Converging Effects of Shrubs on Shadow Dune Formation and Sand Trapping

Shrubs have a patchy distribution under natural conditions. Spacing, as a quantitative parameter used to describe shrub distribution, influences the formation of vegetated aeolian landforms and sand trapping in sandy regions. In this study, the morphology and development of shadow dunes were investigated in a wind tunnel at different wind velocities for three pairs of various-sized artificial shrubs constructed from live branches and with different spacing patterns. The amount of sand trapped by these shrubs was also measured. With decreasing spacing between two adjacent shrubs, independent dunes, separate dunes that mutually curved toward each other, converging dunes, and coalesced dunes were sequentially observed. Two critical thresholds of shrub spacing were determined, for dune deformation (Td) and dune convergence (Tc). Td and Tc represent the spacings between two adjacent shrubs that result in isolated and independent shadow dunes starting to deform and converge in their tail areas, respectively. The spacing thresholds of shrubs increased with shrub size and decreased with wind velocity. The processes of shadow-dune development for two adjacent shrubs were classified into three types: separate, separate-converging, and coalesced. The amount of sand trapped by shrubs at spacings of Td. The converging effects of shrubs on shadow-dune formation and sand trapping decreased with wind velocity and increased with shrub size. The results suggest that the shadow dune morphology and sand trapping capacity of shrubs are governed by both wind regime and distribution pattern and size of shrubs.