Kalahari Sand Soils: Spatial Heterogeneity and Land Degradation
Dougill, A.J.1 and Thomas, A.D2
1
- School of the Environment, University of Leeds, Leeds, LS2 9JT, UK;
adougill@env.leeds.ac.uk; tel - (+ 44) 113 343 6782; fax - (+44) 113 343 6715
2
- Department of Environmental and Geographical Sciences, Manchester Metropolitan
University, Manchester, M1 5GD, UK; a.d.thomas@mmu.ac.uk
Abstract
This paper provides new analysis of the soil nutrient properties and processes of the surface
layer of Kalahari sand soils. We seek to assess the applicability of previous classifications of
resilience to soil degradation and to consider the biogeochemical processes which prevent
extensive soil degradation. We also evaluate the extent to which soil changes regulate
ecological changes, notably bush encroachment and changes in herbaceous grass
composition. Results show that surface biological soil crusts establish rapidly in the coarsetextured soils and are vital in preventing soil degradation. Crusts are responsible for surface
stabilisation and potentially also lead to significant nutrient fixation and adsorption that
retains a near surface supply of key limiting nutrients. Grazing pressures affect the spatial
distribution and the types of crust formed, enhancing spatial heterogeneity in soil
hydrochemical characteristics of grazed areas. Soil-vegetation studies suggest that increased
spatial heterogeneity in soil fertility, notably crust development and nutrient enrichment under
bush canopies, can retain ecological diversity. As such, spatial heterogeneity in soil
biochemical properties should not be immediately viewed as an indicator of degradation.
Keywords:
Kalahari soils; Biological soil crusts; Soil Degradation; Bush
Encroachment; Spatial heterogeneity.
1
Nutrients in Dryland Soils
The spatial and temporal variability of rainfall in dryland areas results in an incomplete
vegetation cover. Soils underneath vegetation have increased organic matter, nutrients and
microbial activity (Bennett and Adams, 1999). The work of Noy-Meir (1973, 1981, 1985)
demonstrated that this concentration of soil nutrients in patches increases the productivity of
dryland landscapes compared to if resources were evenly spread. Similarly, Ludwig et al.
(1996) found that if the without resource patchiness there was a decrease in the capacity of
the landscape to capture rainfall, resulting in a decrease in net primary productivity of 40%.
They concluded that patchiness was a vital component for landscape functioning and
biodiversity in savannas. This phenomenon has been documented in a variety of dryland
environments, such as the regular banding of tiger bush landscapes in West Africa (Valentin
et al., 1999), or more commonly as irregular clumps of vegetation (e.g. Tongway and
Ludwig, 1994; Bennett and Adams, 1999; Puigdefabregas et al., 1999).
However, others have questioned the association between increased spatial heterogeneity and
increased ecological productivity. Schlesinger et al. (1990) associated the development of
spatial heterogeneity in soil and water resources in the southwest United States with land
degradation. They argued that intensive grazing reduced grass cover and resulted in an
invasion of woody shrub species that once established, was difficult to reverse due to the
development of 'islands of fertility' under the canopies.
The processes causing the patchiness of soil and water resources in drylands and their
association to land degradation are only just starting to be understood. Research suggests the
processes can be grouped into two categories: i) vegetation related (where sub-canopy soil
nutrients are elevated by organic matter additions and faunal activity); and ii) sediment
transport related (associated with deposition of eroded material). Landscapes, where solely
vegetation regulation mechanisms predominate are generally regarded as vulnerable to
degradation (Tongway and Ludwig, 1994; Schlesinger et al., 1996; Puigdefabregas et al.,
1999). Tongway and Ludwig (1994) working in western Australian shrublands suggest the
regulation of resource heterogeneity occurs through the density, morphology and spatial
distribution of perennial plants as they obstruct wind and water flows resulting in the
retention of materials. Dean et al. (1999) found that the large tree species of Acacia erioloba
and Acacia haematoxylon in the Nossob River valley of the Kalahari increased the nutrient
content of underlying soil because of the large animals and birds the trees attracted. Both
Tongway and Ludwig (1994) and Schlesinger et al. (1996) found grasses also lead to smalllocalised increases in nutrient availability.
Dryland soil nutrients are preferentially concentrated in the surface layers (Tongway and
Ludwig, 1994; Dougill et al., 1998; Bennett and Adams, 1999). In part, this is due to the
widespread occurrence of biological soil crusts (Eldridge and Tozer, 1996; Rosentreter, 1997;
Belnap et al., 2003), which are able to fix atmospheric nitrogen (Belnap 1994, 1995, 2003;
Evans and Lange, 2003) and sequester carbon (Beymer and Klopatek, 1991; Evans and
Lange, 2003). Nitrogen is often the main limiting factor to plant growth in drylands (Scholes
and Walker, 1993) and, therefore, the distribution of biological soil crusts is vital in
understanding soil fertility. There is an inverse relationship between biological crust cover
and vascular plant cover because they are in direct competition for light and moisture and
certain plants have an allelopathic effect on the microorganisms forming crusts (Skujins,
1984). However, bush canopies can provide protection from disturbance, create shade which
controls the heat and light reaching the soil surface all of which can be beneficial to growth
(Belnap et al., 2001).
The complex relationship between vegetation and biological crust formation and its
relationship to the development of `islands of fertility’ has not previously been studied. To
fully understand the spatial and temporal dynamics of soil fertility and degradation processes
in dryland environments this additional layer of complexity needs to be investigated.
2
Kalahari Sand Soils and Ecosystems
The Kalahari is "the extensive elevated, flat, sand-covered plain that occupies a substantial
part of the southern Africa interior" (Thomas, 2002; p.21). The geological unit of the Kalahari
Sands covers 2.5 million km2 (Thomas and Shaw, 1991), from the Orange River in South
Africa to north of the Congo River in Gabon. Recent intensification of pastoral land use in
the semi-arid core of the Kalahari sandveld (comprising much of Botswana, eastern Namibia
and northern South Africa) based on the drilling of deep boreholes, has led to concerns of
widespread land degradation (Government of Botswana, 1997; SADC, 1997).
Kalahari sand soils typically consist of over 95 % fine sand-sized, aeolian-deposited sediment
(Thomas and Shaw, 1990). Kalahari soils are, therefore, predominantly deep, structureless
and lacking in essential plant nutrients and organic matter (Skarpe and Bergström, 1986;
Dougill et al., 1998). The high infiltration rate of the sandy soils, together with the flat
topography, results in the absence of extensive surface water runoff, implying that water
erosion is not a serious threat. Similarly, wind erosion is not viewed as a significant problem
on Kalahari rangelands (Perkins and Thomas, 1993), or even for the more intensively utilised
mixed farming regions on the southern margins of the Kalahari sandveld (Dougill and
Thomas, 2002).
The natural flora of the Kalahari is well adapted to low water and nutrient availability, leading
to the assumption that vegetation assemblages are sensitive to changes in soil properties
(Skarpe, 1990; Perkins and Thomas, 1993). For example, extensive bush encroachment
associated with grazing intensification has been linked to increased leaching of water and
nutrients (Walker and Noy-Meir, 1982; Perkins and Thomas, 1993). However, research has
failed to identify any association between soil water and nutrient availability and ecological
changes (Dougill et al., 1999). These findings suggest Kalahari sand soils are resilient to
chemical degradation. However, the processes behind this resilience remain unclear.
Degradation Assessment in the Kalahari
Assessment of dryland degradation remains a highly contentious issue (Thomas, 1997;
Warren et al., 2001). In particular, the use of subjective indicators to quantify the extent and
nature of dryland degradation has been widespread in international (e.g. GLASOD, 1990;
UNEP, 1997), national (e.g. NRC, 2000; Hoffmann and Ashwell, 2001) and local-scale
studies (e.g. Rigby et al., 2000; Stocking and Murnaghan, 2001). However, concerns exist
that indicators oversimplify the nature of the environmental processes leading to degradation
(Stocking and Murnaghan, 2001). Furthermore, inconsistencies exist in the relative
importance assigned to soil degradation compared to indicators of ecological change. Many
studies have focused solely on soil degradation indicators (e.g. GLASOD, 1990; Tongway,
1994; NRC, 2001) due to uncertainties over whether ecological changes are effectively
irreversible, or simply indicative of the natural dynamism of dryland ecosystems (Behnke et
al., 1993). This soil-based focus, however, fails to recognise the importance of ecological
changes to the pastoralist communities found across the Kalahari.
The large-scale GLASOD study shows limited soil degradation in the Kalahari (UNEP,
1997). There has been much more concern over the potential economic impacts of ecological
changes (e.g. Cooke, 1983; Skarpe, 1990; Ringrose et al., 1996; Sporton and Thomas, 2002)
and the increased cost of groundwater supply (e.g. de Vries and von Hoyer, 1988). Given that
changes in ecology and groundwater are fundamentally regulated by changes in the soil there
is a need for process-based research on Kalahari soils to assess the factors that control soil
water and nutrient availability. It is these soil hydrochemical processes that control
ecosystem structure and functioning (Scholes and Walker, 1993) and determine the ecological
fodder diversity that is central to pastoral farming sustainability (Scoones, 1995).
Consequently, there is a need to better integrate soil analyses into assessments of degradation
for Kalahari rangelands.
3
The limited evidence of soil degradation processes affecting Kalahari sand soils, in either
rangeland or mixed farming settings, contrasts markedly to soils in many other dryland
regions (UNEP, 1997). This includes soils in Sub-Saharan Africa (e.g. Mortimore, 1998) and
even on other soils in Southern Africa where erosion concerns are greater (e.g. Scoones et al.,
1996; Hoffman and Ashwell, 2001).
Research Objectives and Questions
This paper aims to examine the factors affecting surface soil nutrients on Kalahari sands.
Several locations across the Kalahari are used to provide an insight into the processes
retaining nutrients at the surface, in particular examining the extent and role of biological soil
crusts. Given the soil attributes and resilience mechanisms identified from previous studies
the following objectives were developed:

To determine the extent of biological crust cover in a variety of contrasting Kalahari
environments

To determine the processes leading to spatial heterogeneity in soil biochemical
characteristics.

To assess the association between the spatial heterogeneity in soil biochemical
characteristics and the ecological fodder diversity.
In addressing these issues a clearer assessment of the factors contributing to the resilience of
Kalahari soils to degradation will be obtained.
Research Design and Methods
Site Selection
Four sites across the Kalahari were selected in order to incorporate a range of land uses and
climatic conditions (Figure 1). They include the commercially owned Makoba Ranch Block,
in the Eastern Kalahari; the mixed farming area of the Molopo Basin in the South East
Kalahari; rangelands of Omaheke District, in the Western Kalahari; and in southern
Kgalagadi District, South Western Kalahari. This range of sites provides a first regional
overview of Kalahari sand soil properties and the associations to patterns of ecological change
and fodder diversity.
Field Measurements
Ecological and soil-based studies at the sites have been ongoing for the last decade. As the
research focus and approaches have evolved through time, the exact nature of the methods at
each site differs. The methods are, therefore, detailed by site.
i.
Makoba Ranch Block.
Integrated soil hydrochemical and ecological species analyses have been conducted at plot- (4
m2), site- (30 m x 30 m) and ranch- (8 km x 8km) scales (see Dougill et al., 1999). Topsoil
samples (n = 23) taken from random points across 30 m x 30 m sites in both an intensivelygrazed bush-dominant site (800 m from Uwe Aboo borehole) and a less intensively-grazed
grass-dominant site (4 km from Uwe Aboo borehole) are compared. Soil moisture, organic
matter and inorganic nutrients (NO3--N, NH4+-N and PO43--P) were determined using methods
outlined by Anderson and Ingram (1993).
ii.
Molopo Basin, South Africa and Botswana.
This site is approximately 100 km west of Mafikeng, between the villages of Loporung and
Tsidilamalomo, in an area with a series a low parallel ridges of calcrete and ironstone cutting
across Kalahari sand deposits (see Thomas et al., 2002). Six sites along a 1500 m transect
perpendicular to the ridges were selected and soil crust characteristics determined within 30 m
by 30 m grids. Topsoil samples were taken from soils developed from Kalahari sands,
4
ironstone, alluvium and calcrete. A disturbance index was calculated for each site based on
the number of cattle tracks, surface dung density and grass height. At all sites the percentage
crust cover and type, vegetation species and cover was determined in quadrats every 1.5 m
along three parallel 30 m line transects. The crust type was assigned using a classification
scheme based on crust form and morphology (Figure 2) (Thomas et al., 2002). Soil nutrients
and grass biomass were also determined underneath the canopies of the three main bush
species (Acacia mellifera, Grewia flava and Brachylaena rotundata) at each of the sites and
compared to adjacent open sites.
iii.
Omaheke District, Namibia.
The extent of biological crust cover and the nutrient content of the surface soil was
determined at four locations in Omaheke District of Eastern Namibia. Studies were
undertaken at three communally-managed farms (Netso in Tsjaka Fram Block; Corridor No.4
Farm and Okonyoka in Aminuis Reserve, see Twyman et al., 2001) and a commercially
owned ranch (Hogusrus). At all locations, sampling sites were designated at 0, 100, 200, 400,
800, 1600, 3200 and 4000 m from a waterpoint along a 'piosphere' of declining grazing
intensity (Perkins and Thomas, 1993). At each site, soil crust and vegetation cover was
determined within sixteen systematically located 1 m2 quadrats and crust samples collected
for total-N, total-P and organic matter analysis (Anderson and Ingram, 1993).
iv.
Southern Kgalagadi District, Botswana.
Studies assessing the soil crust occurrence, nutrient properties and vegetation associations
have also been initiated on a commercial ranch (Berry Bush farm), 10 km north east of
Tshabong in Southern Kgalagadi District, Botswana (see Reed and Dougill, 2002). The
methodological framework used in Omaheke District, Namibia was also applied here.
Results and Analysis
Analysis of the findings from the range of locations is provided in an integrated manner
(rather than on a by-site basis) to enable investigation of the specific environmental
objectives.
i.
Distribution of Biological Soil Crusts
Despite coarse-textured soils, the different climatic conditions and the regular surface
disturbances, a significant biological soil crust cover typifies all locations (Table 1). This
suggests that despite variations in the climatic conditions and soil texture, all sites are
conducive to the formation of an extensive surface biological soil crust. The extent of crust
cover is surprising because all sites are characterised by high levels of disturbance by
livestock. This suggests that either crusts are able to recover rapidly and reform after
disturbance or they remain in protected patches free from disturbance. At the Molopo sites,
the extent of all but the most well developed crusts appeared unaffected by the amount of
disturbance (Figure 3). Similarly, significant crust cover is retained throughout ranches in the
Namibian and Kgalagadi rangelands (Figure 4), except around the relatively small disturbed
'sacrifice zone' immediately surrounding waterpoints.
Studies in the Molopo Basin also enable an assessment of the impact of different parent
materials, and therefore soil types, on crust formation. Table 2 summarises the cover of the
different crust types for the soils on the four substrates. This shows that crust formation is
least prevalent, and includes fewer better-developed type 2 and 3 crusts, on solely Kalahari
sand soils. In contrast, on ironstone soils there is over 50 % crust cover and a high proportion
of type 2 and 3 crusts. This implies that any landscape-scale spatial variations in parent
material and, therefore, soil type within the Kalahari will add to the extent and development
of crusts.
ii.
Spatial Heterogeneity in Soil Biochemical Characteristics
5
Previous studies on Kalahari soils have assessed the impacts of grazing regimes on mean soil
nutrient concentrations and ecology (e.g. Skarpe, 1990; de Queiroz, 1993; Perkins and
Thomas, 1993; Dougill et al., 1999). These studies have found no significant differences in
soil chemical characteristics between intensively grazed bush-encroached sites and the grassdominant, less intensively grazed sites. However, these findings could be a consequence of
the statistical methods used in examining differences in mean nutrient concentrations, for
soils with low fertility and high spatial variability. In addition, some studies (e.g. Perkins and
Thomas, 1993) integrate nutrient concentrations from a range of depths, rather than the
surface layer where nutrients are focused.
Here soil nutrient concentrations were determined for 23 randomly located surface soil
samples taken within 30 m by 30 m sites at both intensively grazed bush-dominant, and less
intensively grazed grass-dominant sites at the Makoba site. Table 3 displays the mean
nutrient concentrations, and the coeffiecient of variation, used as an index of the overall
variation in soil properties. The spatial variability within a given site is low (11 to 28 %)
when compared to drylands in the southwestern United States (Schlesinger et al., 1996) where
the coefficient of variation in N and P for shrublands ranged from 59 to 103 %. This suggests
that Kalahari soils are more homogeneous than soils from other dryland areas. The significant
difference in variance of phosphate availability between bush and grass-dominated sites
indicates that intensive grazing and bush encroachment can contribute towards increases in
the spatial heterogeneity of nutrient availability (Table 3). A more detailed analysis of the
impact of bushes on the spatial heterogeneity in soil fertility at the Molopo site supports this
assumption. Table 4 shows that sub-bush canopy sites have enhanced nitrate and phosphate
concentrations, organic matter and, importantly for ecological resilience, grass biomass.
Enhanced grass biomass under bushes was also found at the Makoba study sites (Perkins and
Thomas, 1993; Dougill and Trodd, 1999) and may similarly be linked to nutrient-enriched
sub-canopy niches. Previous work in the Molopo Basin has shown that wind transported
sediments deposited under bushes and forming nebkha dunes are not enriched in plant
nutrients (Dougill and Thomas, 2002) and that elevated nutrient concentrations under bushes
are due to autogenic processes. However, the relatively small increases in organic matter are
not sufficient to explain all the nutrient enrichment. Thus we also investigated the potential
impacts of biological soil crusts to the spatial heterogeneity of nutrients around bushes.
Around all bush species, there is a concentric pattern of unconsolidated soil and crust with the
size of each zone varying with bush species (Table 5). The understorey of Acacia mellifera
was shown to be ideal for crust formation due to a combination of the grazing deterrent effect
of the dense double thorns and the small leaves, which allow light to reach the soil surface.
Grewia flava, in comparison, has no thorns and large leaves producing a thick cover of litter
on the soil. Consequently, despite the similar canopy dimensions, crust development was
greatly reduced under this species. This has important implications for persistence of Acacia
mellifera in the landscape as once established the crust will supply additional nutrients to the
plant.
iii.
Spatial Variability in Vegetation Characteristics
The ecological implications of soil crust formation under bush canopies also depends on the
associated herbaceous grass cover in these protected niches. In addition, to the information on
increased grass biomass under bush canopies (Table 4), studies at the Molopo sites also
investigated the herbaceous species associations in sub-canopy and open locations. Grass
species cover is grouped (Table 6) according to 'Grazing Value' classes, as assigned by van
Oudtshoorn (1999). Grazing values are based on the species palatability, production,
nutritional value, growth vigour, digestibility and habitat preference. There is evidence that
nutritious grasses are protected from grazing under bush canopies (Table 6). Although the dry
season cover of these grasses is low, the enhanced cover in these niches is important in
retaining a seed resource for future germination and maintaining ecological fodder diversity
for livestock.
6
Further investigations of the complex inter-relationships between bush canopies, soil
biochemical characteristics and herbaceous vegetation assemblages therefore remains a key
area for future research to better understand ecological change and potential processes of
degradation in Kalahari rangelands.
Discussion and Conclusions
This study has drawn upon findings from across the Kalahari in order to provide an insight
into the factors controlling soil surface nutrient dynamics. Dryland soils generally are
characterised by low fertility, with a concentration of nutrients at the surface (Bennett and
Adams, 1999) and in 'islands of fertility' (Schlesinger et al., 1990). There is an ongoing
debate on the implications of this spatial heterogeneity in soil nutrients for land degradation.
On the one hand, some authors point to the increased net primary productivity under spatially
heterogeneous ecosystems (e.g. Noy-Meir, 1985; Ludwig et al., 1996). In contrast,
Schlesinger et al. (1990) stress the importance of spatial heterogeneity in reinforcing bush
encroachment. This paper investigates the factors controlling the spatial distribution of
nutrients within Kalahari soils, the links to vegetation cover and the implications for
degradation. This is important because previous assessments of degradation in the Kalahari
have suggested the resilience of the soils to erosion and nutrient depletion (UNEP, 1997;
Dougill and Thomas, 2002).
Previous studies on nutrient heterogeneity in drylands have suggested two main processes
leading to the concentration of nutrients in `islands of fertility'. The most common is through
organic inputs from litterfall and the associated increase in faunal activity under canopies
(Belsky, 1994; Tongway and Ludwig, 1994; Dean et al., 1999). Alternatively, nutrients and
organic matter in eroded sediment are trapped and accumulate around vegetation (Schlesinger
et al., 1996; Puigdefabregas et al., 1999). Our findings suggest that in the Kalahari, the
spatial heterogeneity of soil nutrients is low compared to shrubland areas in the southwestern
United States (Schlesinger et al., 1996). Previous work has shown the nutrient concentration
of wind eroded material is not significantly enriched (Dougill and Thomas, 2002) suggesting
that this is not an important cause of nutrient heterogeneity. Some limited enrichment does
result from organic inputs from vegetation canopies. However, further processes are required
to explain the nutrient heterogeneity observed.
An additional factor influencing the spatial distribution of nutrients in drylands is biological
soil crusts (Belnap, 2003; Evans and Lange, 2003). Our research shows that biological soil
crusts are widespread across the Kalahari. They exist in a variety of settings and are able to
persist in landscapes where there is a high level of disturbance from livestock. This is
surprising given existing models describing crust distribution suggest they are less prevalent
on coarse textured soils, in areas of high disturbance and where there is a high vascular plant
cover (Belnap and Gillette, 1997; Eldridge, 1999).
The implications of the widespread occurrence of biological soil crusts in the Kalahari are
uncertain. They will enhance soil surface stability and, therefore, reduce erosion. In addition,
they are able to fix nitrogen and sequester carbon (Evans and Lange, 2003) thus increasing
soil fertility. Our findings show that crusts develop preferentially under Acacia mellifera,
which is the major bush encroaching species in the Kalahari (Skarpe, 1990; Thomas, 2002).
As such, nutrient enrichment is not evenly distributed across landscapes, but is concentrated
under encroaching bush species and may encourage the persistence of A. mellifera at the
expense of other species. The shallow rooting system of A. mellifera (Skarpe, 1990) secures
many of these nutrients for the bush. Whether this bush encroachment represents degradation
depends upon the degree to which the sub-canopy niche can also support more nutritious
grass species. The evidence presented here suggests that there are small increases in the
cover of the most nutritious grass species under bush canopies that could offset reductions in
ecological fodder diversity associated with A. mellifera encroachment. The net effect of these
processes in the Molopo Basin is to increase the bush cover, but also to retain a grass cover
and seed resource, that enables continued livestock utilisation. Crusts are also important in
7
enabling the seed resource to germinate as previous studies have shown that seedling
emergence in Kalahari soils is mainly from the top 1 cm of soil (Veenendaal et al., 1996).
However, degradation concerns remain due to evidence elsewhere in the Kalahari (e.g.
Skarpe, 1990; Adams, 1996; Twyman et al., 2001) that when A. mellifera occurs in dense
thickets the palatable grasses may no longer be accessible to livestock. It is these ecological
changes that reduce the overall fodder diversity that represent a process of land degradation.
Acknowledgements
Research in Botswana was conducted with the Republic of Botswana Research Permit No.
OP46/1XCVI(87). Funding for these studies has been provided by the Royal Geographical
Society (HSBC Holdings Grant), Royal Society (Dudley Stamp Memorial Fund), Manchester
Geographical Society and the Universities of Leeds, Sheffield and Salford.
Field assistance provided by Kate Berry, Jenny Byrne, George Davies, Liz Metcalfe, Ed
Sherratt, Nigel Trodd and Eleanor van der Linde is gratefully acknowledged.
References
8
Figure Captions:
Figure 1. Locations of study areas and Kalahari sand soils.
9
Figure 2. Soil crust classification scheme based on crust form and morphology.
10
Figure 3. Soil crust cover and type in relation to disturbance index in Molopo Basin (modified
from Thomas et al., 2002)
Crust Cover %
40
30
20
10
0
0
0.5
1
1.5
2
2.5
Disturbance Index
Bio 1 Crust
Bio 2 Crust
Bio 3 Crust
11
Figure 4. Crust cover with distance from waterpoints on Namibian and Kgalagadi ranches.
80
Crust cover (%)
70
60
50
40
30
20
10
0
0
1000
2000
3000
4000
5000
Distance (m)
Netso
Hogusrus
C4b
Okonyoka
Berry Bush
12
Table 1. Soil physical characteristics, crust cover and environmental conditions at each
location (see Figure 1).
Site
Land Use
Climate mean
annual
rainfall
(c. mm
p.a.)
Soil
Texture
Analysis
Sand (%)
Silt (%)
Clay (%)
Soil
Crust
Cover
(mean %
for all
sites)
Vegetation
Status
Makoba
Ranch
Block
Commercial
ranch
380
97.6
1.8
0.6
40.0
Mixed bush:grass
savanna extensive bush
encroachment
Molopo
Communal
rangelands
450
95.9
1.9
2.2
38.9
Mixed bush:grass
savanna extensive bush
encroachment
Omaheke
Communal
rangelands
300
95.6
2.9
1.5
39.3
Grass:bush
savanna extensive bush
encroachment
Omaheke
Commercial
ranch
300
95.2
2.8
2.0
18.9
Grass: bush
savanna remains
due to bush
clearance
Berry
Bush
Commercial
ranch
320
97.9
1.5
0.6
25.7
Grass:bush
savanna extensive bush
encroachment
Sources: Rainfall estimate based on Bhalotra (1987); Particle size data summarised from
Dougill et al. (1998, 2002), Metcalfe (unpublished) and van der Linde (unpublished).
13
Table 2. Soil crust type at sites of different substrate (% ground cover).
Location
Physical
Crust
Type 1
crust
Type 2
crust
Type 3
crust
Alluvial
crust
8.0
16.0
1.3
-
-
25.3
Ironstone
-
7.1
11.9
34.6
-
52.7
Valley Alluvium
-
5.1
4.6
14.6
15.1
39.4
0.4
1.9
10.7
17.8
Kalahari Sands
Calcrete
Total
Crust
30.8
Source: Modified from Thomas et al. (2002).
14
Table 3. Surface soil inorganic nutrient concentrations and variability within bush-encroached
and grass dominant study sites on the Uwe Aboo Ranch, Makoba Ranch Block.
Soil Chemical
Characteristic
Bushdominant
site
Coefficient
of variation
(%) [(s.d. ÷
mean) x
100]
Grassdominant
site
Coefficient
of variation
(%) [(s.d. ÷
mean) x
100]
NO3--N
(mgN100g-1)
4.68 ± 2.53
n = 23
27.6
6.32 ± 1.98
n = 23
NH4+-N
(mgN100g-1)
5.13 ± 1.34
n = 23
13.3
5.07 ± 1.29
n = 23
12.9
x
x
PO43--P
(mgP100g-1)
1.11 ± 0.44
n = 23
20.2
1.19 ± 0.27
n = 23
11.5
x
*
16.0
Statistical
implication
(t-test on
means)
x
Statistical
implication
(F-test on
variance)
x
Source: Data summarised from Dougill et al. (1998).
x No significant difference; * significant difference at 95 % confidence interval
15
Table 4. Mean extractable inorganic nutrient concentrations (and standard deviations) and
grass biomass cover for 'protected' sub-bush canopy and 'unprotected' neighbouring open
quadrats at Molopo Basin study sites.
Environmental Parameter
'Protected' sub-bush canopy
sites
'Unprotected' open sites
NO3--N concentration
(mgN100g-1)
54.2
(40.9)
102.0
(92.4)
NH4+-N concentration
(mgN100g-1)
283.9
(204.4)
45.8
(22.4)
PO43--P concentration
(mgP100g-1)
115.5
(97.0)
87.5
(63.2)
Organic Matter Content (%)
2.8
(1.2)
1.9
(0.6)
Grass biomass cover (gm-2 dry
weight)
17.2
(5.2)
0.9
(0.6)
Source: Nutrient data summarised from Dougill and Thomas (2002).
16
Table 5. Bush canopy characteristics and extent of undisturbed crust cover for the main bush
species at the Molopo Basin sites (mean values with standard deviations in parentheses)
Bush Species
Acacia mellifera
Grewia flava
Brachylaena rotundata
Canopy Height (m)
Canopy Width
(m)
Area of Undisturbed Crust
under bush (m2)
1.50
1.87
2.13
(1.27)
(1.19)
(1.52)
1.26
1.41
1.19
(0.29)
(0.47)
(0.71)
1.57
1.43
1.64
(0.45)
(0.55)
(2.18)
Source: Data summarised from Thomas et al. (2002)
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Table 6. Mean herbaceous species cover estimates (and standard deviations), by grazing value
class (van Oudtshoorn, 1999) for sub-bush canopy and open sites in the Molopo Basin
(species list by class given below).
Herbaceous Species Class (by
grazing value - van
Oudstshoorn, 1999)
Mean cover (%) in sub-bush
canopy sites
(n = 144)
Mean cover (%) in open sites
(n = 125)
High Grazing Value *
1.06
(1.85)
0.32
(0.78)
Average Grazing Value +
2.19
(2.06)
2.29
(1.83)
Low Grazing Value ^
5.45
(6.99)
3.61
(4.36)
*
- species in this class were Anthepora pubescens, Digitaria eriantha and Schmidtia
pappophoroides, Themeda triandra.
+
- species in this class were Enneapogon cenchroides, Eragrostis echinochloidea, Eragrostis
trichophora, Fingerhuthia africana and Stipagrostis uniplumis.
^
- species in this class were Aristida canescens, Aristida diffusa, Aristida stipitata, Eragrostis
biflora, Eragrostis pallens, Pogonarthria squarrosa and Perotis patens.
18