Analysis of the temporal and spatial variations in net primary productivity of vegetation in the grassland region of Inner Mongolia and the factors influencing those changes

Inner Mongolia is an important region of the grassland ecosystem in China. Given that the net primary productivity (NPP) of vegetation is the main parameter of the carbon source and sink of ecosystems, studies of NPP variations and the factors that influence such changes are of great significance for the monitoring and prediction of the carbon sequestration capacity of different ecosystems. Therefore, this study was conducted to investigate the temporal and spatial variations in NPP of natural resources in the grassland region of Inner Mongolia and the factors influencing those changes, using annual MOD17A3–NPP product data from 2000 to 2019 as well as precipitation, temperature, relative humidity, light duration, surface evapotranspiration, elevation, and normalized difference vegetation index (NDVI) data. Through measures of the Theil-Sen median, Mann-Kendall significance level, and Hurst exponent, the temporal and spatial variations and sustainability of the NPP of natural resources in the grassland region of Inner Mongolia from 2000 to 2019 were determined. The factors influencing the NPP changes were analyzed using a spatiotemporal geographical weighted regression model. The results revealed the following: 1) From 2000 to 2019, the average regional NPP value was 207.72 g·(m²·a)⁻¹ and showed an upward trend. The average annual increase was approximately 4.06 g·(m²·a)⁻¹. 2) In terms of the spatial distribution, the areas with a significant rise (|Z| > 1.96) accounted for 79.88%, In terms of the spatial distribution, the areas with a significant rise (|Z| > 1.96) accounted for 79.88%, of which 60.31% showed a moderately significant rise (1.96 < |Z| < 2.576) and 19.57% accounted for an extremely significant rise (|Z| > 2.576). The areas with a significant decrease in this parameter accounted for only 0.15%, showing a sporadic distribution. The persistence of the change in NPP value in the future was determined using the Hurst exponent. Although the NPP value of 94.01% showed an increasing trend, the regions with a continuous increase (showing an upward trend, H > 0.5) accounted for approximately 19.45%, whereas those with a potential decrease accounted for approximately 74.15% (showing an upward trend, H < 0.5). 3) With regard to the factors influencing the change in NPP in the study area, the correlation of the NPP value with precipitation, relative humidity, and NDVI was mainly positive, with the average regression coefficients being 0.368, 0.218, and 0.402, respectively, in 2000 and 0.257, 0.081, and 0.559, respectively, in 2019. The influences of the relative humidity and NDVI on the NPP value had an obvious linear change trend from southwest to northeast. The influence of precipitation on NPP was negatively correlated in the southwest and northeast of the study area, with strong spatial aggregation and heterogeneity. 4) Changes in temperature, light duration, and evapotranspiration mainly had a negative correlation with the NPP value, with the average regression coefficients being −0.027, −0.061, and −0.020, respectively, in 2000 and −0.145, −0.201, and −0.010, respectively, in 2019. The effects of light duration and evapotranspiration on the change in NPP showed an obvious linear trend from southwest to northeast.