# Altered Trends in Light Use Efficiency of Grassland Ecosystem in Northern China
## 4 Discussion
### Driving Factors of Interannual Dynamics of LUE  
This study found a strong relationship between the interannual dynamics of LUE and climate factors. LUE is positively correlated with precipitation, temperature and VPD, and negatively correlated with solar radiation (Figure 6). These results agree with those of previously published studies [8; 9; 14]. In the middle and high Northern Hemisphere, the interannual LUE variability was found to be positively influenced by temperature in a recent study [14]. As temperature increased, vegetation photosynthesis and water use efficiency were improved. The effects of precipitation on LUE might be driven by grassland plants' physiological changes. Changes in precipitation have a direct impact on the availability of water, which controls plant water uptake [6]. More precipitation will result in larger plant leaves [51]. Indirect mechanisms such as changing atmospheric water demand that may control stomatal conductance may also have an impact on LUE [52]. These would have an impact on evapotranspiration and photosynthesis. As a primary catalyst for plant photosynthesis, solar radiation is also not negligible [53]. With high solar radiation levels, vegetation may be light-saturated, and photosynthesis may even be suppressed by excessive light, resulting in low LUE [14; 54]. [PERSON] et al. [8] showed that there is a negative correlation between savanna LUE and VPD in Yunnan Province, China. [PERSON] et al. [14] also showed that LUE was negatively correlated with VPD in global grasslands. Our results are inconsistent with the former, whereby LUE is positively correlated with VPD in all grassland types, except for AS. We speculate that the inconsistent results may be related to the threshold of regional VPD [41].  
According to this study, temperature and precipitation are the dominant factors affecting the interannual LUE variability in the grasslands of northern China in the past 40 years. The interannual dynamics of LUE in the western study area are mainly driven by temperature, while the eastern study area is mainly affected by precipitation (Figure 7a). AM, AS, SG, SS and TM are distributed the western region. This area is generally higher in altitude and more sensitive to temperature than in the east. Previous studies on the QTP and areas surrounding it found that the temperature sensitivity index increases significantly along an elevational gradient [55; 56]. Accelerated warming at high elevations could stimulate vegetation growth by alleviating the restriction of the vegetation growth rate [57]. In addition, subfreezing temperatures can halt photosynthesis, and freezing can directly harm leaves or the hydraulic system that supplies them with water and nutrients [9; 58]. The eastern study area is a typical arid and semi-arid climate area, where TS is mainly distributed. Carbon sink capacity in arid and semi-arid grasslands is water-limited and sensitive to precipitation change [59]. Precipitation can affect grasslands' carbon sink capacity not only through effects on soil moisture and temperature, but also through the regulation of species composition, soil nutrient availability, and plant biomass [60; 61; 62]. Therefore, the LUE of TS was mainly driven by precipitation, and other grassland types were mainly affected by temperature in northern China (Figure 7a).  
Additionally, the relative contribution of drivers changes over time (Figure 7b,c), which may be related to global climate change. The sensitivity of vegetation to temperature change weakened in the Northern Hemisphere, and there was a significant decline in the strength of the relationship between interannual NDVI variability and temperature during the growing season from 1982 to 2011 [63]. Meanwhile, persistent warming may increase evaporation, thus leading to drought [50]. Consequently, we predict that the impact of changes in water status variables on interannual LUE variability will gradually became larger as temperatures continue to increase.