Aeolian sediment transport processes are sensitive to dryland eco‐system change and can contribute to the transition of savannah grasslands to shrub‐invaded and shrub‐dominated states (Okin, Gillette, & Herrick, 2006; Ravi, Breshears, Huxman, & D'Odorico, 2010). In the southern Kalahari savannah, bush encroachment by Senegalia mellifera is a pervasive ecosystem change that creates mosaic landscapes of varying density grasses and bushes that can affect patterns of aeolian sediment transport (Thomas & Shaw, 1991). Herbaceous species losses and bush encroachment in the Kalahari have been associated with increased dune mobility (Wiggs, Thomas, Bullard, & Livingstone, 1995), degraded air quality (Witson, 2017) and potential reactivation of the region as a persistent dust source (Bhattachan et al., 2012). While accelerated wind erosion can be both driver and consequence of ecosystem change in the Kalahari (Bullard, Thomas, Livingstone, & Wiggs, 1997; Mayaud, Bailey, & Wiggs, 2017; Thomas & Leason, 2005), the role of aeolian processes and its regional impacts have not been fully established. Here, we
examine preliminary data on the influence of southern Kalahari ecosystem changes on surface aerodynamic roughness and aeolian sediment transport as a basis for understanding their interactions. Multiple drivers of ecosystem change have been identified in the Botswana Kalahari (Perkins & Thomas, 1993). These include the co‐
incidence of land use pressures from livestock grazing and changing land tenure associated with the first Livestock Development Project (LDP1) in 1970 and the 1975 Tribal Grazing Lands Policy (TGLP; Dougill, Thomas, & Heathwaite, 1999), scarce groundwater resources, highly variable rainfall, altered fire regimes, increasing atmospheric CO2 and regional warming (Saha, Scanlon, & D'Odorico, 2015). Research has investigated the impacts of these drivers on bush encroachment along livestock grazing gradients and at different scales (Dougill et al., 2016; Perkins, 2018; Porporato, Laio, Ridolfi, Caylor, & Rodriguez‐Iturbe, 2003; Thomas, Sporton, & Perkins, 2000), building consensus that the Kalahari savannah is a nonequilibrium system with alternative ecological states (Figure 1).
Bush encroachment in the southern Kalahari typically occurs within areas of concentrated livestock grazing around boreholes and may be associated with a decline in total grass species and cover (Dreber, Rooyen, & Kellner, 2018; Rutherford & Powrie, 2010). Overgrazing can also result in herbaceous species loss without bush encroachment (Thomas & Shaw, 1991). Although grasses can re‐establish after such changes (Bhattachan, D'Odorico, Dintwe, Okin, & Collins, 2014), without livestock exclusion affected areas can become mobile with active sand transport and dust emission (Wiggs, Livingstone, Thomas, & Bullard, 1994). Both fire and bush encroachment have previously been shown to influence surface wind speeds and microclimate in the Kalahari (Saha et al., 2015; Thomas et al., 2018; Wiggs, Livingstone, Thomas, & Bullard, 1996). We hypothesise that vegetation state transitions alter the aerodynamic roughness

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