# Altered Trends in Light Use Efficiency of Grassland Ecosystem in Northern China
## 4 Discussion  
### Altered Temporal Trends in LUE  
In this study, we found that the multiyear mean LUE in grasslands of northern China was 0.88 \(\pm\) 0.047 g C/MJ, which is comparable with the findings of previous similar  
Figure 7: The driving factor that contributes the most to an increase (or decrease) in LUE indicated for each grid cell in 1982–2018 (**a**), 1982–1997 (**b**) and 1998–2018 (**c**). The insets show the area fractions of land dominated by each factor in different grassland types. Gross, grasslands of northern China; AM, alpine meadow; AS, alpine steppe; TM, temperate meadow; TS, temperate steppe; SG, sparse grassland; SS, sparse shrulband. studies [12; 14; 41], indicating that the assessed LUE in grasslands of northern China is reliable. From 1982 to 2018, the LUE in the grasslands of northern China increased at an annual rate of 3.42 \(\times\) 10\({}^{-3}\) g C/MJ. Additionally, there was a difference between two stages (Figure 3a,d), 1982-1997 and 1998-2018, in terms of the mean values and trends of increase in LUE. In accordance with earlier research, the LUE in the northern African savannahs stabilized between 1982 and 2010 as a result of simultaneous and roughly similar increases in both APAR and AGPP [15]. In the grasslands of northern China, however, the rate of increase in AGPP was higher than the increase rate of APAR (Figure 3b,c), leading to imbalances in the growth of AGPP and APAR and a sharp rise in LUE. [PERSON] et al. [42] and [PERSON] et al. [24] showed that climate change and frequent human activities are important reasons for the rapid growth in AGPP and APAR. Our study has confirmed the above, as we found that 1998 represents an abrupt change point (Figure 3d). The slowdown in the growth rate of the interannual LUE variability after 1998 might be related to the ecological protection and restoration initiatives promoted by the Chinese government. The substantial increase in the annual growth rate of APAR after 1998 statistically explains the differences in the interannual LUE variability, judging from the parameters of the LUE estimation model. And the increase in LAI caused by ecological protection and restoration initiatives might be the reason for the substantial increase in APAR growth rate--the mean annual rate of increase in LAI since the implementation of the ecological protection and restoration initiatives is nearly 8 times the former rate [39]. Therefore, in the past 40 years or so, the variability in LUE has generally shown a significant increasing trend due to the influence of climate change and human activities in the grasslands of northern China. Our results also show that the interannual dynamics of LUE in all grassland ecosystem types have undergone a significant increasing trend over the past 40 years. The highest rate of increase was in AM (4.85 \(\times\) 10\({}^{-3}\) g C/MJ/yr), and the lowest increase rate was in TS (1.58 \(\times\) 10\({}^{-3}\) g C/MJ/yr). QTP grassland is one of the most sensitive ecosystems to global climate change [43]. Changes in temperature and precipitation have been suggested as a dominant driver of change in QTP grasslands [44]. QTP grasslands respond strongly to warming, and are also limited by precipitation [45]. AM is mainly distributed in the eastern region of QTP, and is affected by climate factors. Over the past 40 years, the temperature and precipitation in AM regions have increased significantly (Table 1), which is responsible for the highest LUE growth rate being seen in AM. In the case of TS, water availability was identified as the primary limiting factor of LUE. TS is mainly distributed in China's arid and semi-arid climate zones, where moisture is the most basic and important resource for vegetation growth [46]. grasslands in semi-arid climate zones are mainly limited by water resources [17; 22; 33]. The lack of water will inhibit the photosynthesis and respiration processes of the grassland ecosystem [47], thereby affecting their ecological functions. Therefore, water availability is critical to the LUE of TS.