Severely degraded dunes of the southern Kalahari: local extinction, persistence and natural re-establishment of

Michael Charles Rutherford1,2* and Leslie Ward Powrie1 1Applied Biodiversity Research Division, Kirstenbosch Research Centre, South African National Biodiversity Institute, Private Bag X7 Claremont 7735, South and 2Department of Botany and Zoology, Stellenbosch University, Private Bag X1 Matieland 7602,

Une pression particulie`rement intense du paˆturage a ent- Abstract raıˆne´ un de´clin important du couvert de la canope´eetdu This study aimed to quantify and understand the impact of nombre d’espe`ces des formes vivantes annuelles et pe´r- severe land degradation on attributes and diversity ennes, monocotyle´dones et dicotyle´dones, rampantes et on dunes of the southern Kalahari. Heavy grazing pressure dresse´es, et des tiges feuillues, des touffes d’herbes et des in particular resulted in a significant decline of canopy architectures stolonife`res. Cependant, nous n’avons trouve´ cover and species number in annual and perennial life aucun changement significatif dans les arbustes et les forms; forb and graminoid growth forms; erect and pros- formes arbore´es qui survivaient sans nouveau recrutement trate habits; and leafy stem, tussock and stoloniferous visible. Dans ces conditions, la diversite´ des espe`ces de´cli- architectures. However, no significant change was found nait fortement et un certain nombre, surtout des gram- in shrub and tree forms which persisted without apparent ine´es, se sont apparemment e´teintes localement. L’arbuste new recruitment. Under these conditions, species diversity pe´renne Crotalaria cf. spartioides pre´sentait un sche´ma dropped sharply and a number of species, mainly grami- inverse avec une haute fre´quence de jeunes plants qui noids, became apparently locally extinct. The perennial s’e´tablissaient dans les dunes de´grade´es. Des changements shrub, Crotalaria cf. spartioides, showed the converse with a de dominance relative montrent que des traitements high frequency of establishing seedlings on the degraded extreˆmes favorisent les plantes pe´rennes par rapport aux dunes. Changes in relative dominance show that the annuelles, les plantes ligneuses par rapport aux gramine´es extreme treatment favours perennial over annual, woody et aux herbace´es a` fleurs, les plantes dresse´es par rapport over graminoid and forb, erect over prostrate and leafy aux rampantes et les tiges feuillues par rapport aux plantes stem over stoloniferous and tussock. Some of these results stolonife`res et aux touffes d’herbes. Certains re´sultats et and certain species and soil responses differ from those certaines re´ponses d’espe`ces et de sols diffe`rent de ceux qui reported from grazing studies elsewhere, and are possibly ont e´te´ enregistre´s lors d’autres e´tudes sur le paˆturage, no longer directly related to the impact of the primary re´alise´es ailleurs, et ils ne sont peut-eˆtre plus lie´s directe- grazing pressure but to the secondary effect of subsequent ment a` l’impact de la pression meˆme du paˆturage mais instability of the dunes. plutoˆta` l’effet secondaire qu’est l’instabilite´ des dunes qui en re´sulte. Key words: disturbance, extirpation, grazing, plant diver- sity, savanna, traits

Re´sume´ Introduction

Cette e´tude visait a` quantifier et a` comprendre l’impact This study contributes to a pilot research programme on d’une se´ve`re de´gradation de terrain sur les qualite´setla understanding the relationships between plant diversity diversite´ des plantes, dans les dunes du sud du Kalahari. and land degradation. The main cause of biodiversity loss in the arid savannas of southern Africa is land degradation *Correspondence: E-mail: [email protected] (Scholes & Biggs, 2005), which is also one of the major

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environmental issues in the subcontinent (Darkoh, 2009). This programme systematically targets areas of extreme degradation because of heavy grazing pressure within arid and semi-arid parts of South Africa. The focus in this study is on the southern Kalahari dunefield. A number of studies along degradation gradients in the southern Kalahari have indicated important compositional changes in the herba- ceous layer (Van Rooyen et al., 1990; Van Rooyen, Bre- denkamp & Theron, 1991a; Van Rooyen et al., 1994) and predicted woody plant encroachment (Jeltsch et al., 1997). This study aimed to assess which of these diversity-deter- mining results hold under particularly extreme levels of land degradation through overgrazing. Persistence was distinguished from re-establishment. The extent to which plant trait responses to grazing based on a global synthesis (Dı´az et al., 2007) apply under these extreme conditions was also examined. This synthesis has shown that grazing usually favours annual over perennial plants, short over tall plants, prostrate over erect plants and stoloniferous and rosette architecture over tussock architecture. Although our work considers the effects of extreme levels of degradation caused through heavy grazing pres- sure, the results should also relate to the anticipated effects of exposed and re-mobilized Kalahari dunes as a conse- Fig 1 Location of the study site within the southern Kalahari quence of climate warming in this century (Thomas, dunefield Knight & Wiggs, 2005).

occurred under more arid, probably windier circum- Materials and methods stances, most recently between 17,000 and 10,000 years ago (Stokes, Thomas & Shaw, 1997). The aeolian sand of Study area the NW–SE trending parallel linear dunes varies from fairly The Kalahari dunefield in South Africa was systematically compact near the base of the dune to very loose on the scanned for major fence-line grazing contrasts using dune crests. The soil is classified as the Kalahari Family of satellite imagery (mainly SPOT5). There were many can- the Namib Form (Soil Classification Working Group, didate fence-line grazing contrasts, but the site that 1991). The less disturbed dunes lack slip faces. The study showed the greatest contrast was on the adjacent range- was limited to the dunes as they showed a more extreme land farms of Avonds Schijn and Ballater, and located contrast in vegetation than did the inter-dune areas. along a N–S fence near 26o56¢S21o08¢E in the Siyanda Height of dunes sampled varied between 7 and 21 m above District Municipality, Northern Cape Province. This site the inter-dune areas. The dunes are the only physical also complied with the sampling requirement that the features that break the otherwise flat landscape. Mean fence line cross at least 25 dunes to allow for adequate elevation was approximately 895 m amsl. randomized replication. Initial field inspection confirmed The site is situated in the Gordonia Duneveld vegetation that there was no indication of any pre-existing environ- type (SVkd 1) of the Savanna Biome (Mucina & Ruther- mental gradients across the fence. ford, 2006). Vegetation structure is an open, shrubby The site falls within the greater southern Kalahari grassland with occasional small trees. That the vegetation dunefield which is shared between Namibia, South Africa on the north- and south-facing slopes is very similar and (Fig. 1). These dunes were not formed un- (Leistner, 1967) probably relates to the NW–SE dune der current prevailing conditions and their emplacement orientation where solar incidence on the north slope is

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compensated for by the exposure of the south (polar-fac- A total of 32 plots were paired between 25 and 45 m on ing) slope in the afternoons. both sides of the fence spread over a distance of 5.6 km. Mean annual rainfall is approximately 180 mm, falling Each plot was 50 m2 (belts of 20 · 2.5 m) and randomly mainly in summer and late summer from November to placed with the long axis at right angles to the linear dune. April and peaking in March. Summers are hot and severe As the width of dunes varied, randomization took into frost occurs in winter, particularly at ground level. Mean account width to ensure no bias against any section of a monthly maximum and minimum screen temperatures are dune. Minimum distance of plots to riverbeds or pans was 41.5C and )4.0C for December and July respectively, approximately 4 km. There were, however, a few indistinct according to a weather station in Gordonia Duneveld pan-like depressions (deflation basins – Van Rooyen, 1984) (Mucina & Rutherford, 2006). as close as 500 m to some of the plots. The farm Ballater had been stocked for about a decade Canopy cover of each species was esti- with sheep at levels equivalent to approximately 6.5 ha mated using the same observer in all plots to reduce pos- per large stock unit (LSU), which is a few times more sible variation in these estimates between different intense than the recommended 24 ha per LSU (Jeltsch observers (Kercher, Frieswyk & Zedler, 2003). Species were et al., 1997) to 18 ha per LSU (Fourie, De Wet & Page, classed by life form (annual, perennial), growth form (forb, 1987) for the southern Kalahari dune region. The geophyte, graminoid, shrub and tree), habit (erect, pros- stocking level on the farm was relatively recently dropped trate) and architecture (leafy stem, rosette, stoloniferous to within the recommended range and comprised mainly and tussock). Canopy cover of all vegetation of each plot sheep and some springbok antelope. This part of the was estimated independently. Plant specimens were iden- study site is referred to as HU (High Utilization). Avonds tified by the National Herbarium of the South African Schijn’s stocking levels had remained below recom- National Biodiversity Institute. Persistence and seedling mended levels for years, also currently at approximately establishment were noted on HU. 25 ha per LSU, made up of about two-thirds domestic Six soil samples were taken from dunes to a depth of livestock, mainly cattle, and one-third antelope species, 50 mm from each side and analysed for textural classes, mainly springbok, red hartebeest and eland. This part of sand grade classes, pH, conductivity, resistance, percent the study site is referred to as LU (Low Utilization). This carbon and Munsell colour by the Provincial Department terminology is intended to fit with the broader pilot study of Agriculture of the Western Cape (Elsenburg) according in other biomes where both grazing and firewood to methods described by The Non-Affiliated Soil Analysis extraction co-occur. It is acknowledged that LU and HU Work Committee (1990). Water infiltration was deter- are relative terms and do not necessarily reflect long-term mined (pipette method). grazing history. This pilot study was instituted by the Our sampling took advantage of a natural field experi- South African National Biodiversity Institute following ment in which the consequent logistic problems in avoid- the South African National Spatial Biodiversity Assess- ing pseudo-replication (Hurlbert, 2004) were clearly ment (Rouget et al., 2004) that highlighted the inade- immense (Hargrove & Pickering, 1992). The view was quate inclusion of the impact of habitat degradation on followed that extreme contrasts across fence lines that are biodiversity. carefully selected for a high probability of their original environmental uniformity may be regarded as a manipu- lative study. Such studies can be sampled and analysed Sampling with no reason to not constitute strong tests of conjectures Sampling was carried out in April after locally reported (Oksanen, 2004). However, owing to the unavoidable above-average summer rains. April is regarded as the peak experimental design, it should be accepted that inferences germination period in the southern Kalahari, provided may tend to be local (Stohlgren, 2007). adequate rains have fallen previously (Leistner, 1967). During this month, species which normally germinate in Analyses summer (the large majority), winter or any time of year can germinate. It is, however, possible that a few winter Plant canopy cover and soil parameter values were anal- annuals were not present on the site at the time of sam- ysed using two-tailed t-tests after square root [(x + 1)1 ⁄ 2] pling. transformation of the data (Krebs, 1989). Results for plant

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cover were compared with those of the independent Indi- Analysis. The perennial graminoids Aristida meridionalis, cator Species Analysis of Dufreˆne & Legendre (1997) as glauca, Eragrostis trichophora and Stipagrostis given by McCune & Grace (2002). Ordination of plots was amabilis and the annual Limeum fenestratum were indicated carried out using nonmetric multidimensional scaling to become apparently locally extinct. The main persisting (NMS) (PC-Ord) after a number of runs from different perennial species was Hermannia tomentosa, which was starting configurations to avoid the dangers of local min- usually found on HU in the form of large, well-established, ima. The Shannon–Wiener index of diversity (H’) or spreading shrubs often concentrated towards the edge of Shannon entropy was calculated as in Krebs (1989) but the dunes. Acacia haematoxylon also persisted on HU to- using ln base e. The values of this ‘raw’ index were also gether with lower occurrences of other tree species. converted to their number equivalents; this expresses a Whereas cover of the perennial Crotalaria cf. spartioides ‘true’ or species-neutral diversity (Jost, 2009). Alpha increased slightly on HU, its frequency increased sub- diversity (a) was calculated for LU and HU and gamma stantially as young establishing plants, usually with a diversity (c) was calculated for the whole study site by single narrow stem 0.1–0.2 m high (Fig. S1). This form pooling the observations. Independent beta diversity (b) was not noted on LU. The annuals hispidula and across the contrast was calculated by assuming the addi- Requienia sphaerosperma were the next most frequent spe- tive partitioning of diversity, where c = l(a)+b and l(a)is cies establishing on HU, but were smaller in size than those mean alpha diversity (Jost, 2007). Species accumulation on LU. The Indicator Analysis also showed that cover of curves were derived by use of sample-based rarefaction the annuals Brachiaria glomerata and Bulbostylis hispidula (Mao Tau expected richness function in EstimateS 8.0) as and perennial Stipagrostis uniplumis decreased significantly described by Colwell, Mao & Chang (2004). (P < 0.05) on HU. Soil texture was pure sand with medium sand grade, with soil Munsell colour mainly reddish brown (5YR 6 8 or 5YR Results ⁄ 7 ⁄ 7 dry). On HU, there was a significant decrease in carbon Species diversity indices declined by close to an order of and increase in pH, resistance and silt (Table 1). Water magnitude on HU (Table 1). Beta species-neutral diversity infiltration was instantaneous on LU and HU. Dunes on HU was 5.09 out of a total gamma diversity of 10.67. Both were generally lower and wider than those on LU and often species richness (mean number of species) and evenness had a dune slip face of 1–3 m high, which was absent on LU. declined several fold and mean canopy cover almost twentyfold on HU. At scales from 50 to 800 m2, species Discussion number remained consistently lower on HU (Fig. 2). The ordination of plots showed a clear separation of LU and Responses of life form, height of woody plants, habit and HU, with a greater scatter on the latter (Fig. 3). Most of the some architecture attributes did not accord with the main species on HU were shared with those on LU. conclusions of the global synthesis of plant trait responses to Both species number and canopy cover declined signifi- grazing (Dı´az et al., 2007). Thus grazing did not favour cantly on HU for annual and perennial life forms; forb and annual over perennial plants, or prostrate over erect plants, graminoid growth forms; erect and prostrate habits and or lower woody shrubs over the taller trees or stoloniferous leafy stem, tussock and stoloniferous architectures, with plants over tussock plants. Dı´az et al. (2007), however, last-mentioned architecture type becoming apparently qualify their findings for arid areas. Thus in dry systems with locally extinct (Table 1). No significant change was found in a long evolutionary history of herbivory, no significant cover of shrub and tree growth forms. However, these pattern for annuals was detected. Similarly for dry systems, together constituted the vast majority of plant cover on HU but with a short evolutionary history (e.g. some Australian (Fig. 4). Other changes in relative cover show that the sites), response to grazing did not differ significantly between extreme grazing treatment favoured perennial over annual, short and tall plants. There was no increase in stoloniferous erect over prostrate, and leafy stem over stoloniferous and plants with grazing in dry systems. The positive response of tussock. Geophytes and rosette plants were very rare. prostrate plants to grazing was weaker in dry than in humid Of the fifteen species with frequency greater than 30% systems. Despite these caveats, in our study, annuals and on either LU or HU, the cover of eight declined significantly stoloniferous plants declined unequivocally and prostrate on HU (Table 2), which was confirmed by the Indicator plants showed an opposite response to that of the synthesis.

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Table 1 Species diversity, abundance of Parameters LU HU P plants and plant guilds and soil properties Total number of species 38 16 under low- and high-utilization grazing Shannon–Wiener index 2.30 0.21 intensities Species-neutral diversity index 9.93 1.23 Evenness index 0.63 0.07 ) Mean number of species ± SE 12.00 ± 1.00 3.38 ± 0.46 1.2 · 10 7* Life form ) Annual 3.81 ± 0.36 0.50 ± 0.18 1.8 · 10 9* ) Perennial 8.19 ± 0.84 2.88 ± 0.33 9.9 · 10 6* Growth form Forb 3.88 ± 0.65 1.50 ± 0.26 0.0016* Geophyte 0.00 ± 0.00 0.06 ± 0.06 0.33 ) Graminoid 6.19 ± 0.46 0.44 ± 0.13 8.2 · 10 14* Shrub 1.25 ± 0.19 0.88 ± 0.15 0.15 Tree 0.69 ± 0.22 0.50 ± 0.13 0.55 Habit ) Erect 9.81 ± 0.70 2.75 ± 0.31 8.2 · 10 9* Prostrate 2.19 ± 0.45 0.63 ± 0.22 0.0023* Architecture ) Leafy stem 5.81 ± 0.68 2.88 ± 0.35 8.5 · 10 4* Rosette 0.00 ± 0.00 0.06 ± 0.06 0.33 ) Stoloniferous 1.38 ± 0.13 0.00 ± 0.00 9.2 · 10 10* ) Tussock 4.81 ± 0.38 0.44 ± 0.13 1.0 · 10 12* ) Mean canopy cover (%) ± SE 75.31 ± 2.87 3.96 ± 1.16 9.0 · 10 16* ) Mean sum of canopy covers (%) ± SE 91.81 ± 5.10 4.58 ± 1.18 8.5 · 10 12* Life form ) Annual 37.24 ± 4.50 0.05 ± 0.02 1.3 · 10 9* ) Perennial 54.57 ± 2.95 4.53 ± 1.18 1.3 · 10 12* Growth form ) Forb 20.81 ± 4.63 0.15 ± 0.03 7.7 · 10 5* Geophyte 0.00 ± 0.00 0.01 ± 0.01 0.33 ) Graminoid 65.29 ± 3.78 0.04 ± 0.01 5.0 · 10 15* Shrub 2.99 ± 0.83 3.10 ± 1.17 0.87 Tree 2.72 ± 0.93 1.28 ± 0.58 0.22 Habit ) Erect 74.56 ± 3.94 4.51 ± 1.18 2.1 · 10 12* ) Prostrate 17.26 ± 4.33 0.06 ± 0.02 2.4 · 10 4* Architecture ) Leafy stem 26.53 ± 4.20 4.53 ± 1.18 9.5 · 10 5* Rosette 0.00 ± 0.00 0.01 ± 0.01 0.33 ) Stoloniferous 21.19 ± 2.44 0.00 ± 0.00 6.9 · 10 10* ) Tussock 44.10 ± 4.56 0.04 ± 0.01 5.4 · 10 11* Soil properties pH 4.7 ± 0.15 5.7 ± 0.38 0.040* Conductivity (mS m)1) 4.6 ± 0.77 2.3 ± 0.09 0.028* Resistance (ohm) 6125 ± 900 11082 ± 435 0.002* Carbon (%) 0.075 ± 0.019 0.018 ± 0.007 0.032* Coarse sand (%) 2.3 ± 0.99 2.8 ± 1.33 0.769 Medium sand (%) 55.2 ± 4.07 58.3 ± 3.96 0.590 Fine sand (%) 40.5 ± 3.72 35.2 ± 3.65 0.330 Total sand (%) 98.0 ± 0.00 96.3 ± 0.33 0.004* Silt (%) 1.0 ± 0.00 2.7 ± 0.33 0.004* Clay (%) 1.0 ± 0.00 1.0 ± 0.00 1.000

LU, low-utilization grazing intensity; HU, high-utilization grazing intensity; P, probability level using two-tailed t-test; SE, standard error; mS m)1, millisiemen metre)1. *Significantly different at P < 0.05. Sum of covers of individual species, includes overlapping canopies.

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The Kalahari vegetation is acknowledged to be floristi- cally impoverished (Van Rooyen & Van Rooyen, 1998) relative to the floristic diversity in many other regions of the subcontinent (Cowling et al., 1989). Mean species richness of 17.2 estimated at a 100 m2 scale for LU (Fig. 2) compares to 22.0 on four same-sized plots on dunes 70– 100 km to the north-west of the site (Leistner & Werger, 1973). Beta diversity is regarded as extremely low across the sandy areas of southern Kalahari (Van Rooyen & Van Rooyen, 1998), but severe levels of degradation can clearly result in much higher levels of beta diversity that reach almost half that of gamma diversity in this study. Fig 2 Species accumulation curves derived by use of sample-based The apparent local extinction of the most palatable rarefaction for low-utilization (LU) and high-utilization (HU) perennial grass species, , in the southern grazing intensities Kalahari dunefield (Leistner, 1967) on HU is as anticipated. Predictions from a modelled decade of increased grazing Less expected is the apparent local extinction of the less pressure around an artificial waterhole for Twee Rivieren palatable, southern Kalahari endemic, Stipagrostis amabilis in the southern Kalahari dunefield (Jeltsch et al., 1997) that Van Rooyen, Bredenkamp & Theron (1991) indicate show a sharp decline in total plant cover closest to the should survive well under ‘severely overgrazed’ conditions. waterhole (typically situated in the inter-dune areas). The unpalatable Aristida meridionalis also became appar- Annual plants constitute at least half the plant cover in ently locally extinct and, although this is supported by Van this zone. Shrub cover declines sharply but more than Rooyen, Bredenkamp & Theron (1991), it has also been doubles beyond this zone, resulting in bush or shrub regarded as a species that increases under heavy grazing encroachment. The marked decline in total plant cover on pressure (Fourie, De Wet & Page, 1987). The sharp decline of HU is in keeping with this model’s prediction with the most Schmidtia kalahariensis on HU suggests that conditions here intensive grazing impact. However, the concomitant are more extreme than the grazing and trampling around decline in annual plant cover and relative dominance of an artificial watering point in the Kalahari dunefield, where woody plants are contrary to that predicted for this zone by this species was observed to increase (Van Rooyen et al., the model. The comparative constancy of absolute cover of 1990). This species is often considered an indicator of dis- woody plants on HU would correspond to a transient point turbance (Van Rooyen, Theron & Bredenkamp, 1991). It somewhere between the model’s most extreme impacted was overwhelmingly dominant in the inter-dune areas on zone and the domain of predicted shrub encroachment. HU, which suggests that its rare occurrence on the dunes is not directly because of grazing but of substrate instability. Crotalaria spartioides, Hermannia tomentosa and Requienia LU sphaerosperma are associated with overgrazing or distur- HU bance (Leistner, 1967; Skarpe, 1986; Van Rooyen et al., 1990). Crotalaria spartioides is poisonous to stock (Van Der Lugt, Fourie & Liddell, 2002), confined to loose or very loose sand grades in the Kalahari and is possibly helped to Axis 2 re-establish on HU by its very narrow aerial parts (Fig. S1) that offer little wind resistance (Leistner, 1967). Even on the exposed dunes, the reduced wind speed near the ground within the boundary layer because of surface resistance (Monteith & Unsworth, 1990) assists establish- Axis 1 ment (Leistner, 1967). The persistence of the toxic Her- Fig 3 Ordination of sample plots using nonmetric multidimen- mannia tomentosa on HU is facilitated by its bipartite root sional scaling (NMS) for low-utilization (LU) and high-utilization system with a deep tap root and shallow horizontal lateral (HU) grazing intensities roots that enables utilization of both deep, more permanent

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(a) (b) 100 100

80 80

60 60

40 40

Percent cover (%) 20 20

0 0 LU HU LU HU LU HU LU HU LU HU Annual Perennial Forb Graminoid Woody (c) (d) 100 100

80 80

60 60 Fig 4 Relative dominance (relative canopy 40 40 cover) of plant traits on low- and high- utilization grazing intensities (LU and HU

Percent cover (%) 20 20 respectively) for: (a) life forms, (b) growth forms, (c) habits and (d) architectures. Cover values of the geophyte growth form 0 0 LU HU LU HU LU HU LU HU LU HU and rosette architecture round to zero and Erect Prostrate Leafy stem Stoloniferous Tussock are omitted water supplies and water from light showers. In addition, it and relative decline of annual plants on HU accords with is able to sprout from vegetative buds on root portions Leistner’s (1967) observation that wind kills annual plants exposed to sunlight (suckering) (Leistner, 1967). The on unstabilized Kalahari dune crests. persistence of this species on HU appears to be at variance The decrease in soil carbon content on HU is expected with its negative association with grazing intensity on given the substantial decline in plant cover. The increase in dunes reported for a different locality in the Kalahari (Van pH (and possibly also the decline in carbon) on HU may Rooyen et al., 1994). The persistence of trees such as result from the removal of the top layers of sand by wind Acacia haematoxylon on HU is probably facilitated by its erosion. At increasing depth on the dunes of the southern deep roots (30 m or greater – Leistner, 1967) which pro- Kalahari, pH reportedly increases, whereas organic carbon vide access to water throughout the year. declines sharply (Van Rooyen, 1984). An alternative The normally vegetated and stable southern Kalahari explanation for the increase in pH is the mobilization of lime dunes have only limited sand movement on their crests. underlying the disturbed pan-like depressions on HU and The cover of only 4% on HU is well below the threshold of wind transport on to the dunes. However, this might not be approximately 14% for dune mobility (sediment move- supported by the decline in soil conductivity on HU dunes if ment) in the region (Wiggs, Thomas & Bullard, 1995) and an increase in soluble salts might have been expected from the slip faces present on dunes on HU may be expected to the same source area. In addition, directional variability of continue to advance. That some of the plant responses to wind in the area is relatively high (Bullard et al., 1996), grazing differ from those reported before may not relate to which should allow aeolian transport of any particles to the effects of extreme levels of herbivory, trampling, defe- both HU and LU in the vicinity of the fence line, unless the cation and urination so much as to the secondary effects of vegetation cover on LU impedes such movement. It has also the unstabilized sand substrate. For example, the absolute been shown on a very sandy site with low vegetation cover

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Table 2 Frequency and canopy cover of species with a frequency greater than 30% on low- or high-utilization grazing intensities [Corrections made in Table 2 after initial online publication]

Frequency (%) Canopy cover (%) Mean ± SE

Species LU HU LU HU P

) Eragrostis trichophora 100 0 15.94 ± 2.15 0.00 ± 0.00 2.14 · 10 6* Schmidtia kalahariensis 100 6 17.38 ± 5.20 0.01 ± 0.01 0.0045* ) Stipagrostis amabilis 94 0 18.63 ± 2.78 0.00 ± 0.00 7.00 · 10 6* Bulbostylis hispidula 81 25 0.62 ± 0.30 0.03 ± 0.01 0.065 Crotalaria cf. spartioides 6 81 0.06 ± 0.06 0.08 ± 0.01 0.77 Hermannia tomentosa 81 75 1.93 ± 0.65 2.93 ± 1.19 0.47 Limeum arenicolum 75 13 15.63 ± 3.82 0.01 ± 0.01 0.0010* Limeum fenestratum 69 0 3.20 ± 0.87 0.00 ± 0.00 0.0023* Aristida meridionalis 56 0 3.32 ± 1.22 0.00 ± 0.00 0.016* Requienia sphaerosperma 56 44 0.11 ± 0.04 0.04 ± 0.01 0.099 Acacia haematoxylon 50 25 1.97 ± 0.66 0.89 ± 0.57 0.22 Centropodia glauca 44 0 2.56 ± 1.07 0.00 ± 0.00 0.030* Aristida stipitata 38 6 1.91 ± 0.81 0.01 ± 0.01 0.033* Stipagrostis uniplumis 38 0 3.19 ± 1.61 0.00 ± 0.00 0.067 Brachiaria glomerata 31 0 0.26 ± 0.19 0.00 ± 0.00 0.19

LU, low-utilization grazing intensity; HU, high-utilization grazing intensity; P, probability level using two-tailed t-test; SE, standard error. *Significantly different at P < 0.05. that approximately 15% of organic carbon from the top References 50 mm of soil can be removed by wind in a single wind Bullard, J.E., Thomas, D.S.G., Livingstone,I.&Wiggs, G.F.S. season (Li et al., 2007). On Kalahari sand flats over 300 km (1996) Wind energy variations in the southwestern Kalahari to the ESE, opposite trends to those on HU were found in pH, Desert and implications for linear dunefield activity. Earth Surf. resistance and organic carbon near water points with Process. Landforms 21, 263–278. heavy grazing and trampling by domestic stock (Smet & Colwell, R.K., Mao, C.X. & Chang, J. (2004) Interpolating, Ward, 2006). This suggests that the soil changes on HU extrapolating, and comparing incidence-based species accu- may also be secondary and no longer directly related to the mulation curves. Ecology 85, 2717–2727. Cowling Gibbs ussell Hoffman Hilton primary grazing pressure impact, but rather to the sub- , R.M., R , G.E., , M.T. & -Taylor, C. (1989) Patterns of plant species diversity in south- sequent instability of the dunes. ern Africa. In: Biotic Diversity in southern Africa: Concepts and These possible secondary effects need further research. Conservation (Ed. B.J. Huntley). Oxford University Press, This should include addressing colonization issues identi- Cape Town. fied by Leistner (1967): (i) seed abundance, mobility, Darkoh, M.B.K. (2009) An overview of environmental issues in ability to settle in hollows or against other plants and rapid southern Africa. Afr. J. Ecol. 47(Suppl. 1), 93–98. germination; (ii) ability of plants to withstand partial Dı´az, S., Lavorel, S., McIntyre, S., Falczuk, V., Casanoves, F., MiIchunas Skarpe Rusch Sternberg exposure and submergence under sand and (iii) other , D.G., , C., , G., , M., Noy-Meir, I., Landsberg, J., Zhang, W., Clark,H.&Campbell, effects of the major hazards of wind on unstable substrates, B.D. (2007) Plant trait responses to grazing – a global synthesis. extreme soil surface temperatures and rapid desiccation of Glob. Chang. Biol. 13, 313–341. the upper soil layers after rain. Dufreˆne,M.&Legendre, P. (1997) Species assemblages and indi- cator species: the need for a flexible asymmetrical approach. Acknowledgements Ecol. Monogr. 67, 345–366. Fourie, J.H., De Wet, N.J. & Page, J.J. (1987) Veld condition and Landowners Sakkie Burger (Avonds Schijn) and Jonny trend in Kalahari duneveld under an extensive stock production Brink (Ballater) are thanked for permission to carry out system. J. Grassld. Soc. South. Afr. 4, 48–54. Hargrove Pickering research on their farms and for providing historical grazing , W.W. & , J. (1992) Pseudoreplication: a sine qua non for regional ecology. Landscape Ecol. 6, 251–258. and background information.

2009 Blackwell Publishing Ltd, Afr. J. Ecol., 48, 930–938 938 Michael Charles Rutherford and Leslie Ward Powrie

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Department of Agricultural nical support issues arising from supporting information Development, Pretoria. Stohlgren, T.J. (2007) Measuring Plant Diversity. Lessons from the (other than missing files) should be addressed to the authors. Field. Oxford University Press, New York. Stokes, S., Thomas, D.S.G. & Shaw, P.A. (1997) New chrono- (Manuscript accepted 11 October 2009) logical evidence for the nature and timing of linear dune development in the southwest Kalahari Desert. Geomorphology doi: 10.1111/j.1365-2028.2009.01194.x 20, 81–93.

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