Land surface roughness affected by vegetation restoration age and types on the Loess Plateau of China

Land surface roughness (LSR) plays a critical role in hydrological and erosion processes. The changes in near soil surface characteristics induced by vegetation restoration likely affect LSR greatly. However, few studies have been performed to quantify these potential effects. This study was conducted to investigate the changes in LSR under different restoration age and types, and identify the influencing factors contributing to those changes on the Loess Plateau, China. The photogrammetric method was applied to measure random roughness (RR) in one sloped farmland (as control), six abandoned farmlands with different restoration age, and five abandoned farmlands with different restoration types. The results showed that RR increased significantly after vegetation restoration. Random roughness increased exponentially with vegetation restoration age (R-2 = 0.93), and reached a steady state when the restoration age was approximately 25 years. Random roughness was greatly influenced by vegetation restoration types. The mixed forest of Amotpha fruticosa and Pinus tabuliformis had the maximum RR, followed by Caragana korshinskii, Pinus tabuliforrnis, Robinia pseudoacacia, and Artemisia sacrorum. Compared to control, RR increased by 1.66 to 4.68 mm and 3.68 to 6.67 mm for different restoration age and types. The variations in RR were closely related to the changes in near soil surface characteristics induced by vegetation restoration. Random roughness increased linearly with plant litter coverage, thickness, density, stem diameter, soil organic matter content, soil median grain size, sand content, and water stable aggregates. While, it decreased linearly with soil bulk density, silt and clay contents. The measured RR could be well estimated by plant litter density, soil organic matter content, silt content, and water stable aggregates (NSE = 0.77). These results are helpful to understand the influencing mechanism of vegetation restoration on hydrological and erosion processes on hillslope.