Dominant drivers of vegetation changes in key ecological barrier of northeastern Tibetan Plateau since 2000: Human impacts or natural forces?

Human activities and natural forces have profoundly influenced vegetation ecosystems in the Tibetan Plateau over the recent decades. However, contributions of these two driving forces to vegetation changes remain controversial, especially in the ecological barriers like the Qilian Mountains (QM) in the northeast where many protecting measures and strategies were applied to enhance ecosystem stability and services. Our study employed a process-based model and a multi-perspective assessment method to determine dominant drivers of vegetation changes in the QM since 2000. The result indicated that human activity changes contributed 40.74% to the significant vegetation amelioration of the QM as a whole, which is comparable to natural forces. Areas dominated by human activity changes accounted for 18.42% of the entire region, which were mainly distributed in grassland restoration area, oasis agricultural area and forestry protection area. In these areas, increased human activities (including desertified grassland restoration, agricultural irrigation and fertilization, forest protections and afforestation) mainly promoted the vegetation amelioration. Yet, natural forces resulted in amelioration in partial alpine grasslands in southeastern QM, accounting for 10.53% in area. Areas dominated by both human activity changes and natural forces, mainly in grazing grasslands, accounted for 47.80%. Variations in grazing intensity and climate jointly determined fluctuations in vegetation therein. Additionally, there remained 23.25% areas lack of obvious drivers, generally with sparse vegetation. Ecological protections and agricultural measures significantly promoted the vegetation amelioration of the QM since 2000. The findings could provide essential insights for protection and construction of the ecological barriers in the Tibetan Plateau.