Abstract
In desert dune environments, vegetation may be an indicator of dune stability and rates of sediment transport. As topography and underlying controls of vegetation vary over large spatial areas, variations in dune form make scaling up of field research difficult. To mitigate this, and to identify spatial variations in vegetation distributions in a Saudi Arabian sand sea, spectral information from high resolution satellite images was classified to map polygons of shrub vegetation over 360 km2 of well-defined linear dunes, broken linear dunes and dome dune forms. When compared to topographic characteristics of the landscape, extracted from a digital elevation model, vegetation densities were often highest on 10–20° slopes elevated above interdune salt deposits on dune flanks. Spatially this was confined to small areas, and density was not always related to dune form, more to access of groundwater which encouraged colonisation on the tops of some dunes. Field observations identified shrubs of mainly Calligonum genus whose size is related to the amounts of salts, moving sediment, and access to the water table that varies within and between dune forms. Shrub density is likened to surface roughness (z0) to better understand sediment movement and modelling in dunes.
Highlights
Vegetation distribution in desert dune systems is often related to long term dune stability (Stokes et al, 1997), short term interaction with airflows trapping and releasing sediment (Ash and Wasson, 1983; Wolfe and Nickling, 1993), and rapid changes in sediment transport rates when vegetation is removed (Bullard et al, 1997; Thomas and Leason, 2005)
Shrub vegetation cover made up 0.9 % to 1.5%, shaded sand ranged from 3.7 % to 12 %, illuminated sand, other sand and interdunes ranging from 87.1 % to 95.3 % with negligible contributions from buildings and roads
The Normalised Differential Vegetation Index (NDVI) threshold in the class training stage reduced the potential of confusion between vegetation inside shadow as shadow as inspection showed that there were instances where vegetation was detected inside areas of dune shadow
Summary
Vegetation distribution in desert dune systems is often related to long term dune stability (Stokes et al, 1997), short term interaction with airflows trapping and releasing sediment (Ash and Wasson, 1983; Wolfe and Nickling, 1993), and rapid changes in sediment transport rates when vegetation is removed (Bullard et al, 1997; Thomas and Leason, 2005). The loss of vegetation reduces soil fertility and as moisture and plants recede sand becomes mobile (Schlesinger et al, 1990; Ludwig and Tongway 1995; Valentin et al, 1999). The coincidence of these factors may cause clustering of vegetation in an oasis (Gimingham, 1955) and in arid regions spatial variations of these factors may cause spatial patterning of the vegetation (Aguir and Sala 1999). In arid environments and deserts predominant factors such as slope angle and aspect (Parker 1991, Valentin et al, 1999; Aguir and Sala 1999), salinization of groundwater and soils
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