# Altered Trends in Light Use Efficiency of Grassland Ecosystem in Northern China
## 3 Results  
### Interannual Dynamic Characteristics of LUE  
The interannual LUE variability in the grasslands in northern China increased (3.42 \(\times\) 10\({}^{-3}\) g C/MJ/yr) during 1982-2018, and the multi-year mean LUE was 0.88 \(\pm\) 0.047 g C/MJ (Figure 3a). Around 1998, the LUE's multi-year mean and trends altered. After 1998, the upward trend in LUE slowed and the mean LUE significantly increased (Figure 3a). The Mann-Kendall test was used to further determine the abrupt change point to verify the accuracy of the changes in LUE trends (Figure 3d). The results show that the period from 1997 to 1998 saw significant changes in LUE (Figure 3d). During 1982-1997, the mean LUE was 0.84 \(\pm\) 0.030 g C/MJ, and the annual rate of increase in LUE was 2.75 \(\times\) 10\({}^{-3}\) g C/MJ/yr. From 1998 to 2018, the mean LUE was 0.91 \(\pm\) 0.029 g C/MJ, and the yearly rate of increase in LUE was 1.63 \(\times\) 10\({}^{-3}\) g C/MJ/yr (Figure 3a).  
In the past 40 years or so, the growth rate of AGPP has been greater than that of APAR (Figure 3b,c). Compared with 1982-1997, the average AGPP and APAR values for 1998 to 2018 increased significantly, and the growth rate of AGPP and APAR also increased during 1998-2018. It is noteworthy that the change in the growth rate of APAR is much greater than that for AGPP (Figure 3b,c).  
We found that 42.16% of grassland areas in northern China experienced significant (\(p<0.05\)) changes in LUE between 1982 and 2018, of which 40.43% showed an upward trend, and 1.73% a downward trend (Figure 4a). The interannual variability in the LUE of all grassland types presented significantly increasing trends (Figure 4a and Figure S1). AM showed the fastest rate of LUE growth, increasing at a rate of 4.85 \(\times\) 10\({}^{-3}\) g C/MJ/yr (\(p<0.05\)) (Figure S1a). TS had the slowest upward trends of LUE, with a rate of 1.58 \(\times\) 10\({}^{-3}\) g C/MJ/yr (\(p<0.05\)) (Figure S1e). TM's LUE presented a significantly increasing trend from 1982 to 1997 (\(p<0.05\)), but the growth trends of other grassland types were not significant (Figure 4b). Contrarily, from 1998 to 2018, the interannual LUE of all grassland types tended to be stable, except for SG (\(p<0.05\)) (Figure 4c). Additionally, all grassland types' average LUE increased significantly after 1998 (\(p<0.05\)), except for TS (Figure S2).  
Figure 3: Interannual dynamics of LUE (**a**), AGPP (**b**), APAR (**c**) and the results of the Mann–Kendall test for LUE (**d**), AGPP (**e**), APAR (**f**) in northern China grassland from 1982 to 2018. The dashed lines and shaded areas indicate the trends and 95% confidence intervals obtained by the simple linear regression model, respectively, where black dashed lines indicate the trends during 1982–2018, and red and blue indicate the trends during 1982–1997 and 1998–2018, respectively. The multi-year means from 1982 to 1997 and 1998 to 2018 are shown in insets. The letters in the insets indicate the results of non-parametric test interval differences. Data with the same letter are not significantly different (\(p>0.05\)). UF depicts the positive sequence statistics; UB depicts the inverse sequence statistics.