Spatiotemporal variation of net primary productivity and its response to drought in Inner Mongolian desert steppe

The desert steppe in Inner Mongolia is an important part of the temperate grasslands in Central Asia and plays a key role in sustaining the regional pastoral livelihood and securing the ecological environment in northern China. Grassland net primary productivity (NPP) is an important indicator to ecosystem health, and is largely affected by the variation of precipitation, especially the frequent droughts in the desert steppe region. Monitoring the spatiotemporal changes of the NPP in relation with climate variation, especially with the drought events, is of great significance for assessing the total amount of grassland resources and predicting its future change trend, which is critically important for the development of grassland management regimes with the consideration of current and future climate changes. In the present study, we estimated the spatiotemporal variation of the NPP of the desert steppe in Inner Mongolia for the period from 2000 to 2019. We calculated the standardized precipitation evapotranspiration index (SPEI) of the study area, and analyzed its effects on vegetation NPP, especially for the drought effects. We found that the temporal trend line and Mann-Kendall statistical curve of vegetation NPP were basically consistent. Over the 20-year period, the NPP showed an overall downward trend during the plant growing season, or in summer and autumn period, but a slight upward trend in spring. Spatially, the plant growing season NPP slightly decreased on the 74.89% of the studied desert steppe area, the NPP in summer slightly decreased on the 72.25% of the area, while the NPP in spring slightly increased on the 61.8% of the area, and NPP in autumn significantly decreased on the 25.9% of the area. From the perspective of the persistence on different time scales, the proportion of moderate persistence-slight increase area covered 11.72% of the study area, and weak persistence-slight increase in spring covered 50.10% of the study area. The proportion of weak persistence-slight decrease in summer was 59.68%. The proportion of moderate persistence-slight decrease covered 18.83% in autumn. In the growing season, a positive correlation between NPP and SPEI accounted for 91.18% of the desert steppe area. Over 8% of the area showed a highly significant positive correlation, and 25.21% showed a significant positive correlation. Overall, 92.56% of the area showed a positive correlation in spring, and 87.26% showed a positive correlation in summer. A positive correlation was observed over 60.71% of the area in autumn. The research results show that the NPP results of CASA model simulating desert vegetation in Inner Mongolia are reliable. The SPEI drought index has good applicability in the study area. On the whole, the dynamic changes of NPP are closely related to drought. This research has important reference significance for regional ecosystem management and construction.