# Response of Grassland Vegetation Growth to Drought in Inner Mongolia of China from 2002 to 2020
## 3 Results  
### Spatiotemporal Patterns of NDVI, EVI, GOSIF, and GPP from 2002 to 2020  
Figure 2a illustrates the change trends in monthly grassland NDVI and EVI from 2002 to 2020, revealing apparent periodical characteristics with a one-year cycle. Notably, the maximum values of grassland NDVI, reaching around 0.40, consistently occurred in July or August each year, signifying the period when grassland growth and metabolism were most vigorous. Conversely, the minimum values of grassland NDVI, dropping below 0.1, were consistently observed in January, indicating minimal grassland growth during that period. The change trends of monthly grassland EVI exhibited similar patterns to NDVI, but the fluctuation range of EVI values was lower than NDVI (Figure 2a). Monthly grassland GOSIF and GPP also displayed clear periodic characteristics throughout the period from 2002 to 2020 (Figure 2b). GOSIF exhibited patterns similar to those of GPP, with the maximum value appearing in August 2018, reaching \(0.1371\,\text{W}\cdot\text{m}^{-2}\cdot\text{\mu}\text{m}^{-1}\cdot\text{sr} ^{-1}\). Conversely, the minimum GOSIF value was recorded in February 2014, with a value of \(-0.0121\,\text{W}\cdot\text{m}^{-2}\cdot\text{\mu}\text{m}^{-1}\cdot\text{sr} ^{-1}\). In the case of grassland GPP, its maximum value was observed in July 2012, reaching \(126.61\,\text{gC}\cdot\text{m}^{-2}\cdot\text{month}^{-1}\). Overall, the monthly variations in grassland NDVI, EVI, GOSIF, and GPP consistently exhibited a one-year cycle in Inner Mongolia between 2002 and 2020. Figure 2: Temporal trends of monthly average grassland (**a**) NDVI, EVI, (**b**) GOSIF, and GPP from 2002 to 2020. In Figure 3, the yearly trends of NDVI, EVI, GOSIF, and GPP from 2000 to 2020 are depicted. All four variables exhibited significant increasing trends during this period, with change rates of 0.0024 a\({}^{-1}\), 0.0018 a\({}^{-1}\), 0.0012 W\(\cdot\)m\({}^{-2}\cdot\)um\({}^{-1}\cdot\)sr\({}^{-1}\cdot\)a\({}^{-1}\), and 4.6950 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\), respectively (\(p<0.01\)). The peak values for grassland NDVI, EVI, GOSIF, and GPP were observed in 2018, reaching 0.4617, 0.3145, 0.1394 W\(\cdot\)m\({}^{-2}\cdot\)um\({}^{-1}\cdot\)sr\({}^{-1}\), and 447.7892 gC\(\cdot\)m\({}^{-2}\), respectively. Conversely, the minimum lowest values for grassland NDVI and EVI were recorded in 2009 at 0.3593 and 0.2381 (Figure 3a), while the lowest values for grassland GOSIF and GPP were observed in 2007, with values of 0.0856 W\(\cdot\)m\({}^{-2}\cdot\)um\({}^{-1}\cdot\)sr\({}^{-1}\cdot\)a\({}^{-1}\) and 322.67 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) (Figure 3b). Figure 4 illustrates the spatial patterns and change trends of grassland NDVI, EVI, GOSIF, and GPP in Inner Mongolia between 2002 and 2020. The annual mean values of these variables showed similar spatial distribution characteristics, gradually increasing from the northwest to the southeast (Figure 4a-d). High values were primarily concentrated in the eastern part of Inner Mongolia, including LM regions and TMS regions (Figures 1 and 4a-d), while lower values prevailed in the western regions, which are predominantly TD and TSD (Figures 1 and 4a-d). Spatial variability in grassland change trends across Inner Mongolia from 2002 to 2020 was evident. For NDVI and EVI, approximately 82.13% and 80.18% of the region displayed increasing trends. Notably, 25.83% and 21.18% of the grassland NDVI and EVI values showed significant increasing trends, primarily concentrated in Tongliao, Ordos, and Alxa League (Figures 1 and 4e,f). Conversely, only 0.91% and 0.82% of the grassland NDVI and EVI values exhibited significant decreasing trends, scattered in areas such as Chifeng and Alxa League (Figures 1 and 4e,f). As for grassland GOSIF and GPP, the areas showing increasing trends (84.42% and 96.89%) were substantially higher than those with decreasing trends (15.58% and 3.11%).