Wind tunnel study of the effect of planting Haloxylon ammodendron on aeolian sediment transport

Highlights

• Influence of vegetation density and pattern at a regional scale investigated.

• Geostatistical interpolation used to analyse the spatial variation of sand depth.

• Density and pattern of artificial shrubs significantly affected sediment transport.

• Wind erosion still occurred around shrubs regardless of vegetation density and pattern.

• Optimal planting pattern for reducing aeolian transport different in the 3 densities.

Revegetation is a widely used way to minimise aeolian sediment transport in desertified land. An optimal vegetation density and planting pattern reduces the economic investment for revegetation and competition for limited water resources. This study aimed to ascertain the effects of vegetation density and planting pattern on aeolian sediment transport at the regional scale. A series of wind tunnel tests were implemented with artificial shrubs representing Haloxylon ammodendron. These tests were designed with three planting densities (i.e., 600 plants ha⁻¹, 1200 plants ha⁻¹, and 1650 plants ha⁻¹) and four patterns (uniform distribution, random distribution, two rows/one belt, and one row/one belt pattern). Geostatistical interpolation methods available in ArcGIS were used to analyse the spatial variation of sand heights. Vegetation density and planting pattern and their interaction all significantly affected the aeolian sediment transport but vegetation density had the greatest impact. With increasing density of vegetation, wind erosion rate decreased significantly following quadratic function, and wind erosion still occurred around the artificial shrubs regardless of vegetation density and planting pattern. For wind erosion rate, erosion and deposition area, the uniform distribution pattern had little protective benefit at the three planting densities, whilst the combinations of the random distribution pattern with 600 plants ha⁻¹, two rows/one belt pattern with 1200 plants ha⁻¹, and one row/one belt pattern with 1650 plants ha⁻¹ had the best protective benefit in controlling the aeolian sediment transport.

Introduction

Aeolian desertification is a serious environmental problem in northwest China and is characterised by aeolian sediment transport. It not only brings about land degradation but also results in loss of biodiversity, pollutes the environment, adversely impacts on human health and destroys living facilities, eventually restricting economic and social sustainable development (Reynolds et al., 2007; Schlesinger et al., 1990), which in turn intensifies regional poverty. Therefore, seeking suitable approaches to control aeolian sediment transport is essential.

Sand-fixing vegetation involves drought resistance, wind erosion and sand burial tolerance and is the most effective measure for controlling aeolian sediment transport (Li, Qu, Han, Fang, & Wang, 2013; Sun, 2000; Yang et al., 2006). Vegetation can prevent airflows from acting directly on the ground, reduce wind speed near the surface layer, alter the erosion and deposition rule of wind-sand flow, capture saltating grains and enhance the threshold wind velocity for movement of sand (Cleugh, 1998; Dong, Gao, & Fryrear, 2001; Eldridge & Leys, 2003; Vigiak, Sterk, Warren, & Hagen, 2003; Wolfe & Nickling, 1993). Vegetation can also reduce wind erosion by improving microhabitat and accelerating the process of soil formation (Perroni, Montaña, & García-Oliva, 2006; Ruiz, Zaragoza, & Cerrato, 2008). Thus, knowing the impact of vegetation in counteracting aeolian sediment transport is important for the revegetation of aeolian desertified land.

Plant morphology (plant height, porosity, canopy width, leaf area, etc.), the spatial structures of shelterbelt (forest belt width, section form, plant spacing, etc.), and vegetation density are all key factors controlling wind-blown transport (Banzhaf, Leihner, Buerkert, & Serafini, 1992; Brandle, Hodges, & Zhou, 2004; Chepil & Woodruff, 1963; Hagen & Skidmore, 1971; Wilson, 1987). For shelterbelts constructed with one species of vegetation, planting density is likely the most important design parameter; the higher the vegetation density, the better the protective benefit in controlling aeolian sediment transport, but in arid region vegetation density is mainly limited by water resources. For a given vegetation density, planting pattern should be fully considered because when shelterbelts are constructed planting pattern significantly affects the wind flow distribution and the distance protected (Erktan & Rey, 2013). To summarise, determining the suitable vegetation density and the optimal design of planting pattern is an effective way to help reduce the economic investment required for revegetation and deal with the water shortage.

The effect of vegetation on aeolian sediment transport has been studied both in the field and in wind tunnels. Field measurements of aeolian sediment transport in vegetated areas have been conducted on different scales and this has driven many advances in our understanding of the role of vegetation in controlling wind erosion but it is often difficult to draw general conclusions because the impacts of vegetation on wind erosion cannot be easily isolated and results are often highly site-specific (López, Sabre, Gracia, Arrúe, & Gomes, 1998; Ma, Wang, Qu, & Liu, 2010). Wind tunnel tests can be a suitable alternative to studying the effect of vegetation on aeolian sediment transport by controlling wind speed, topography, soil properties and other factors. Previously, most wind tunnel tests regarding the protective benefit of vegetation focused mainly on the morphological characteristics of the individual plants, patterns of multiple plants such as single row, multi-rows and uniform distribution (Bitog et al., 2012; Burri, Gromke, Lehning, & Graf, 2011; Cheng et al, 2018, 2019; Lee & Lee, 2012; Liu, Zheng, Cheng, & Zou, 2018; Wu, Zou, Zhou, Zhang, & Shi, 2015; Youssef et al., 2012), and the simulation of field conditions at the point scale (Li et al., 2008; Musick, Trujillo, & Truman, 1996). Wind tunnel tests have contributed greatly to improving the understanding of the relationship between vegetation and wind erosion. However, to date they have been insufficient to understand the effects of vegetation on aeolian sediment transport at a regional scale. Therefore, further studies are required to fill the gaps in knowledge between the relationships between vegetation density, pattern and wind erosion rate.

This study aimed to investigate the impacts of vegetation density and planting pattern on aeolian sediment transport at the regional scale by wind tunnel tests. This paper presents (1) the best geostatistical interpolation method regarding the spatial variation of sand heights, (2) the wind erosion rate under different vegetation density and planting pattern conditions, and (3) the size and spatial distribution of erosion and deposition area along the test board.