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).