# Early Evidence That Soil Dryness Causes Widespread Decline in Grassland Productivity in China
## 3 Results  
### Characteristics of Long-Term Changes in Chinese Grassland SM and Its Constraints on Ecosystem GPP  
Th long-term trend of soil moisture simulation in Chinese grasslands is not insignificant over the past 100 years. The annual SM in the historical period had an insignificant soil-degenerating aridity trend, and future projections were largely constant (Figure 1a). The warm-season SM showed a similar trend, but the moisture anomaly domain was significantly higher in the warm season than in the annual (Figure 1b). Compared to the trends, the interannual variability of soil moisture in Chinese grasslands is highly significant, as can be seen in periods such as 1930-1950 and 2050-2060 when soil moisture is significantly higher. In conclusion, the long-term change of soil moisture is mainly controlled by its interannual fluctuations. Therefore, the next analysis will focus on the regulation of the interannual variability of soil moisture on ecosystem GPP. Figure 2 shows that the interannual variability of SM and GPP is similar, i.e., years with low SM correspond to low GPP, and the two are strongly positively correlated. The correlations between SM\({}_{\text{IAV}}\) and GPP\({}_{\text{IAV}}\) are 0.171 and 0.477 for the whole year in the historical and future periods, respectively, and the fluctuation patterns of both are found to have been basically consistent since 1950. The degree of correlation was significantly stronger in the warm season than in the whole year, with correlations of 0.48 and 0.63 between them year by year for the historical and future periods, respectively, and both passed the significance level (\(p<0.05\)) test. We concluded that soil moisture is an important factor that strongly regulates ecosystem GPP, and when soil moisture is below a certain threshold, it significantly constrains the accumulation of ecosystem GPP, because an insufficient soil moisture supply significantly forces a decrease in plant photosynthetic capacity, which in turn leads to a decrease in ecosystem GPP. Moreover, this effect more strongly restricts plant growth during the growing season. Next, the correlations of SM\({}_{\text{IAV}}\) and GPP\({}_{\text{IAV}}\) were calculated using a sliding window of 41 years for the whole year and warm season from 1901 to 2099 (Figure 3). The bivariate correlations were found to be consistently positive in most periods and regions of Chinese grasslands, and the correlation values were higher for the warm season than for the whole year. The correlations between SM\({}_{\text{IAV}}\) and GPP\({}_{\text{IAV}}\) in Mongolia, the Loess Plateau, and Xinjiang all exceeded 0.4 and passed the significance test, while the correlation in the Tibetan Plateau grasslands gradually increased to 0.2 from year to year. This reflects that the regulation of GPP by soil moisture is always positive, i.e., it suppresses plant growth in water-scarce years and promotes it in water-abundant years. In comparison, the positive correlations in the warm season are higher in all regions and stages than in the whole year, and the correlations in Chinese grasslands are always around 0.4 throughout the year, while the correlations in the warm season have been over 0.5 since 1970. In the Xinjiang and Loess Plateau regions in particular, the correlations of the warm-season bivariate exceeded 0.6. This indicated that the regulation of GPP by soil moisture is higher in the warm season than in the whole year; in other words, the warm-season soil moisture deficit can trigger more serious ecosystem GPP loss events in most cases. Given that our study discusses the effect of soil moisture constraint events on ecosystem GPP, the choice of warm-season bivariate better reflects the limiting effect of extreme moisture constraints on carbon uptake in grasslands, and, therefore, the SM and GPP in the warm season were chosen for further analysis. Figure 1: Long-term variation in soil moisture anomalies based on process-model simulations. (**a**) Annual SM; (**b**) warm-season SM.