Variable vegetation cover and episodic sand movement on longitudinal desert sand dunes

Geomorphology 81 (2006) 276 – 291 www.elsevier.com/locate/geomorph

Variable vegetation cover and episodic sand movement on longitudinal desert sand dunes Paul P. Hesse ⁎, Rebecca L. Simpson Department of Physical Geography, Macquarie University, Sydney, NSW 2109, Australia Received 5 August 2005; received in revised form 13 January 2006; accepted 20 April 2006 Available online 30 May 2006

Abstract Longitudinal (linear) sand dunes of the Simpson and Strzelecki dunefields in eastern central Australia present a paradox. Low levels of activity today stand in contrast to luminescence dating which has repeatedly shown deep deposits of sand on dune crests dating to within the late Holocene. In order to investigate the nature of dune activity in the Simpson–Strzelecki dunefield, vegetation and sand mobility were investigated by detailed vegetation survey and measurement of rippled area and loose sand depth of dunes at three sites along a climatic gradient. The response of both vegetation and sand movement to inter-annual climate variability was examined by repeat surveys of two sites in drought and non-drought conditions. Projected plant cover and plant + crust cover were found to have inverse linear relationships with rippled area and the area of deep loose sand. No relationship was found between these measures of sand movement and the plant frontal area index. A negative exponential relationship between equivalent mobile sand depth on dune surfaces and both vascular plant cover and vascular + crust cover was also found. There is no simple threshold of vegetation cover below which sand transport begins. Dunes with low perennial plant cover may form small dunes with slip faces leading to a positive feedback inhibiting ephemeral plant growth in wet years and accelerating sand transport rates. The linear dunefields are largely within the zone in which plant cover is sufficient to enforce low sand transport rates, and in which there is a strong response of vegetation and sand transport to inter-annual variation in rainfall. Both ephemeral plants (mostly forbs) and crust were found to respond rapidly to large (> 20 mm/month) rainfall events. On millennial time-scales, the level of dune activity is controlled by vegetation cover and probably not by fluctuations of wind strength. Land use or extreme, decadal time-scale, drought may destabilise dunes by removing perennial plant cover, accelerating wind erosion. © 2006 Elsevier B.V. All rights reserved. Keywords: Sand dune; Wind erosion; Aeolian; Arid; Geomorphology; Simpson Desert

1. Introduction Several recent investigations of the age of sand dunes in the Simpson and Strzelecki Desert dunefields of Australia (Nanson et al., 1992; Twidale et al., 2001; Lomax et al., 2003) have found deep bodies of sand on ⁎ Corresponding author. Tel.: +61 2 9850 8384; fax: +61 2 9850 8420. E-mail address: [email protected] (P.P. Hesse). 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.04.012

some dune crests dating to the Holocene, including the last millennium. However, these young deposits are not coeval across the dunefield and appear to indicate patchy, isolated sand movement rather than a widespread period of climatically induced sand dune activity. Research in southern Africa has suggested an intermediate mode of activity of longitudinal (or linear) sand dunes between complete stability and complete mobility, enabled by climate variability in dunefields poised near the threshold of sand movement (Livingstone and

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Thomas, 1993; Wiggs et al., 1995; Bullard et al., 1997). The Australian dating results are consistent with the model of an intermediate state of episodic, partial activity rather than the model of binary states of stability and inactivity which has been described previously (Ash and Wasson, 1983; Wasson, 1984). Although the role of short-term climate variability has been indicated as responsible for the episodic partial activity of southern African dunes, the role of climate variability in determining vegetation cover and susceptibility to wind erosion has received little direct attention and the application of these notions to Australian dunefields is untested. The larger part of the Australian dunefield is comparatively stable, usually with only small patches of mobile sand on dune crests (Ash and Wasson, 1983). Yet there are numerous examples of quite active dunes (Wopfner and Twidale, 1967; Tseo, 1990; Bishop, 2001) of small extent which show that dunes may become locally active over their entire surfaces, most obviously in areas of present or past vegetation disturbance. These examples can be found in many areas, including the Simpson and Strzelecki Desert dunefields in central Australia, the area of lowest average rainfall but also low frequency of sand-moving winds (Ash and Wasson, 1983). Anecdotally (e.g., Ratcliffe, 1936), sand drift has been a major threat to European pastoralism in the arid zone in the extended droughts of the 1890s and 1930s, compounded by overstocking of sheep and overgrazing by an exploding population of feral rabbits. All of these examples illustrate that far from being stable and inactive, the dunes of the Simpson and Strzelecki dunefields may also be episodically activated by factors, such as climate variability or land use, which modify plant cover. This paper presents results of an investigation of the vegetation and sand mobility on sand dunes in the Simpson and Strzelecki dunefields. Repeat surveys were made with the aim of assessing the responsiveness of the vegetation and sand transport to short-term fluctuations of climate. 2. Study sites in the Simpson and Strzelecki Desert dunefields The Simpson and Strzelecki Deserts comprise large dunefields, nearly separated by a low bedrock ridge, in the lower Lake Eyre Basin of central eastern Australia (Fig. 1). Very long, regularly spaced narrow-crested linear dunes dominate the dunefield (Wasson et al., 1988) however large areas of ‘white’ dunes, created by the addition of small quantities of clay deflated from the

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floodplains of the major rivers and groundwater-fed playa lakes, disrupt the better known bright red, claypoor dunes (Wasson, 1983). The clay content leads to greater soil coherence, and therefore, in this study, only red sand dunes were studied to better isolate the effects of vegetation cover on sand mobility. There is a strong south to north gradient in wind direction, dune orientation, rainfall seasonality and vegetation in the Simpson–Strzelecki dunefield. Three sites were selected across this climatic gradient, in particular to sample a range of vegetation cover. In the south, dunes are smaller and more closely spaced (Wasson et al., 1988), oriented WSW–ENE with a sparse tree canopy (Callitris glaucophylla, Dodonea attenuata, Acacia ligulata) and mixed shrub (Crotolaria eremaea), forb (mostly daisies including Myriocephalus stuartii) and grass (Aristida contorta) understorey. Rainfall is evenly distributed throughout the year on average but large summer storms, although infrequent, may lead to local flooding and strong growth of both ephemeral and perennial vegetation (240 mm mean annual rainfall at Fowlers Gap). One site was established in this area, Teilta (30°56′S141°12′E), near the southeastern edge of the Strzelecki dunefield and surveyed in July 2002. In the northern Strzelecki dunefield the dunes swing markedly through to SSW– NNE orientation, and eventually west of north, and are larger and more regular in morphology. Trees, mostly in swales, are sparser and lower than further south but more diverse. Canegrass (Zygochloa paradoxa), a rigid clumping perennial grass, is the dominant perennial species on the dune crests while spinifex (Triodia basedowii, another rigid perennial clump forming grass) is found on the lower flanks of many dunes. In addition, there is a fluctuating population of ephemeral species (especially M. stuartii, Blennodia canescens, Calandrinia balonensis, A. contorta) which flourish after large rainfall events. There is little clear rainfall seasonality in the northern Strzelecki; however, summer rainfall becomes relatively more important while total rainfall is less abundant and less reliable. Several quadrats were surveyed at a site near Della gas field in July 2002 and in September 2004, including a repeat survey of one site, Della Crest (28°09′S, 140°40′E). The dunes of the Simpson Desert are some of the most spectacular examples of long, linear dunes available, trending SSE– NNW for up to 600 km. Rainfall is increasingly summer-dominated but less reliable than in southern areas. Trees are rare on dunes (some umbrella mulga, Acacia brachystachya) but form sparse, low woodlands (Georgina gidgee, Acacia georginae) in the interdune corridors in the northeastern Simpson. At the study site

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Fig. 1. Location of the field sites (filled circles) within the Simpson and Strzelecki dunefields (shaded) in the Lake Eyre Basin.

established in the Simpson Desert National Park of southwestern Queensland, Z. paradoxa is the dominant perennial plant on the dune crests and spinifex is rare, becoming dominant only in the northern Simpson. Flushes of ephemeral forbs (especially M. stuartii, B. canescens) and grasses (A. contorta) periodically cover the dunes following heavy rains. The crest and flanks of Mayan Dune (25°54′S, 138°34′E) were surveyed in July 2002 and the Crest resurveyed in September 2004

as well as additional quadrats on neighbouring Aztec Dune (Fig. 2). In all parts of the dunefield potential evapotranspiration is extreme and the area experiences true aridity. The ratio of annual potential (pan) evaporation to precipitation at Birdsville ranges from 10 to 40 (mean = 16). Episodic rainstorms, mostly in summer, are the only means by which soil moisture is replenished. These rainstorms are also responsible

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Fig. 2. Maps showing the location of the field sites and topographic sections of dunes at each site. Sections follow the convention of ‘looking’ downwind with the left flank to the left and right flank to the right. Dunes increase in length, width and height from south to north.

for periodic runoff and ephemeral water ponding in the interdunes at all three sites. There is no evidence of modern groundwater discharge or interaction at these sites, although deflated scalds in the interdune

surfaces at the Simpson Desert site have sparse lags of carbonate nodules, possibly indicating past higher groundwater tables (possibly Pleistocene) (Magee et al., 2005).

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3. Methods The aim of this study was to investigate the processes occurring on geomorphic spatial and temporal scales of relevance to the formation of small aeolian forms (such as ripples and small slip faces) which might be expected to show change on inter-annual time-scales. Quadrats were established on the sand dunes at each of the three sites (Fig. 2). The survey of July 2002 measured quadrats on the crest and right and left flanks (using the conventional orientation of looking along the dune crest in a downwind direction) of vegetated dunes at the Simpson Desert and northern Strzelecki Desert sites. These quadrats (30 × 30 m at Mayan Crest, 25 × 20 m at Della Crest and 20 × 20 m on flanks) were surveyed on a 1-m grid. The Teilta site (50 × 130 m) in the southern Strzelecki Desert incorporated both crest and flanks on a 5-m grid to accommodate the small trees and lower number of forbs and shrubs occurring on the dune. The survey of September 2004 returned to the crest quadrats at the Simpson and northern Strzelecki Desert sites and measured additional quadrats (25 × 20 m) on the crests of neighbouring dunes (Aztec and Bosca, respectively), focusing on areas of noticeably sparser vegetation cover. Sampling strategies were designed to accommodate the need for instantaneous measures of both plant cover and mobile sand without instrumentation of the plots. Wiggs et al. (1995) defined measures of dune activity based on repeated line intercept surveys of stakes across linear dunes in the Kalahari Desert, and Tseo (1990) applied a similar method in the Strzelecki Desert. Like Wiggs et al., in this study several sites with a range of vegetation cover were sampled to explore the relationship with sand movement. In this study the presence of ripples and the depth of loose sand at each grid point in the quadrats were recorded as measures of recent sand transport and accumulation, respectively. Wind measurements were not made during this study but sandmoving winds were observed on many sampling days and ripple formation observed. Ripples have been used previously as evidence of fresh sand movement and are ideal for mapping wind movement over the dune surfaces (Bishop, 2001). While differences in sand grain size between sites may affect the saltation threshold locally, and therefore, the readiness with which ripples are formed, these differences were not thought to be critical because of the widespread occurrence of ripples on all dunes. The longevity of the ripples is not known. Certainly ripple orientation can be observed to change over a short period (hours) as wind direction changes. Between sand moving events the ripples tend to be dissipated by

animals and rainsplash but these act only slowly and the ripples can survive until succeeding sand-moving winds occur. In an extreme case, the ripples at Mayan Crest in 2004 were stabilised by soil crust formation. The depth of loose sand is, possibly unusually, quickly responsive in this environment because of the observed growth of cyanobacterial crusts over the dune surfaces, possibly following large rain events. Shallow pits on the dune surfaces showed stacked surfaceparallel laminae of bacterially encrusted sand. Results (Section 4.3) indicate that there are short-term responses of crust cover and vegetation cover and loose sand depth. These observations support the idea that this loose sand is a good indicator of recent sand accumulation, on the time-scale of plant response to rain events. The major measurement limitation was found in slip faces and other areas of deep sand accumulation where the crust could not be reached or detected by shallow digging or the crust may not be able to cover the surface sand as readily. In these cases, a depth of ‘greater than 100 mm’ was recorded. The minimum depth of loose sand is found where a crusted (or pedogenicaly bonded) surface has been swept free of any loose sand, although this was usually found to account for less than 5% of the plot areas, reaching a maximum of 15%. Even crusted surfaces will often have a thin layer of 1 mm or so of loose sand. This may reflect desiccation and the strength of inter-grain bonds or dislodgement by fauna rather than wind transport. All sites show equivalent sand depths greater than can be accounted for by this process. Vegetation cover was measured by three methods: point contact at each grid point (Kershaw, 1973) expressed as a percentage of all points in the grid; line intercept (McDonald et al., 1990) along each 1 m (or 5 m at Teilta) spaced section of the grid expressed as a percentage of the total line length; and frontal (silhouette) area of the plants encountered in the line intercept survey (length × height) expressed as a ratio of total plant frontal area to the quadrat area (frontal area index) (Raupach, 1992). These measures are compared in Fig. 3. In addition, condition (living or dead) and species, where identifiable, were recorded. The point contact method overestimates the vegetation cover, compared with the line intercept method, by more than 100% at 12% (by line intercept method) cover. The point contact method is particularly prone to overestimation because of the small size of much of the vegetation; that is, a dominance of small bushes and forbs much smaller than the 1-m grid. Although there is a very strong linear relationship between the two measures, the over-estimation by the point contact

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Fig. 3. Comparison of measures of vegetation cover. (A) indicates overestimation of cover by point contact method relative to the line intercept method, but a strong and significant correlation. (B) shows no apparent relationship between the frontal area index (a measure of object size and density) and the line intercept method of projected cover.

method is particularly unwelcome in this study of the effects of sparse vegetation cover. There is no apparent relationship between the projected cover (line intercept) and frontal area index (FAI) (Fig. 3B). A close relationship may be expected if the vegetation were relatively uniform, approximating the spheres and cylinders conceptualised in models (e.g., Raupach, 1992). Instead, the vegetation observed on all the Simpson and Strzelecki dunes is much more heterogenous, explaining the very different results from FAI and line intercept measurement of plant cover. 4. Results 4.1. Vegetation cover and structure There are three very distinct components of the vegetation (Fig. 4): compact low perennial shrubs and

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clump-forming grasses (generally less than 1.5 m) are the tallest element on all the sites except Teilta but cover only between 5% and 25% of the surface; low (< 50 cm) forbs are the other major group of vascular plants and contribute 10–70% areal coverage but relatively low frontal area; third, cyanobacterial crust covers an additional 25–55% of the surface but does not contribute to FAI. The variable contribution of the first two elements to total vascular plant cover (subsequently simply referred to as projected plant cover) explains the large scatter in Fig. 3B. At Teilta, there is, in addition, a significant fraction of small trees. These have a distinct ‘vase’ shape with very small basal area (only the trunk) and compact form. Scour is observed around the bases of these plants at all stages of growth from seedlings to senescent trees and there are no noticeable wake sand accumulations. At death, all tree species drop limbs which then become sites of growth of ephemeral forbs and sand accumulation. A perennial shrub species (Crotalaria eremaea) has a similar shape and scour (rather than sheltering) effect to the trees but forms only a small component of the cover at any site. The dominant perennial grass (Z. paradoxa), however, is a renowned sand stabiliser with a very broad basal area afforded by multiple small stalks emerging from the surface. Canegrass clumps are often the sites of accumulations of mounds of sand, while living or dead. None of the measures of plant cover incorporates the porosity of the vegetation, which may significantly modify the drag. Both perennial and ephemeral plants have considerably higher porosity when dead, or dry, than when growing vigorously. Living forbs form a near-continuous cover with limited porosity and obvious accumulation of sand. However, dying or dead forbs become sparser and, although uncommon, sand may be transported between isolated individuals until the dead plants are themselves blown away. It is impractical to measure the frontal area of each living or dead forb (numbering thousands on each plot) and so they were treated as a mass. This is reasonable when they are dense enough to trap sand but becomes less secure as they shrink and become broken up (i.e., porosity increases). Canegrass is noticeably less dense when dry or dead than when growing vigorously. Nevertheless, even when dead for more than a year, the standing leafless stalks are dense enough to trap sand. Canegrass itself is rarely dense enough to cause frontal scour. Usually, this results from the build-up of sand within the canegrass clump. Spinifex traps sand in the centre of each clump but also promotes scour around its outer edge by virtue of its very dense foliage. These strong species

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Fig. 4. Vegetation structure and cover on study sites. Tallest vegetation present is read from the y-intercept while total vascular plant cover (line intercept method) is read from the x-intercept. Unfilled symbols represent the total surface protection (vascular plants+crust) (point contact method).

differences suggest that the behaviour of wind and sand movement on dunes will be specific to each desert bioregion. Total projected plant cover on these quadrats ranged from 12.8% to 77.7% and FAI from 0.03 to 0.24 (Table 1). Thus, a very broad range of cover levels was sampled, although the sites with the sparsest projected plant cover barely reach the 14% level suggested by Wiggs et al. (1995) as the threshold for significant sand movement on Kalahari Desert sand dunes. However, several quadrats have cover less than 30%: the figure proposed by Ash and Wasson (1983) as marking an effective threshold below which sand transport was significantly enhanced. Comparisons are not straightforward as Wiggs' figure is for the entire dune surface (flanks and crest), and conversely, the figures given for the quadrats in this study disguise the barer crests by inclusion of slightly better vegetated upper flanks at the edges of the grids. The FAI values range from those typical of sparse, isolated elements (< 0.1) through the

value predicted to give maximum surface protection (0.12) to high levels where skimming flow is predicted with displacement of the plane of zero velocity to above the plant height (Raupach et al., 1993; Greeley et al., 1995). Although these values of either measure cannot be taken as representative of the entire dunefield, in July 2002, the sites were selected, subjectively, as being typical of the surrounding area. The new 2004 sites, on the other hand, were selected as the most conspicuously bare areas of crest in the surrounding area. 4.2. Sand movement The observations of sand transport (rippled area) and accumulation (sand depth) are compared with vegetation cover indices in Fig. 5. Frontal area index does not perform well in explaining the range of values of either ripple cover or area of deep (> 100 mm) sand (Fig. 5A and D). The expected inverse relationship between cover and sand transport (Wolfe and Nickling, 1993) is

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Table 1 Summary of plot vegetation cover and mobile sand measures Location

Projected contact Projected contact cover-point cover-length (% area) (% area)

Total protected surface (% area)

FAI (m2/m2)

Ripples (% area)

Sand depth > 100 mm (% area)

Mayan Crest 2004 Della Crest 2004 Aztec North 2004 Aztec South 2004 Bosca Central 2004 Bosca Crest 2004 Bosca South 2004 Mayan Crest 2002 Mayan Right 2002 Mayan Left 2002 Della Crest 2002 Della Right 2002 Della Left 2002 Teilta 2002

88.1 80.4 41.9 45.1 28.0 28.6 26.7 29.8 61.5 40.1 54.3 72.3 60.3 43.1

99.1 100.0 65.2 68.3 78.4 69.6 66.5 55.8 90.7 81.8 76.2 99.1 91.2 48.4

0.13 0.24 0.10 0.08 0.03 0.20 0.03 0.11 0.04 0.06 0.21 0.06 0.14 0.12

74.1 7.5 78.9 65.4 75.5 55.3 94.1 66.4 15.6 78.0 22.9 1.8 8.2 56.1

0.0 0.0 36.6 20.0 20.7 30.6 47.1 44.9 2.7 8.4 46.3 1.4 24.5 75.9

77.7 60.9 36.5 37.2 15.6 14.2 12.8 22.2 43.3 24.3 46.1 63.1 51.7 19.5

not observed. This could be because the measured parameters (rippled area and area of deep sand above the biotic crust) are not good proxies of sand transport flux or because the porosity of the vegetation was not taken into account. Nevertheless, such considerations would equally apply to the other two measures of plant cover which do have clear relationships with both measures of

Equivalent sand depth (mm) 8.2 4.4 49.1 52.8 79.3

22.0 32.8 14.9 58.9

sand movement. The projected plant cover (i.e., only the vascular plant component) has a strong relationship with the area of rippled sand (r2 = 0.75, excluding Mayan Crest 04, p = 0.000) (Fig. 5B). A single outlier (Mayan Crest 04) has a large area of ripples under sparse dead or dying forbs, but these ripples are largely stabilised by crust and the quadrat has a very low area of deep sand

Fig. 5. Comparison of vegetation cover with sand movement. Dashed lines are least squares regressions (excluding one outlier in (B)). No significant relationship is seen between FAI and either the area of rippled sand or area of deep (>100 mm) loose sand. Both projected plant cover and total protected surface (vascular plants + crust) show the expected inverse relationship with the area of rippled sand or area of deep (>100 mm) loose sand.

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(Fig. 5E). Other plots cluster into two groups, suggestive of a threshold, but different groupings are apparent when projected plant cover is compared with the area of deep sand. Plots with the highest cover have very little deep sand (Fig. 5E) but for plots with cover lower than around 50% there is a very large range of values, also suggestive of a threshold in sand movement behaviour. However, comparison of the area of deep sand with the area with some level of cover provided by either vascular plants or cyanobacterial crust (total protected surface) shows a much simpler and very strong inverse relationship (r2 = 0.78, p = 0.000) (Fig. 5F ). In this case there is apparently no threshold: even the smallest area of unprotected surface (i.e., total protected surface < 100%) will generate sand movement. The same

observation holds for the relationship with rippled area (Fig. 5C), despite the greater spread of points (r2 = 0.32, p = 0.034). The nature of the relationship for dunes with less than 50% total surface protection is unclear. Logically, the same linear relationship cannot continue until there is no surface protection but, rather, very poorly protected dune surfaces are expected to have close to 100% coverage of deep sand suggesting a different relationship below some lower level of cover than measured here. Given the difficulty of finding very active bare dunes in Australia, it may be problematic to explore this behaviour. A more meaningful measure than either rippled area or area of deep sand is the volume of mobile sand or, alternatively, the equivalent depth of mobile sand. From

Fig. 6. Equivalent depth of mobile sand for all plots. Panel A shows polynomial regressions fitted to values of cumulative area versus sand depth from which equivalent depth is calculated (solid lines) by integration of the fitted functions. Plots considered too unreliable for calculation are indicated by dashed lines. Panels B and C display the calculated equivalent sand depths against projected plant cover and total surface protection for plots with slip faces (triangles) and without (circles). The negative exponential relationships are significant (p < 0.05) and are consistent with the relationships determined in Fig. 5.

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the data collected the equivalent sand depth, that is the volume of mobile sand per unit area of dune surface (m3 m−2), can be calculated by integration of the curve fitted to the relationship between sand depth and cumulative area with sand deeper than a given depth (Fig. 6). For most plots, a third-order polynomial function gives a realistic result, including the predicted maximum sand

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depth. However, for the quadrats with the deepest sand, the extrapolation to the maximum depth is too great to be reliable (dashed lines in Fig. 6A). For the remaining nine plots, there is a strong negative exponential relationship between equivalent sand depth and either projected plant cover (r2 = 0.60, p = 0.014) or total protected surface (r2 = 0.66, p = 0.008) (Fig. 6B and C). The predicted equivalent sand depth for zero projected plant cover of only 111 mm (Fig. 6B) is unrealistically low. There is evidence of a change in the behaviour of dunes with low cover and the development of small slip face dunes which is not recorded here because of the necessary exclusion of plots with the greatest depth of loose sand. This morphodynamic transition is discussed below. Nevertheless, these preliminary results support two of the observations made above: there is apparently a strong relationship between plant cover and sand transport, and there is apparently no upper cover threshold for sand movement. Some depth of loose sand is apparent even at nearly complete coverage of plants or crust, and equivalent sand depth increases steeply with declining cover but apparently conforms to a negative exponential function. 4.3. Temporal variability: response to rainfall We have made measurements of the response of vegetation and sand movement to inter-annual variations in precipitation (Bureau of Meteorology data) through repeat surveys of two dune crest quadrats in July 2002 and September 2004 (Fig. 7). The initial survey in July 2002 was after 12 months of very low rainfall in the Simpson Desert (68 mm: third lowest July–June annual rainfall at Birdsville since 1960– 1961; mean annual rainfall = 162 mm [S.D. 95]). In the preceding 4 months only 3.4 mm fell at Birdsville and a major drought afflicted most of eastern Australia. Rainfall at Moomba in the Strzelecki Desert for the same 12-month period was not particularly low (134 mm compared with mean of 192 mm [S.D. 110]) Fig. 7. Short-term changes in vegetation cover and sand mobility in response to inter-annual rainfall variation at Mayan Crest (triangles) and Della Crest (circles) in July 2 (solid) and Sept 4 (empty). (A) monthly rainfall at Birdsville—arrows indicate survey times. Rainfall in the 12 months preceding July 2 = 68.4 mm (01/02 third driest year since 1960–1961), 12 months preceding Sept. 4 = 155.8 mm, mean J–J rainfall = 162 mm (S.D. 95 mm) (1961–2003). (B) Vegetation structure —showing a dramatic increase in ephemeral cover in 2004 compared with 2002 following greater rainfall (A). (C) Sand depth distribution— greatly reduced in 2004 compared with 2002 due to the formation of surface crust and the protection of ephemeral plants. (D) Area of deep sand against vascular plant cover showing the marked change in status of the two plots from July 2002 to September 2004.

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but only 2 mm fell in the preceding 4 months. The September 2004 survey followed a wetter (156 mm) year in the Simpson and a 45-mm fall in May but only 11 mm in the preceding 4 months. At Moomba, only 90 mm fell in the 12 months preceding September 2004 and 26 mm in the preceding 4 months. The response of the vegetation was particularly clear (Fig. 7B) with an abundance of forbs, many still alive but in decline, widely distributed in both dunefields and on both Della and Mayan crest sites in 2004. Ephemeral forbs made up 12% of all vegetation at Mayan Crest in 2002 but 80% in 2004. At Della Crest ephemeral forbs increased from 48% in 2002 to 66% in 2004. In 2002 most forbs at both sites were dead but the perennial canegrass was a roughly equal mixture of live and dead (leafless dried stalks, possibly with viable roots) individuals at Mayan and dominantly live at Della. In 2004, however, despite the flush of ephemeral growth, the canegrass at both sites had diminished slightly in areal coverage and in height (Fig. 7B) as the now dominantly dead plants had begun to be broken down. The aerodynamic effect of the perennial component remained roughly the same, but its long-term viability appears questionable in the continued absence of deep, soaking rain. Ephemerals (nearly entirely dicotyledonous forbs) are the most rapidly responsive component of the vegetation cover to fluctuations in rainfall. At Della 18 mm of rain in August (recorded at Moomba, 50 km away), following 40 mm of rain in April and May 2004, was apparently enough to promote a near-blanketing cover of forbs. Conversely 4 months of dry conditions, following 50 mm of rain in February 2002, was apparently long enough for the flush of ephemerals to die and be largely removed from the dune crests. Very little of this dead material remains as litter on the dune; what is not consumed by various grazers must be blown from the site. At Mayan also, 5 months of little rain in 2002 saw almost complete removal of the dead ephemerals but after 4 dry months in 2004, the forbs were still standing, although dying. The different vegetation components have different response times and clearly respond differently to rainfalls of any given intensity. The ephemeral plants have a life cycle of around 4 months and can germinate after monthly rainfall (most likely 1 or 2 days of rain) of 20 mm or so, which can be expected perhaps two or more times a year. The canegrass, however, must require wetter than average years to regenerate and probably particularly heavy and/or prolonged wet periods which replenish deep soil moisture. Conversely, a dry period of substantially more than 2 years is required to remove the dead canegrass and create a smoother dune surface.

Both the Mayan and Della sites are currently only lightly grazed by native animals and wild camels (Mayan) or cattle (Della) and there is no obvious case for dieback of the canegrass in response to overgrazing which has been observed with chenopods in arid Australia (Read, 2004). Because of the different behaviour of the ephemeral and perennial components, vascular plant cover can vary dramatically and rapidly from close to 100% (certainly>80%) down to a minimum value set by the largely unchanging perennial plant cover; this is 15– 20% on the Mayan and Della Crest sites but as low as 1.5% on the east flank of Della dune or 4–5% on several of the barer crest sites. The depth of loose sand on the Mayan and Della Crest sites decreased markedly between the July 2002 and September 2004 surveys (Fig. 7C) and appears to be a direct response to the changed status of the vegetation. The mobile sand present in 2002 was not exported from the quadrats but was stabilised largely by the growth of crust. The crust observed in 2002 (53% of the area of Mayan Crest and 55% of Della Crest) was formed by conspicuous cyanobacterial sheath material. In 2004 surface crust was much more widespread (97% and 99% at Mayan and Della, respectively) but less cohesive and without conspicuous sheath material. The crust is possibly abiotic (in which case it is most likely held together by clay bridges between sand grains) (Bishop, 2001) or formed by fine cyanobacterial strands not visible without magnification (A. Thomas and A. Dougill, personal communication). In either case, soil crust has a great capacity to grow and stabilise loose sand on these dune crests, and both the stronger crust observed in 2002 and the weaker crust in 2004 appear to be effective. These data show a short-term response of sand mobility to variation in protective plant cover and, ultimately, the variable precipitation of the Australian deserts (Fig. 7D). Together with the general relationship found between sand mobility and vegetation (Fig. 4), they provide evidence of both a strong role for vegetation in stabilising sand dune surfaces in Australia and an explanation of periodic, patchy dune mobilisation today and during the Holocene. 5. Discussion 5.1. Vegetation cover and dune stabilisation The results of the vegetation surveys presented here show that vascular plant cover on Australian desert dunes can be very high, expressed as either projected contact cover or frontal area, but is very variable on

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short time-scales. The more locally typical sites (Mayan and Della) have levels of cover above those usually given as the threshold for significant sand transport (Ash and Wasson, 1983; Wolfe and Nickling, 1993; Wiggs et al., 1995) for months following heavy rains but approaching threshold levels during intervening dry spells. Likewise Mayan and Della crests have FAI values above the value quoted for complete surface protection (Raupach, 1992) in both wet and dry years, because of the relatively invariable cover of the large perennial component of the vegetation. Ash and Wasson found quite low levels of total projected cover in the Simpson–Strzelecki dunefields (5–30% on crests; 10–45% on flanks), below their predicted value for stabilisation of 30%. However, these data were derived from air photograph interpretation and it is not recorded whether these photographs were taken during a wet or dry period. The low levels of observed cover could be due to photography taken during a drought year but also the difficulty of detecting the small forbs (or soil crust) from 1:50 000 scale air photography. They also found up to 40% of dune crests covered with mobile sand in the Simpson Desert and an inverse relationship between projected plant cover and sand mobility and give numerous anecdotal references to sand mobility in disturbed areas. The finding of a strong relationship between plant cover and mobile sand in this study is similar to that of Ash and Wasson (1983). However, higher levels of cover explain the low level of sand mobility observed on dune crests whereas Ash and Wasson concluded that sand movement was limited by infrequent strong winds. The consistent inverse relationships found between measures of vegetation cover and measures of sand movement strongly suggest that vegetation is instrumental in stabilising these dunes. This relationship is observable at any scale from 20-m line intercept surveys to entire dunes. If there is no causal link it seems remarkable that such a pattern should appear so consistently. This supports the conclusion that vegetation cover is very much a controlling factor in sand movement on Australian dunes and that most Australian desert dunes are inactive for most of the time because of the abundance of vegetation. 5.2. Role of drag versus surface shear strength The observations presented here have shown that the vegetation on these dunes is much more complex than the uniform arrays of uniform roughness objects so often modelled (Raupach et al., 1993) or the shrubdominated mesquite communities which have been the

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subject of field measurement of shear velocity ratios (Wolfe and Nickling, 1996). Perhaps because of this complexity, the frontal area index does not appear to explain the observed sand mobility. Even at the lowest levels of cover, these results do not show the expected inverse relationship between FAI and sand movement (Fig. 5). Projected plant cover or total surface protection from plants and crust provide a much better explanation of sand mobility. This result, especially the apparent dependence on the soil crust which has no role in increasing drag at the surface, suggests that in this environment the vegetation (of all forms) is effective in stabilising the dunes because it confers greater shear strength on the dune surface which is then able to suppress sand entrainment and resist saltation bombardment. Even the relatively weak crust observed in 2004 seems to be effective in this, consistent with observations of the effectiveness of crust of any strength in suppressing liberation of particles from soil surfaces (Hupy, 2004). Although there is wide recognition of the importance of crusts of various types in stabilising surfaces, affecting entrainment thresholds and particle emissions, models based on aerodynamic considerations (Raupach, 1992; Wolfe and Nickling, 1996) downplay the role of surface shear strength, even those which explicitly recognise soil erodibility based on particle size distribution (Shao et al., 1996). 5.3. Temporal and spatial variability in sand movement Like Ash and Wasson (1983) patches of mobile sand, with small active slip faces, were found on the crests of dunes which are otherwise well vegetated. Plots without active slip faces (Mayan and Della Crests) showed remarkable expansion of ephemeral forbs and nearly complete surface protection in 2004 but the slip face plots only a few hundred metres away showed very little growth of ephemerals (Fig. 4) despite receiving the same rainfall. The sites without slip faces have perennial shrubs which act both as nodes of sand deposition and later scour, as the mounds grow and begin to develop moats. This ‘mound and moat’ dune morphology is characteristic of the well-vegetated dune crests and flanks (Figs. 4 and 8). On these dunes the mobile sand layer is shallow even in dry periods and is readily colonised by forbs and cyanobacteria after rain. All the bare sites contained small slip-face dunes and much greater equivalent depth of mobile sand (Figs. 4 and 8). The positive feedback between actively mobile sand and suppressed plant germination and growth is most likely responsible for

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long droughts have occurred in the 1890s and 1930s to 1940s. These extreme droughts, coupled with heavy sheep grazing beforehand and the destructive grazing impact of European rabbits, which reached plague proportions across the area until controlled by myxomatosis (1951) and rabbit calicivirus (1995), contributed to extensive wind erosion in the Simpson and Strzelecki dunefields (Ratcliffe, 1936). Once established, positive feedback would ensure the growth and continued activity of the mobile bare patches until stabilised, perhaps by extreme wet periods similar to those of the early 1950s and 1973–1974. 5.4. Wind as a limiting factor Fig. 8. Morphodynamic model of sparsely vegetated sand dunes. Data from Fig. 6B with two morphodynamic zones superimposed. Bare plots with expected high equivalent sand depth which could not be reliably determined (Fig. 6A) are expected to plot within the hatched area. The change in behaviour from vegetation control to insensitivity is governed by the development of dune slip faces at low vascular plant cover levels but is not defined by a fixed cover level.

the failure of the heavy rainfall events to encourage ephemeral plant growth on these sites. There is a threshold evident in dune surface morphodynamic behaviour, remembering that equivalent sand depth could not be estimated for the sites with the deepest sand (Figs. 6 and 8). It is likely that there is some critical patch size of bare sand necessary to generate slip faces, which explains the transition from one state to another. It follows that this morphologically imposed behaviour threshold divides better vegetated sites with a strong climatic response from less well-vegetated dunes whose activity is independent of most short-term fluctuations in rainfall. Given the resilience of the canegrass, its absence from some dunes or patches requires an explanation beyond the normal drought cycle. The perennial plant cover on these dunes has response time longer than the usual inter-annual variations of rainfall. Canegrass and spinifex are well adapted to both the aridity and variability of the Australian deserts and, other than a reduction in leaf area, show little response (in height, area or porosity) to the standard drought cycle. In the wider region there are obvious point causes of vegetation disturbance such as vehicle tracks or piospheres around artesian bores. Teilta, for example, has a history of heavy grazing by sheep and cattle since the 1870s which has led to nearly complete vegetation turnover and long periods of bare dune surfaces and blowouts. Fire is effective in spinifex-covered areas but does not occur in Zygochloa-covered dunefields under modern conditions. In the historical period extreme,

In this study wind flow was not measured, nor were the meteorological records of wind observations specifically examined. A future paper will address the climate record and dune mobility. However, the observations reported here support the anecdotal reports of Ash and Wasson that the wind is certainly strong enough to at least move the sand on the dune crests periodically. Ash and Wasson (1983) and Wiggs et al. (1995) suggested that linear crests of the Australian and Kalahari desert dunefields (respectively) are only active because of the wind speed-up effect acting on winds flowing obliquely across the dunes and that, generally, the lack of strong winds is the explanation for their apparent stability. However, the use of annual average frequency of strong winds by Ash and Wasson underplays the frequency of strong winds in summer. Observations of some other phenomena, some of which were used as criteria for judging the effectiveness of wind erosion by Wiggs et al. (1995), suggest that winds are sufficiently strong, at least seasonally or periodically, to move large amounts of sand. As noted in the Introduction, at sites of disturbance, for example, around stock watering points or heavy vehicle use, there is strong sand movement over the entire surface of the dune including the lower flanks. At least two such sites have been monitored for sand movement (Tseo, 1990; Bishop, 2001) and found to be very active. The Teilta site fits this pattern of disturbance by grazing, triggering the formation of blowouts deflated to interdune level alternating with active small transverse dunes along the axis of the former dune and extending from lower flank to lower flank (the transect in Fig. 2 is through an undisturbed section of dune at the margin of the quadrat). Active sand movement was also observed in strong winds in interdunes in the Simpson Desert, as was the formation of ripples and small build-ups of sand in sheltered areas in flat, wide lightly wooded interdune corridors which

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have only a thin covering of sand and extensive winddeflated scalds. Freshly rippled sand was also observed on dune flanks at Mayan and Della. The coverage of these ripples was small in three of four quadrats but 78% on the left flank of Mayan despite very low sand volume. All of these flanks had high total surface protection but this was largely contributed by forbs and crust rather than perennial shrubs. Where the crust was broken, as at Mayan Left, sand was observed to be deflated and moved along the dune surface, generally obliquely toward the crest. At the sites reported here, sand movement is not restricted to the crests; however, it certainly is most common there, coincident with the area of lowest surface protection from plants or crust. Topographically induced wind speed-up is detectable on bare or very sparsely vegetated dunes (Wiggs et al., 1996) but not on better vegetated dunes, where deceleration occurs. The arrangement of plant species, and cyanobacterial crust, on the dunes suggests that there are also strong edaphic controls on plant growth which seem to predispose the crests to have lower plant coverage and greater sand movement. The more stable dunes have less perennial plant cover on their flanks than on the crests (c.f. Ash and Wasson, 1983), which are the preferred habitat of canegrass. The arrangement of forbs, ephemeral grasses, shrubs and trees in regular patterns, with different species within the structural classes occupying distinctive ranges in interdune, flank or crest, points to a plant response to some properties which vary over the dune surface. These could be temperature, sun exposure, soil moisture or other factors. Because this gradient of plant distribution is the same as the expected gradient which might arise from wind stress it is difficult to separate the two satisfactorily. 5.5. Episodic mode of dune activity in the Holocene Although the last glacial maximum appears to have been a time of significant dune construction and sand movement (Hesse et al., 2004) there are a significant number of sites with surficial sand units dating to the Holocene, including the last millennium (Nanson et al., 1992; Twidale et al., 2001; Lomax et al., 2003). Within the limits of the available dating, it does not appear that these dunes were active simultaneously during the Holocene. Rather than responding to an external forcing of largescale climate change the dunes appear to have responded in a more stochastic pattern to local disturbances, possibly triggered by short-term climate variability. The morphology of the sand dunes also points to the limited scale of Holocene activity compared with earlier

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episodes of dune mobilisation and formation. Many authors have noted the asymmetry of the sand dunes over large areas of the Simpson and Strzelecki Deserts (Wopfner and Twidale, 1967; Wasson, 1983; Nanson et al., 1992). Mayan and Aztec dunes in the Simpson Desert also showed a much steeper right flank (Fig. 2) and all three sites showed net sand movement from left to right across the dune. Evidence of reversal of sand movement direction over short periods, including oblique transport toward the crest on the right flanks, was also noted. The dunes also exhibited exposed palaeosol on the left flanks, previously noted by Nanson et al. (1992), scoured mounds on the upper flanks and some oblique grooves leading to the crest, all formed by stripping of the surface sand, above the palaeosol B horizon, and transfer to the dune crest. The palaeosol surface can be quite firm, with a polygonal structure developed in the red brown sands probably bonded by clays. When undermined (for example in rabbit warrens) the massive polygonal columns have limited structural strength but their surfaces appear to be quite resistant to saltation bombardment. The palaeosol horizon yielded OSL ages around 40–48 ka in the Strzelecki Desert (Lomax et al., 2003) and TL ages of 9 ka and 78 ka in the Simpson Desert (Nanson et al., 1992). It is not clear if the palaeosol itself forms the ultimate limit on sand supply today, but it demonstrates the limited nature of Holocene (post-palaeosol formation) sand movement on most dunes. For many areas the effects of drought or fire disturbance are to periodically expose the surficial sand for transport and accumulation on the dune crest where it is reworked occasionally into small dunes. At other sites, including Teilta and other examples noted above, the disturbance is more dramatic and mobility is not limited by a palaeosol (not observed at Teilta). The transition from one state to the other may depend on the degree of disturbance, especially strong localised disturbances which are today mostly associated with various human land uses. 6. Conclusions The sand dunes examined here showed a complex vegetation structure including rapidly responsive ephemeral forbs and cyanobacterial crust and a more stable perennial shrub component. The ephemeral forbs and crust can apparently flourish and die (with near complete removal) in 4–5 months following monthly rainfall of greater than around 20 mm. The total plant cover fluctuates rapidly from nearly complete cover (including crust) to a lower limit set by the perennial shrubs (as low as 3% on disturbed crest sites and 15% on

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‘stable’ crest sites). The perennial component appears to respond to cyclical droughts on time-scales of years, perhaps requiring several years of drought for plants to die and be removed completely but also above average rain to regenerate after drought. Sand movement responds to short-term variations in vegetation cover (including crust). There is an inverse relationship between measures of mobile sand and both projected (vascular) plant cover or total surface protection (vascular plants plus crust). The negative exponential relationship found between equivalent sand depth and total surface protection shows a very rapid increase in the volume of mobile sand as the dune surfaces become denuded, without a formal threshold. However, there is a transition in dune morphology as sand depth increases and dunes with slip faces are able to form. At this stage the higher level of sand mobility appears to inhibit the growth of ephemeral plants and these patches of more mobile sand remain mobile while nearby areas are carpeted in ephemeral growth following rainstorms. Episodic activity over limited areas on these linear sand dunes appears to be possible because of extreme droughts within the range of the modern climate regime which are able to kill and then see the removal of perennial vegetation. Less extreme drought cycles are shorter than the response time of the perennial vegetation. Additional local causes of disturbance in the historical period are vehicle tracks and grazing by introduced domestic and feral animals. The mobility of dune crests in the Holocene determined from sediment dating may be explicable by extreme drought events, such as those of the 1890s or 1930s to 1940s. Acknowledgements We thank Christian Alscher, Deanne Bird, Vivien Howard, Levi Roberts and Lee Seymour for their hard work and good company in the field and Russell Field for assistance with logistics. We also thank Don Rowlands and the Queensland National Parks and Wildlife Service for permission to undertake fieldwork in Simpson Desert National Park; Rob Batterham and SANTOS for permission to undertake fieldwork at Della and John Scobie of Avenel for permission to undertake fieldwork at Teilta. Geoff Humphreys and Marshall Wilkinson, as well as two anonymous reviewers, helped improve the paper. References Ash, J.E., Wasson, R.J., 1983. Vegetation and sand mobility in the Australian dunefield. Zeitschrift fur Geomorphologie, N.F., Suppl. Bd. 45, 7–25.

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