Temporal and spatial evolution of vegetation cover in the wind erosion field on the Qinghai–Xizang Plateau

Abstract

Soil wind erosion is one of the key earth surface processes in arid and semi-arid regions. Soil wind erosion not only leads to land desertification but also serves as an important source of fine particulate matter in the atmospheric environment. Accurate assessment of soil wind erosion and its temporal and spatial distributions is critical for planning and implementing soil conservation measures. As an important factor in wind erosion control, vegetation coverage has been included in almost all the major wind erosion models. The traditional models, however, usually overestimate wind erosion rate because they rely solely on photosynthetic vegetation coverage (PVC) but overlook nonphotosynthetic vegetation coverage (NPVC), such as fallen leaves and branches covering and protecting the soil. In the current study, field surveys, phenological data and fractional vegetation coverage (FVC) derived from the normalized difference vegetation index (NDVI) were employed to examine the temporal and spatial evolution of both PVC and NPVC in the wind erosion region on the Qinghai–Xizang Plateau (QXP). The results reveal significant variations in phonology across QXP. During 2000–2020, the growing season started on Julian Days 124–150, i.e., corresponding to the last month of spring, and ended on Julian Days 242–296, i.e., covering almost the first half of autumn, in the wind erosion-prone areas of QXP. The vegetation greening initially began in the northern basins and southern river valleys with lower elevations and higher air temperatures, followed by the plateau areas with higher elevations and lower temperatures. Whereas, an opposite trend was manifested in the evolution of senescence. Approximately 40.7% of the area in arid, semi-arid and extreme arid regions had never been observed greening. Owing to the combined influence of topography and climate, the vegetation coverage exhibited a decreasing trend from the southeast to the northwest of QXP. The mean annual FVC during the growing and nongrowing seasons were 36.2% and 24.4%, respectively. During the growing season, moreover, the FVC was approximately 1.04–1.37 times greater than the PVC. Regarding the interannual trend, the vegetation coverage increased from 2000 to 2020 in general. The mean annual FVC over the entire study region increased by 0.15% and 0.14% during the growing and nongrowing seasons, respectively, over the past 20 years. The temporal trend, however, varied among different areas. During the growing season, FVC remained basically unchanged in 37.2%, experienced mild improvements in 42.0% and underwent mild degradations in 20.8% of the study region. These findings hold important implications for understanding soil wind erosion processes and improving wind erosion models on QXP.