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 .