*2.4. Changes and Trends in Grassland Response to Drought*

The area of grassland Fv, Fb, and NPP responding to scPDSI in the control response for grassland increase accounts for 36.55, 37.99, and 28.23% of the total area, respectively (Figure 5A–C). The control response to grassland increase from Fv, Fb, and NPP to scPDSI appear in similar areas, mainly concentrating on central, north, and west MG, and west IM. On the contrary, the control response to grassland decrease accounts for 10.09, 17.10, and 14.98% (Figure 5a–c). It is mainly concentrated on the south Sayan Mountains, south Hangai Mountains, and Dornod in MG, and northeast and south IM. The area of grassland Fv, Fb, and NPP responding to scPDSI in the counter response for grassland increase accounts for 44.73, 37.76, and 40.34% of the total area, respectively (Figure 6A–C). The counter response to grassland increase from Fv, Fb, and NPP to scPDSI appear in similar areas, mainly concentrating on the northeast and west MG and northeast and south IM. On the contrary, the counter response to grassland decreases accounts for 8.63, 7.15, and 16.45% (Figure 6a–c). It is mainly concentrated on central MG and west IM. *Plants* **2022**, *11*, x FOR PEER REVIEW 7 of 17

**Figure 5.** The control response of Fv, Fb, and NPP to PDSI changes in the Mongolian plateau from 2000 to 2013. ((**A**—**C**) are the correlation coefficient spatial distribution and (**a**–**c**) corresponding its pixel frequency). **Figure 5.** The control response of Fv, F<sup>b</sup> , and NPP to PDSI changes in the Mongolian plateau from 2000 to 2013. ((**A**–**C**) are the correlation coefficient spatial distribution and (**a**–**c**) corresponding its pixel frequency).

**Figure 6.** The counter response of Fv, F<sup>b</sup> , and NPP to PDSI changes in the Mongolian plateau from 2000 to 2013. ((**A**–**C**) are the correlation coefficient spatial distribution and (**a**–**c**) corresponding its pixel frequency).

**Figure 5.** The control response of Fv, Fb, and NPP to PDSI changes in the Mongolian plateau from 2000 to 2013. ((**A**—**C**) are the correlation coefficient spatial distribution and (**a**–**c**) corresponding its

#### **3. Discussion**

pixel frequency).

#### *3.1. Methodology*

The current study used the slope-combined analysis based on multi-index to simulate grassland vegetation dynamic and monitor grassland response to droughts. The hypothesis is that grassland F<sup>v</sup> and NPP dynamic are a positive feedback, whereas F<sup>b</sup> is on the contrary. Previous studies applied single index, such as NDVI, Fv, and NPP, to simulate the grassland dynamic. However, many uncertainties remain due to the inversion model or the uncertainty of dataset itself [34]. The advantage of the current method is the reference of F<sup>b</sup> index. Our findings show that 12.93% of the grassland in the MP experiences an increasing trend compared with 0.73% of the grassland that experienced a decreasing trend during the study period. Several studies about grassland NPP showed that grassland has an increasing trend in the similar area during the study period [35,36]. Similarly, studies on vegetation indexes, such as NDVI, Fv, and EVI, show an increasing trend of grassland vegetation [37–39]. Thus, the present studies confirmed that the grassland shows a recovery trend in the MP, which agrees with our findings.
