Spatial distributions of optimal plant coverage for the dominant tree and shrub species along a precipitation gradient on the central Loess Plateau

The Loess Plateau in China has the most severe soil erosion in the world. Increasing plant coverage can effectively control soil erosion; however, low water availability in this region limits plant growth. The objective of this study was to determine the optimal plant coverage for the two non-native plants mainly used in vegetation restoration (Robinia pseudoacacia and Hippophae rhamnoides) on the Loess Plateau. We analyzed the spatial distribution of the mean actual evapotranspiration (AET), net primary productivity (NPP) and maximum leaf area index (LAI) along a precipitation gradient transect on the central Loess Plateau. The modified Biome-BGC model was used to simulate the dynamics of AET, NPP, and LAI for the two plants. The model was assessed by using the only available parameter that had been continuously determined in the field (i.e., AET) that pertained to the two plants growing at two sites that had validated physiological parameters. The validated model was subsequently used to simulate the dynamics of AET, NPP and maximum LAI for the two plants at 75 representative sites along the transect. The results indicated that annual NPP and maximum LAI did not present significant trends over time for either plant. Spatial distributions of the mean AET, NPP, and LAI exhibited decreases along the southeast to northwest precipitation gradient on the Loess Plateau, which was consistent with the spatial distribution pattern of the mean annual precipitation (MAP) in the studied area. In the non-native tree zone where MAP was greater than 550 mm, the optimal plant coverage (given by the mean maximum LAI value) ranged from 2.5 to 3.5. In the non-native shrub zone where MAP ranged from 250 to 350 mm, the optimal plant coverage ranged from 0.8 to 1.5. In the mixed zones of non-native trees and shrubs where MAP ranged from 350 to 550 mm, the optimal plant coverage ranged from 1.5 to 2.5. These quantitative findings giving optimal plant coverages for different precipitation regions should be useful for guiding non-native vegetation restoration on the Loess Plateau.