Biogenic crust dynamics on sand dunes

Abstract

Sand dunes are often covered by vegetation and biogenic crusts. Despite their significant role in dune stabilization, biogenic crusts have rarely been considered in studies of dune dynamics. Using a simple model, we study the existence and stability ranges of different dune-cover states along gradients of rainfall and wind power. Two ranges of alternative stable states are identified: fixed crusted dunes and fixed vegetated dunes at low wind power, and fixed vegetated dunes and active dunes at high wind power. These results suggest a cross-over between two different forms of desertification.Sand dunes have been the subject of active research for many years, largely because of their fascinating shapes and dynamics [1][2][3][4]. Current studies have increasingly addressed the question of sand-dune stability in relation to climate change and anthropogenic disturbances [5][6][7]. Sand dunes are considered "stable" when they are fixed in place or are stationary [31]. Their stability is strongly affected by the degree of vegetation coverage. High coverage reduces the wind power at the dune surface and thereby acts to immobilize the dunes. The remobilization of fixed dunes, either by vegetation mortality or clear-cutting, often has detrimental effects on the unique ecosystems that develop in stable dunes [8,9], leading to alternative ecosystems associated with active sand [10]. Active dunes may also pose a threat to human settlement as they can block roads and cover residential areas and agricultural fields [11,12].Sand dunes are also stabilized by biogenic soil crusts. These crusts comprise a variety of organisms, including cyanobacteria, lichens and mosses, which live at the surface of desert soils [13]. Biogenic crusts enhance the aggregation of sand grains, prevent saltation, and reduce wind erosion. Since most sandy soils are located in drylands where the vegetation is patchy and generally sparse [3], the role of biogenic crusts in stabilizing dunes is important and often crucial [14]. Despite their significance and vast presence in the Kalahari, Australian and Central Asia deserts [15][16][17], soil crusts have rarely been considered in studies of dune dynamics [18,19].Depending on wind power and precipitation level different dune-cover states are observed. Figure 1 shows several typical states from regions of relatively weak winds. At very low precipitation levels (Fig. 1a), dunes are active due to low crust and vegetation coverage. As the precipitation increases, the dunes are gradually stabilized. At low precipitation levels, the dominant stabilizing agent is the soil crust (Fig. 1b,c), while at high levels, the stabilizing agent is predominantly vegetation (Fig. 1d). Although vegetation and biogenic crusts have similar effects in stabilizing dunes, they lead to ecosystems that differ vastly in their bioproductivity.Motivated by these observations, we ask whether the transition from crusted to vegetated dunes along the rainfall gradient is gradual or abrupt, and how it is affected by the wind power. Studying t...

The effect of wind and precipitation on vegetation and biogenic crust covers in the Sde-Hallamish sand dunes

“…Recent modeling studies [ Yizhaq et al , , ] have suggested the bistability of vegetation cover on sand dunes under given climatic conditions. Our recent modeling studies suggest a similar bistability for crust cover on sand dunes [ Kinast et al , ]. The field observations reported here support this prediction, as nearby plots 2 and 3 (with 1 m distance between them) on the north slope had quite different soil surface coverage, with zero crust cover for plot 2 while plot 3 had more than 40% crust cover (see Figure b).…”

The effects of psammophilous plants on sand dune dynamics

“…Our model for psammophilous plants (coupled to vegetation and BSC) follows previously suggested mean field models [ Yizhaq et al , ; Kinast et al , ] for the dynamics of vegetation and BSC cover of sand dunes. The dynamical variables in our model are the fractions of regular vegetation cover, v , BSC cover, b , and psammophilous plant cover, v p , where v p is a new variable added to the model described in Kinast et al [].…”

“…Our model for psammophilous plants (coupled to vegetation and BSC) follows previously suggested mean field models [ Yizhaq et al , ; Kinast et al , ] for the dynamics of vegetation and BSC cover of sand dunes. The dynamical variables in our model are the fractions of regular vegetation cover, v , BSC cover, b , and psammophilous plant cover, v p , where v p is a new variable added to the model described in Kinast et al []. The model does not account for the geomorphology of the sand dunes and adopts a very simplified view of the various interactions involved in dune dynamics (see, for example, the much more complicated models considered in Baas and Nield [], Eastwood et al [], and references therein).…”

“…These mathematical and physical models usually require a long integration time, therefore only enabling the simulations of relatively small dune fields. An alternative approach is to model the vegetation and BSC cover of the dunes, ignoring dune patterns and 3‐D dune dynamics, and to determine dune stability (active or fixed) according to the fraction of vegetation and BSC cover; bare dunes are active, while vegetated and/or BSC‐covered dunes are fixed [ Yizhaq et al , ; Kinast et al , ; Yizhaq et al , ]. Such models require a relatively short computation time and have been used to explain the bistability of active and fixed dunes under similar climatic conditions.…”

“…In addition, it is possible to model the development of a 2‐D vegetation cover by considering the spatial effect of the wind and the diffusion of vegetation [ Yizhaq et al , ]. Both observations [ Veste et al , ] and models [ Kinast et al , ] indicate that BSC plays an important role in dune stabilization in arid regions with relatively weak winds.…”