Nangqian, Xinghai, Tongde, Majin, Gande, Dari and Bamna) have a relatively moderate AGB of approximately 60-120 g C m\({}^{-2}\) covering approximately 24.2% of the study area. The AGB distribution is mostly concentrated at 0-60 g C m\({}^{-2}\), and the number of AGB values larger than 100 g C m\({}^{-2}\) is relatively low. Figure 8 presents the spatial patterns of the multiyear average seasonal AGB distribution. The AGB reached its highest value (320-360 g C m\({}^{-2}\)) in summer (JJA, June, July, and August). The change trend of the AGB spatial distribution in summer (Figure 8b) is similar to the multiyear patterns of AGB during 1982-2018 (Figure 7). The AGB is less than 80 g C m\({}^{-2}\) in spring (MAM, March, April, and May) and winter (DJF, December, January, and February). The seasonal AGB has overall moderate values of less than 200 g C m\({}^{-2}\) in autumn (SON, September, October, and November). Figure 6: Importance of input variables to AGB upscaling model depicted by increased RMSE percentage, including NDVI, precipitation (P), downward shortwave radiation (Rs), air temperature (Ta), and elevation. Figure 7: Spatial pattern of average AGB in the TRHR during 1982–2018. Figure 9 presents the turning point of AGB temporal variation was found in 1998 by the sequential Mann-Kendall test and the interannual variation in AGB, P, and NDVI over the period of 1982-2018. The annual AGB, P and NDVI in the TRHR increased over 1982-2018 with a linear fit with slopes of 0.37 g C m\({}^{-2}\) year\({}^{-1}\), 3.66 mm year\({}^{-1}\), and 0.9 \(\times\) 10\({}^{-3}\) year\({}^{-1}\), respectively (\(p\) < 0.05). An increase in annual AGB and NDVI occurred during 1982-1998, with linear trends of 0.16 g C m\({}^{-2}\) and 0.30 \(\times\) 10\({}^{-3}\) year\({}^{-1}\) (\(p\) < 0.05). Such an increase in the annual AGB is consistent with the NDVI increase in response to global warming. Coincident with the El Nino event in 1998, the positive trend in AGB seems to have accelerated (0.21 g C m\({}^{-2}\) year\({}^{-1}\), \(p\) < 0.05) during 1998-2018. The time slope of P was found to have changed from being negative (\(-\)2.81 mm year\({}^{-1}\), \(p\) < 0.05) during 1982-1998 to positive (5.02 mm year\({}^{-1}\), \(p\) < 0.05) during 1998-2018. However, since 1998, the increase in the annual NDVI seems to have stalled, with a 95% confidence interval trend of \(-\)0.50 \(\times\) 10\({}^{-3}\) year\({}^{-1}\) (\(p\) < 0.05). Figure 10a shows the spatial distribution of AGB trends in the TRHR during 1982-2018. Our results illustrate that the AGB of the TRHR increased by 0-1.5 g C m\({}^{-2}\) year\({}^{-1}\) during 1982-2018. A total of 82.3% of the pixels in the grassland of the study area show an increasing trend in AGB. Except for the far west of the study area, the other regions have a significantly increasing trend (\(\sim\)79.4%, \(p\) < 0.05). The largest AGB increases occurred in the eastern TRHR (Figure 10a), where the annual AGB is also the highest, as shown in Figure 7. The overall increment in the annual AGB can largely be attributed to the upward trend of NDVI and P during 1982-2018 Figures 11a and 12a). Figure 8: Multiyear seasonal spatial patterns of grassland AGB in the TRHR.