Spatio-temporal variations in non-photosynthetic vegetation cover on the Qinghai-Tibet Plateau grassland

Estimating the fractional cover of non-photosynthetic vegetation (f_NPV) is of great importance for managing grassland resources, calculating carbon balance, and adapting to climate change. The Qinghai-Tibet Plateau has unique climatic characteristics and rich vegetation types that play an important role in the global ecosystem. In this study, we first developed a method for estimating f_NPV using a dimidiate pixel model and normalized difference tillage index (NDTI) without field measurements. The model assumes that a mixed pixel is a linear combination of non-photosynthetic vegetation and bare soil, and their coefficients represent the proportion of the area occupied by non-photosynthetic vegetation and bare soil in that pixel. The model was then used to estimate the grassland f_NPV on the Qinghai-Tibet Plateau from the year 2000 to 2020 using MODIS surface reflectance data at 500 m resolution (MOD09A1). Statistical analyses were used to further examine the spatio-temporal variations in grassland f_NPV on the Qinghai-Tibet Plateau and their relationship with climatic factors. The results showed that the proposed dimidiate pixel model was capable of estimating f_NPV of the Qinghai-Tibet Plateau grasslands. The grassland f_NPV on the Qinghai-Tibet Plateau gradually increased from northwest to southeast. This trend was closely related to the topography and climatic factors of the Qinghai-Tibet Plateau. The grassland f_NPV on the Qinghai-Tibet Plateau was positively correlated with precipitation, and negatively correlated with precipitation in parts of the southwest. Temperature and f_NPV were positively correlated in the central and eastern parts of the study area, whereas in the south, southeast, and southwest, f_NPV was negatively correlated with temperature. The correlation between f_NPV and temperature in the study area was not significantly affected by precipitation, the correlation between f_NPV and precipitation was almost unaffected by temperature, and f_NPV was significantly affected by the mean precipitation received between May and September and the accumulated precipitation received between January and December. Our study provides a new approach for estimating the f_NPV and a new idea for the dynamic monitoring of non-photosynthetic vegetation on the Qinghai-Tibet Plateau.