2.1. Site description
The study area (103°52′-105°13′ E, 34°26′-35°35′ N) is located in the middle of Gansu Province in northwest China (Fig. 1) and possesses the hills and gullies typical of the middle of the Loess Plateau. Its average elevation is about 1947 m. It has a semi-arid climate and average annual precipitation of about 390.99 mm, which falls mostly from July to September following the harvest season. The soil is a typical loess soil, which is soft and prone to erosion. In recent decades, large-scale vegetation restoration was implemented by the government to address ecological degradation. As such, a large area of sloped cropland was replanted with trees and shrubs. The major tree species planted during each period of reforestation are Xanthoceras sorbifolia ,Caragana korshinskii, and Hippophae rhamnoides .