Research on temporal-spatial distribution differences and formation mechanisms of NPP in the Lanzhou section of the yellow river mainstream

The Lanzhou section of the Yellow River mainstem is a typical area for vegetation restoration in the ecologically fragile region of Northwest China. The spatiotemporal differences in Net Primary Productivity (NPP) of vegetation in this area serve as an important criterion for measuring the effectiveness of assessing vegetation growth. We analyze the spatiotemporal differentiation characteristics and driving mechanisms of NPP in the northern and southern mountain along the Lanzhou section of the Yellow River mainstream from 2000 to 2020 with the methods of the CASA model, Sen-MK trend analysis, and geographical detector. The results are as following: (1) Between 2000 and 2020, the overall trend of NPP in the North and South Mountain showed an increasing state, with the proportions of extremely significant growth areas in the North and South Mountain being 66.62% and 61.01%, respectively. But NPP per area is dominated with lower grades and presents a slowly increasing, especially in the North Mountain, the area with lower altitude and having stronger human activities. (2) The average NPP of the South Mountain from 2000 to 2020 was 202.01 gC·m⁻²·a⁻¹, while that of the North Mountain was 118.69 gC·m⁻²·a⁻¹. Analyses of total quantity differences, grade distribution differences, temporal variation differences, and seasonal differences all indicated that the Vegetation NPP of South Mountain was superior to that of the North Mountain; (3) This differentiation was influenced by a combination of natural factors (topographic factors, climatic factors, vegetation coverage) and human activities (land use types); (4) The geographical detector results further confirmed that NDVI was the most significant factor influencing NPP (q-value of 0.654938 for the southern mountains and 0.466094 for the northern mountains), and the interaction between NDVI and temperature had the most significant impact on NPP. This study helps to better understand the mechanisms behind regional NPP differentiation and reveal the extent which natural changes and human activities could influence NPP. It suggests that, in addition to implementing proven ecological restoration measures, efforts to improve vegetation cover quality in Lanzhou section of the Yellow River mainstem should comprehensively consider both natural and human factors, promote ecological comprehensive governance work according to local conditions in the future.