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 .Xanthoceras sorbifolium is a promising bioenergy crop as its seed oil is a suitable feedstock for biodiesel. It belongs to theXanthoceras Bunge (Sapindaceae Juss. ) and is a unique woody oil tree species unique to China with a planting area of more than 170000 ha (Ma et al., 2020). Caragana korshinskii is a cultivated leguminous shrub belonging to the legume. Its unique stress resistance characteristics make it an important forage, industrial raw material and afforestation species, with various economic and ecological values (Zhang et al., 2009). Hippophae rhamnoides belongs to the familyHippophae rhamnoides . It has the characteristics of drought tolerance, cold tolerance and barren tolerance. It is one of the few tree species that can grow in the forest zone, forest steppe zone and typical steppe zone at the same time. Sea buckthorn berries have been used as traditional medicine for a long time due to their richness in various bioactive ingredients (Sharma et al., 2019; Wei et al., 2019).