Quantitative analysis of the spatiotemporal dynamics and driving forces of vegetation NPP in Wulan County of China from 1982 to 2020

The driving effects of climate change and human activities on vegetation NPP (net primary productivity) is known as a research hotspot in the context of global climate change, of which the consensus cannot be reached at different spatiotemporal scales. The vegetation ecosystem in cold and arid pastoral areas of northwest China is fragile and highly sensitive to climate change and human activities. In this study, Wulan County, Qinghai Province was taken as the representative study area. Based on the long sequence NDVI (normalized difference vegetation index) dataset constructed by GIMMS-NDVI and MOD-NDVI, combined with the CASA (carnegie-ames-stanford approach) model, the vegetation growing season NPP of the study area from 1982 to 2020 was obtained. The spatiotemporal evolution characteristics of growing season NPP in the study area over the past 40 years were explored using the Sen+MK method, and then the proposed quantitative attribution method of ADE+Sen was used to quantitatively identify the driving forces of different climate factors and human activities to the dynamics of growing season NPP. The results showed that the annual mean of growing season NPP in the study area was (205.9±11.5) g/(m²·a) (measured in C), and the interannual variation of growing season NPP had no significant trend. The interannual variation of growing season NPP for different vegetation types was basically consistent with the regional growing season NPP. Over the space, the multi-year mean of growing season NPP gradually increased from west to east. The change patterns of growing season NPP exhibited significant spatial heterogeneity and were mainly characterized by degradation with the average rate of -0.151 g/(m²·a²). The areas with severe and slight degradation accounted for 31.7% and 29.5%, respectively. Climate change was the dominant driving factor for the dynamics of growing season NPP, with a dominant vegetation area accounting for 85.2%. The absolute average value of the contribution of climate change was 1.025 g/(m²·a²), which was nearly twice the absolute average value of the contribution of human activities. Solar radiation, precipitation, average temperature, and average wind speed were the main climate factors affecting the dynamics of growing season NPP. In conclusion, the vegetation in the study area was mainly in a degraded state, and climate change was the dominant driving factor leading to this phenomenon. The findings can provide reference for the sustainable utilization and protection of vegetation ecosystem in cold and arid pastoral areas of northwest China.