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The Namibian Journal of Environment is a scientific e-journal published by the
Environmental Information Service, Namibia for the Ministry of Environment
and Tourism, the Namibian Chamber of Environment and the Namibia
University of Science and Technology.
The Namibian Journal of Environment (NJE) covers broad environmental areas of ecology, agriculture, forestry,
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Editor: BA CURTIS
SECTION A: Peer-reviewed papers
Recommended citation format:
Strohbach BJ (2017) Vegetation of the Auas-Oanob
Conservancy in the Khomas Hochland of Namibia.
Namibian Journal of Environment 1 A: 19-42.
Namibian Journal of Environment 2017 Vol 1. Section A: 19-42
19
Vegetation of the Auas-Oanob Conservancy in the Khomas Hochland of
Namibia
BJ Strohbach
URL: http://www.nje.org.na/index.php/nje/article/view/volume1_p19-42_strobach
Published online: (to be completed by editor)
Faculty of Natural Resources and Spatial Sciences, Namibia University of Science and Technology,
P/Bag 13388, Windhoek, Namibia. bstrohbach@nust.na
Date received: 30th June 2017; Date accepted 24th October 2017.
ABSTRACT
The Auas-Oanob Conservancy is situated in the very rugged, botanically highly diverse Khomas Hochland in
central Namibia. A number of vegetation-related studies have been undertaken in this highland before, but none
covering the full extent of the conservancy, and with different interpretations of the syntaxonomy of the vegetation.
The current study aimed to describe and map the vegetation of the conservancy for practical management
purposes. A total of 229 relevés was compiled within the conservancy, and 72 relevés from other studies (falling
either within the conservancy, or the nearby Auas Mountain range) were added. The data were classified with a
Modified TWINSPAN. Three large groups were identified: (a) the high-altitude veld of the Auas Mountain range,
with three associations being recognised on the basis of altitude and aspect; (b) the lowlands and valley veld with
five associations, and (c) the Khomas Hochland veld with five associations. These 13 associations are described,
mapped and compared to descriptions of the vegetation in the vicinity of the conservancy. A higher-order
syntaxonomy, with three orders, one subdivided into two alliances, is suggested. This higher-order syntaxonomy
needs to be further investigated, taking into account vegetation descriptions of the greater Khomas Hochland. The
high-altitude veld of the Auas Mountain range has a unique composition with a high degree of endemisim and a
sub-alpine character. Due to its restricted range, there is an urgent need for formal protection. Two associations
within the lowland and valley veld are under threat, being the Platycarphella carlinoides - Chloris virgata association
of the omiramba (erosion) and the Schmidtia kalahariensis - Acacia erioloba association in the bottom lands at
Omeya (development). Bush encroachment is a general threat to various upland associations.
Keywords: Auas Mountains; freehold conservancies; Highland Savannah; Khomas Hochland; phytosociology;
vegetation classification; Namibia
INTRODUCTION
The Auas-Oanob Conservancy consists of a cluster of freehold (commercial) farms south and
south-west of Windhoek (Figure 1a), with a total area of 105 644 ha (Shaw & Marker 2010).
The farms are used mainly for cattle farming, but horse breeding, game farming, hunting and
ecotourism are also practised. In forming a freehold conservancy, the owners subscribe to the
principle of "... a legally protected area of a group of land-occupiers practicing co-operative
management based on a sustainable utilisation strategy, promotion of the conservation of
natural resources and wildlife, and the desire to reinstate the original biodiversity with the basic
goal of sharing resources amongst all members" (Shaw & Marker 2010). With more than 88%
of all wildlife in Namibia occurring outside national parks, it was deemed necessary to form
such conservancies to aid in the protection and sustainable utilisation of this renewable natural
resource (NACSO 2010, Shaw & Marker 2010).
The Auas-Oanob Conservancy is situated in the Khomas Hochland (‘Khomas Highland’, often
referred to as the ‘Central Highlands’) in central Namibia. This very rugged, high altitude
(reaching 2,000 m asl) landscape was formed through extensive faulting and erosion of the
Namibian Journal of Environment 2017 Vol 1. Section A: 19-42
20
soft schists of the Damara Sequence, and forms a broad south-west to north-east oriented
band through central Namibia (Swart & Marais 2009). The ruggedness of the landscape gives
rise to a plethora of habitats, which, in turn, result in a high diversity of plant and animal species
(Craven 2001, Mendelsohn et al. 2002). Giess (1998) referred to the vegetation of this
landscape as the 'Highland savannah'.
To date, three studies have been undertaken to describe portions of this Highland savannah
vegetation type: (a) Volk and Leippert (1971) studied the farms Binsenheim and Voigtland to
the south-east and east of Windhoek; (b) Kellner (1986) reported on the Daan Viljoen Game
Reserve, as well as portions of the farms Claratal and Bergvlug (west, south-west and east of
Windhoek) and (c) Burke and Wittneben (2007) investigated the Auas Mountain range south
of Windhoek. Burke and Wittneben (2007) give an overview of composition and structure of
the high-altitude Auas Mountain range vegetation, but provide no formal classification. Both
Volk and Leippert (1971) and Kellner (1986) provide phytosociological descriptions of the
vegetation at lower elevations in similar habitats, however, they differ in interpretation and
syntaxonomy of the vegetation. Kellner (1986) describes the vegetation of the northern part of
the farm Claratal as dominated by the Acacia hereroensis - Brachiaria nigropedata
1
savannah.
This is the only part of the Auas-Oanob Conservancy that has an existing description of the
vegetation.
This study aims at describing and mapping the vegetation of the entire Auas-Oanob
Conservancy in order to serve as baseline information for the management of this important
resource.
STUDY AREA
Topography and geology
The Lichtenstein Mountains, which form a southern extension of the Auas Mountain range,
dominate the landscape of the central-eastern part of the conservancy. The highest peaks of
the Lichtenstein mountains rise to 2 400 m, whereas the Molkteblick, as highest peak in the
Auas Mountains, rises to 2 450 m (Jarvis et al. 2008). To the east, these taper out into the
Omeya valley, which is closed in the far south-east of the study area by the Oamites Mountain.
To the west of the Lichtenstein Mountains, an undulating plateau forms the upper catchment
of the Oanob River. This catchment is characterised by a number of omiramba (flat
watercourses without discernable gradients - King 1963, Strohbach 2008), which feed into the
endoreic Oanob river. The Oanob plateau ends in the north-west along the watershed to the
Kuiseb valley, which forms the typical moderately steep to steep mountainous highland of the
Khomas Hochland. To the south and south-west of the Oanob plateau, the mountainous
highland of the Khomas Hochland sets in again, less steeply, forming a rolling landscape.
The geology is dominated by various formations of the Damara Sequence. The Oanob plateau
and adjacent Kuiseb catchment are formed by highly erodible quartz-biotite ('mica') schists of
the Kuiseb Formation (Geological Survey 1980, South African Committee for Stratgraphy
1980). The Auas Mountains and Lichtenstein Mountains form a band of the Auas Formation,
which consists of interbedded layers of quartzite. To the south of the Auas Formation is an
extensive band of the Chuos Formation, bounded by a narrow band of the Kudis Subgroup.
Both these formations are also characterised by schists, whilst the Chuos Formation contains
a variety of other lithological layers (South African Committee for Stratigraphy 1980). The Aris
/ Omeya valley is underlain by gneisses and schists of the Hohewarte Complex, which is to
1
Recent name changes to the genus Acacia (Kyalangalilwa et al. 2013) have not been recognised by prevailing
taxonomic literature in Namibia (Klaassen & Kwembeya 2013). To avoid confusion, also in reference to older
vegetation descriptions, the name Acacia is thus used throughout this paper. Alternative names are provided in
the appendices.
Namibian Journal of Environment 2017 Vol 1. Section A: 19-42
21
the south, overlain by quaternary alluvial deposits (South African Committee for Stratigraphy
1980). In the far south, the Oamites mountain forms part of the Billstein Formation,
characterised by grey quartzites and acidic igneous rocks (South African Committee for
Stratigraphy 1980).
Figure 1a (above): Altitude and topography of the study area. 1b (below): Geological formations of the study area.
Data sources: background image: LandSat TM scene path 178 row 076 band 7, dated 4 May 2001; Auas-Oanob
Conservancy boundaries: “Environmental Information Service Namibia” (2014); DEM for altitude map: Jarvis et al.
(2008); rainfall isohyets and rivers: (NARIS 2001); geological information: Geological Survey (1980).
Namibian Journal of Environment 2017 Vol 1. Section A: 19-42
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Climate
The climate of the Auas-Oanob Conservancy can best be described as a subtropical steppe
climate (BS), following Köppen (1936). The mean annual rainfall ranges from below 300 mm
in the far south-west to about 350 mm in the north-east (Figure 1a). No information is available
on the orographic effect of the high mountain ranges on the precipitation, but it is expected to
be considerable. Temperatures can rise to about 36° C in summer, whilst frost can be
expected during the winter months from May till as late as October (Figure 2).
Figure 2: Climate diagram following the convention of Walter et al. (1975) and wind rose of Claratal within the
Auas-Oanob Conservancy. Data source: SASSCAL (2014).
METHODS
Field survey
Surveying followed the general method employed for the Vegetation Survey of Namibia project
(Strohbach 2001, 2014). Sampling of 229 plots throughout the study area took place during
April 2000 and again in April 2002. At each survey plot of 20 x 50 m, a Braun-Blanquet type
relevé was compiled. All vascular plant species occurring were noted down, as well as their
typical growth forms and estimated crown cover. Habitat descriptors included: the position, by
way of a GPS-reading (referenced to Schwarzeck), the landscape, local topography, slope
and aspect, lithology, degradation indicators, as well as a photograph. Unknown species and
reference specimens were collected, identified and deposited at the National Herbarium of
Namibia (WIND). The relevé data were captured on TurboVeg (Hennekens & Schaminée
2001). The data form part of GIVD AF-NA-001 (Dengler et al. 2011, Strohbach & Kangombe
2012). Two further sets of data were available for the northern part of the conservancy: 19
relevés compiled by Kellner (1986) on the farm Claratal (also part of GIVD AF-NA-001), as
well as 19 relevés from the Claratal BIOTA observatory (Jürgens et al. 2010, forming part of
AF-00-003 - Muche et al. 2012), collected in 2005 by the author of this paper. As the high
Lichtenstein mountains form a southern extension of the Auas mountain range, 34 relevés
available as part of GIVD AF-NA-001 from Burke and Wittneben (2007), collected in 2004 in
the Auas mountains, were also included in the extended data set.
Essential differences in data collecting were as follows: Kellner (1986) collected his relevés
on a 625 m2 plot (25 x 25 m). His original field notes were not available: the data were captured
Namibian Journal of Environment 2017 Vol 1. Section A: 19-42
23
from phytosociological tables. No GPSs were available at the time of collecting, meaning that
the exact location of his sample plots is unknown. Burke and Wittneben (2007) collected their
relevés on undefined plots, essentially a short transect, along an altitudinal gradient (Burke
2017, pers. com.). This was necessary due to the extremely steep and rough terrain. GPS
positions for the start of each transect are known, and the habitat descriptions followed largely
the standards of the Vegetation Survey. Missing habitat data (e.g. geology) could be extracted
from GPS sources based on the GPS position. The Claratal BIOTA observatory relevés were
collected on 20 x 50 m plots, following the standards of the Vegetation Survey of Namibia
project. The only difference is that their position was predetermined, and the plots strictly
oriented in an east-west direction. This results in an occasional mixture of habitats. In this way
one of the original 20 relevés from the observatory had to be excluded. To avoid observer
bias, data were cleaned before processing following methods described by Strohbach (2014).
The final, extended data set consists of 301 relevés, with 514 species.
Data analysis
Modified TWINSPAN (Roleček et al. 2009), using average Sørensen dissimilarity as diversity
measure, was used for classification. For the first classification, pseudospecies cut levels were
set at 0 and 10% since many species occurred widely spread, but abundance was low outside
their typical niche habitats. Crispness values (Botta-Dukát et al. 2005) for this first
classification indicated a highly reliable subdivision into three clusters. Due to highly varying
internal diversity within each cluster, it was necessary to split the data set into three subsets,
and classify each subset separately. These three subsets represent (a) the high-altitude
mountain veld of the Auas range, (b) the lowlands and valley habitats and (c) the upland
Khomas Hochland veld.
The three subsets were further classified as follows: For the first cluster (high mountain veld
of the Auas range), best results were achieved without the use of pseudospecies.
Classification procedures were stopped with four subdivisions based on crispness values
(Botta-Dukát et al. 2005). Pseudospecies (set at 0 and 10%) were utilised in the classification
of the subsequent two data sets. Classification of the second subset was stopped, again based
on crispness values, after five subdivisions. However, especially the riparian vegetation
proved to be ill-defined. For this reason, Cocktail (Bruelheide & Flintrop 1994) was utilised to
select relevés with a combination of Ziziphus mucronata, Searsia lancea, Cynodon dactylon
and a high abundance of Acacia karroo, based on a description by Kellner (1986).
Classification of the third subset was stopped after three subdivisions, as these represented
the best ecologically interpretable results. However, rocky outcrops, highlighted by both Volk
and Leippert (1972) and Kellner (1986) as distinctly different from the vegetation of the rolling
hills of the Khomas Hochland, were not defined at all. Cocktail (Bruelheide & Flintrop 1994)
was thus employed to select 11 relevés from cluster 2 to define a new group based on the
occurrence of Manuleopsis dinteri, Triraphis rammosissioma, Pennisetum foermeranum and
Ozoroa crassinervia. Furthermore, during field work it was observed that the vegetation of the
far south-western uplands subtly changed to a more xeric form, with Panicum arbusculum
replacing Brachiaria nigropedata. This trend was subsequently confirmed with further field
work to the west and south of the present study area, but did not reflect in the present
classification of the third data subset (uplands of the Khomas Hochland). Therefore, Cocktail
(Bruelheide & Flintrop 1994) was again employed to define a new group based on the
presence of Aizoon schellenbergii, Aptosimum albomarginatum, Panicum arbusculum and
Hibiscus discophorus. Seven relevés were selected this way from group four and made into
an additional group five.
Diagnostic species were determined using the phi co-efficient of association (Chytrý et al.
2002). For this calculation the numbers of relevés were standardised following Tichý and
Chytrý (2006). Species with phi ≥ 40 were considered as diagnostic and with phi ≥ 60 as highly
Namibian Journal of Environment 2017 Vol 1. Section A: 19-42
24
diagnostic; however, species with a non-significant fidelity at α = 0.05 using Fisher’s exact test
were omitted. Species occurring with at least a 60% frequency were regarded as constant and
with at least an 80% frequency as highly constant.
The average structure for each grouping (i.e. average tree, shrub, dwarf shrub, perennial
grass, annual grass and herb cover) was calculated using the available growth form data. The
Shannon Index (as an index of evenness) (H' = -Σ pi ln pi) and Simpson's Index (as an index
of dominance) (D = Σ(n/N)2 (Peet 1974) were calculated for each relevé using Juice (Tichý et
al. 2011). For the species density (number of species per 1,000 m2), the relevé data from
Kellner (1986) were excluded, as these were sampled on 625 m2 plots (25 x 25 m), not 1,000
m2 plots as all other relevés.
A Nonmetric Multidimensional Scaling ordination (NMS) (Kruskal 1964) was calculated with
PC-ORD version 6.08 (McCune et al. 2002). The data set was reduced by removing the data
from Kellner (1986), as these had the most incomplete habitat data, and no lat/long localities
to derive data from GIS sources (e.g. altitude, annual precipitation, slope, etc.) The resulting
database had 282 relevés with 14 environmental factors, in addition to the classification
results. Average Sørensen dissimilarity was used as distance measure, and the ordination
was calculated in three dimensions (i.e. three resulting axes), based on an initial scree plot of
stress versus dimensions (McCune et al. 2002, Peck 2010). The solutions were calculated
with 250 iterations using real data. To aid the interpretation of the resulting scatter plots of the
ordination results, the environmental parameters were overlain in a joint plot.
Mapping
Mapping was performed using a Landsat 7 ETM satellite scene path 178 row 076, dated 4
May 2001. The satellite image was clipped to an area slightly larger than the study area before
further processing. This clip was imported into the Definiens software package (Definiens
2006) and segmented into relatively homogenous areas. The segments were classified using
the classified sample sites as ground truth data. In a final step, the resulting shape file was
clipped according to the farm boundaries and areas for each landscape calculated using the
QGIS 2.4.0 Chugiak software package (QGIS 2014).
RESULTS
Classification results
The classification results are depicted in Figure 3 as a dendrogram.
Observer bias could not be detected, with relevés from various data sources being classified
in eight of 13 associations (Table 1). No outlying groups (which is a tell-tale indication of
observer bias) could be detected (Figure 6a) (Edwards 2000, Strohbach 2014). It needs to be
remembered that Burke and Wittneben (2007) focused specifically on the High Mountain Veld,
and their relevés thus dominate the classification results of those associations. Conversely,
two of their relevés classified into the Khomas Hochland Veld, representing the foothills to the
High Mountain Veld. Also, Kellner's focus (1986) was specifically on the Khomas Hochland
part of Claratal, and his relevés were accordingly classified into the associations of the
Khomas Hochland Veld.
The full phytosociological table is presented as downloadable online Appendix 1, the synoptic
table as downloadable online Appendix 2. The classification yielded 13 associations in three
higher-order syntaxa, one with two subassociations. The associations are not described
formally according to the International Code for Phytosociological Nomenclature (ICPN)
(Weber et al. 2000), pending further descriptions of the vegetation of the greater Khomas
Hochland as well as a review of the syntaxonomy of these. Within the description, highly
Namibian Journal of Environment 2017 Vol 1. Section A: 19-42
25
diagnostic species (with phi coefficient >60) and highly constant species (occurring in more
than 80% or relevés) are indicated in bold.
Table 1: Overview of classification results, indicating the source of relevé data on which the description of
associations is based.
Veld type
Association
(no of relevés)
Data source
Strohbach
Kellner (1986)
Burke &
Wittneben
(2007)
BIOTA
(Jürgens et al.
2010)
1. High Mountain
Veld
1.1 (8)
8
1.2 (10)
10
1.3 (19)
5
14
2. Lowlands and
valley habitats
2.1 (12)
11
1
2.2 (6)
6
2.3 (9)
9
2.4 (8)
7
1
2.5 (16)
16
3. Khomas
Hochland Veld
3.1 (12)
12
3.2 (13)
10
3
3.3 (99)
65
16
2
16
3.4 (80)
79
1
3.5 (9)
9
1 Digitaria eriantha - Osyris lanceolata high mountain veld
Provisionally recognised at the level of an alliance, the high mountain veld (above 2,000 m
asl) represents a unique cluster of associations related to high altitudes. Three associations,
one with two subassociations related to aspect, were recognised.
1.1 Eriocephalus dinteri - Danthoniopsis ramosa high altitude mountain veld of the
south-facing slopes
A total of eight relevés has been classified into this association. It is characterised by the
diagnostic species Eriocephalus dinteri, Jamesbrittenia pallida, Cheilanthes hirta,
Selago alopecuroides, Frankenia pomonensis, Leucas glabrata, Senecio inaequidens,
Namacodon schinzianum, Eragrostis rigidior and Tagetes minuta. These are constantly
associated with Hypoestes forskaolii, Digitaria eriantha, Danthoniopsis ramosa, Oxalis
purpurascens, Tarchonanthus camphoratus, Eragrostis scopelophila and Eragrostis
nindensis. A total of 47 species has been observed in this association, with, on average, 27
species per 1,000 m2.
Namibian Journal of Environment 2017 Vol 1. Section A: 19-42
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Figure 3: Dendrograms depicting the classification results. (a) subset 1, representing the high Auas mountain
vegetation; (b) subset 2, representing the vegetation of the lowlands and valleys, and (c) subset 3, representing
vegetation of the uplands of the Khomas Hochland.
This association occurs on the upper southern slopes of the Auas mountain range, from about
2,150 m upwards. The southern slopes are extremely steep (> 50°), often with steep cliff faces.
The relatively high shrub cover, mostly confined to rock crevices, and the virtual lack of annual
grass species are conspicuous (Figure 4a).
1.2 Danthoniopsis ramosa - Olea europaea high altitude mountain veld of the north-
facing slopes
Namibian Journal of Environment 2017 Vol 1. Section A: 19-42
27
Ten relevés have been classified into this association. It is characterised by the diagnostic
species Selago angustibractea, Lopholaena cneorifolia, Cheilanthes multifida,
Adromischus species, Jamesbrittenia lyperioides, Eriocephalus scariosus, Babiana
hypogea, Anthospermum species, Cineraria canescens, Calostephane marlothiana,
Hypoxis iridifolia, Felicia muricata, Wahlenbergia denticulata, Tristachya superba, Silene
burchellii var. burchellii, Helichrysum obtusum, Gladiolus saccatus, Crassula capitella subsp.
nodulosa, Monsonia burkeana, Euphorbia spartaria, Thesium lacinulatum, Brachiaria serrata,
Ebracteola montis-moltkei and Cotyledon orbiculata. These are constantly associated with
Tarchonanthus camphoratus, Acacia hereroensis, Hypoestes forskaolii, Digitaria eriantha,
Danthoniopsis ramosa and Chascanum pinnatifidum. A total of 88 species has been observed
in this association, with an average of 48 species per 1,000 m2.
This association occurs on the upper northern slopes of the Auas mountain range, again from
2,200 m upwards. The slopes are very steep, flattening off near the mountain peak (but never
forming a 'flat' plateau). The soil surface is rock-strewn, with a 50 – 80% cover of blocky rock
boulders. With increasing altitude, the shrub layer (up to 3 m high) recedes and the vegetation
becomes dominated by grasses (Burke & Wittneben 2007) (Figures 4b and 5a).
1.3 Acacia hereroensis - Tarchonanthus camphoratus mid-altitude mountain veld
A total of 19 relevés has been classified into this association. It is characterised by the
diagnostic species Anthephora pubescens, Kyphocarpa angustifolia, Heteropogon
contortus, Combretum apiculatum subsp. apiculatum subsp. apiculatum, Ziziphus
mucronata, Andropogon chinensis, Brachiaria nigropedata, Schmidtia pappophoroides,
Microchloa caffra, Phyllanthus pentandrus, Hibiscus sulfuranthus, Dombeya rotundifolia,
Talinum caffrum, Cenchrus ciliaris, Geigeria ornativa, Hibiscus pusillus, Fingerhuthia africana,
Dyschoriste pseuderecta, Oxygonum species and Eragrostis echinochloidea. These are
constantly associated with Tarchonanthus camphoratus, Acacia hereroensis, Pellaea
calomelanos, Hypoestes forskaolii, Eragrostis scopelophila, Eragrostis nindensis,
Searsia marlothii, Digitaria eriantha and Commelina africana. A total of 125 species has been
observed in this association, with, on average, 47 species per 1,000 m2.
The Acacia hereroensis - Tarchonanthus camphoratus association occurs on the mid-slopes
of the Auas mountain range, at a mean altitude from 2,000 - 2,250 m asl. The slopes are steep
to very steep (up to 50°), but less rock-strewn than at higher altitudes (rock and large stone
cover each up to 40%). Gravel cover on these slopes becomes more prominent, also up to
40%. The association can be subdivided into two subassociations based on aspect.
1.3.1 Acacia hereroensis - Tarchonanthus camphoratus - elephanthorrhiza suffruticosa
mountain veld of the northern slopes
A total of seven relevés has been classified into this subassociation. It is characterised by the
diagnostic species Elephantorrhiza suffruticosa, Vigna frutescens, Eragrostis porosa,
Elephantorrhiza elephantina, Trochomeria macrocarpa, Solanum delagoense, Albuca
species, Cyperus margaritaceus, Tephrosia rhodesica, Sida chrysantha, Evolvulus
alsinoides, Ipomoea holubii, Lantana dinteri, Montinia caryophyllacea, Lycium eenii,
Dicoma macrocephala, Stipagrostis uniplumis var. uniplumis, Dipcadi glaucum and Ipomoea
obscura var. obscura. These are constantly associated with Tarchonanthus camphoratus,
Searsia marlothii, Pellaea calomelanos, Melinis repens subsp. repens, Hypoestes
forskaolii, Heteropogon contortus, Eragrostis scopelophila, Eragrostis nindensis,
Commelina africana, Brachiaria nigropedata, Acacia hereroensis, Digitaria eriantha and
Chascanum pinnatifidum. A total of 105 species has been observed in this subassociation,
with, on average, 58 species per 1,000 m2.
Namibian Journal of Environment 2017 Vol 1. Section A: 19-42
28
This subassociation occurs on the northern slopes of the Auas Mountain range. Shrub cover
is relatively low, forming an open shrubland between 2 - 3 m high. The perennial grass cover
is variable, but can reach more than 50% (Burke & Wittneben 2007) (Figure 4c).
1.3.2 Acacia hereroensis - Tarchonanthus camphoratus - Monelytrum luederitzianum
mountain veld of the southern slopes
A total of 12 relevés has been classified into this subassociation. It is characterised by the
diagnostic species Monelytrum luederitzianum, Thesium xerophyticum, Bidens
biternata, Raphionacme velutina, Phyllanthus species, Melinis repens subsp. grandiflora,
Eragrostis lehmanniana, Solanum capense, Pogonarthria squarrosa, Hermannia affinis,
Drimia sanguinea and Aristida effusa. These are constantly associated with Tarchonanthus
camphoratus, Acacia hereroensis, Pellaea calomelanos, Melinis repens subsp. repens,
Hypoestes forskaolii, Eragrostis scopelophila, Eragrostis nindensis, Digitaria eriantha,
Searsia marlothii, Solanum lichtensteinii and Anthephora pubescens. A total of 104 species
has been observed in this subassociation, with, on average, 40 species per 1,000 m2.
This subassociation occurs on the southern and western mid-slopes of the Auas Mountain
range including the Lichtenstein mountains. Shrub cover is denser than on the upper reaches,
forming a moderately closed, tall shrubland. Grass cover is less variable, from 50 to 60%
cover, but occasionally even reaching 70% cover (Figures 4d and 5b).
2. Lowlands and valley habitats
The vegetation of the lowlands, particularly the riverine vegetation, alluvial plains of the Omeya
valley as well as the foot slopes of the Khomas Hochland form a mixture of vegetation types,
which cannot all logically be associated with a clear, single syntaxon. Five associations have
been identified here as follows:
2.1 Acacia karroo - Cynodon dactylon riparian vegetation
Twelve relevés have been classified into this association. It is characterised by the diagnostic
species Gomphocarpus fruticosus, Cynodon dactylon, Ziziphus mucronata, Achyranthes
aspera var. sicula, Searsia lancea, Felicia muricata and Acacia karroo. These are constantly
associated with Bidens biternata, Tagetes minuta, Chloris virgata, Cenchrus ciliaris and
Schkuhria pinnata. A total of 127 species has been observed in this association, with an
average of 39 species per 1,000 m2.
This association typically occurs as a moderately closed bushland to sub-continuous thicket
sensu Edwards (1983), dominated by Acacia karroo, Ziziphus mucronata and Searsia lancea
trees between 8 to 15 m high along the banks of ephemeral rivers both within the steeper
Khomas Hochland landscapes as well as the flatter plains landscapes (Figure 4e and 5c). The
herbaceous layer is often dominated by weedy (often annual) and/or shade-loving species. As
this association occurs along riverbeds, seeds and other propagules from other, nearby
associations are often washed in, resulting in a highly diverse species composition. In this
way, even typical sand bank species like Stipagrostis namaquensis and Schmidtia
kalahariensis (which are not known shade species) occur here occasionally.
2.2 Stipagrostis namaquensis sand banks
Six relevés have been classified into this association. It is characterised by the diagnostic
species Stipagrostis namaquensis, Felicia clavipilosa, Indigofera alternans,
Hermbstaedtia odorata and Melianthus comosus. These are constantly associated with
Acacia karroo, Pogonarthria fleckii, Bulbostylis hispidula, Bidens biternata and Acrotome
species. A total of 38 species has been observed in this association, with, on average, 21
species per 1,000 m2.
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This association occurs within the river bed on sand banks that are stable enough to support
a fairly dense grass layer, but frequently threatened by flash floods, so that only a few trees
become established. Acacia karroo is thus a typical pioneer species in this community,
occurring mostly only as juvenile plants or small shrubs. The structure is typically a tall,
moderately-closed grassland sensu Edwards (1983) (Figure 4f and 5d).
2.3 Platycarphella carlinoides - Chloris virgata floodplains and omiramba
A total of nine relevés was classified into this association. It is characterised by the diagnostic
species Hypertelis salsoloides, Tragus berteronianus, Talinum caffrum, Oxalis
depressa, Tragus racemosus, Monsonia angustifolia, Eragrostis pilgeriana, Aptosimum
glandulosum, Platycarphella carlinoides, Talinum arnotii, Digitaria eriantha, Pentzia incana,
and Eragrostis nindensis. These are constantly associated with Kyllinga alata, Ursinia nana,
Pogonarthria fleckii, Eragrostis echinochloidea, Chloris virgata, Mollugo cerviana, Melinis
repens subsp. grandiflora, Geigeria pectidea, Eragrostis porosa, Commelina benghalensis
and Acacia karroo. A total of 109 species has been observed in this association, with, on
average, 43 species per 1,000 m2.
This association is found in the shallow drainage systems (omiramba) within the Oanob
plateau. These end in rivers and tributary streams of the Oanob River. Typically, they are
dominated by a dense, tall grass sward consisting of a combination of annual and perennial
species (Figure 4g and 5e). Occasional tree clumps (Acacia karroo and/or Acacia erioloba)
indicate the close relationship to the riverine system, in particular the Cynodon dactylon -
Acacia karroo riverine woodlands. The soils are fine-grained with virtually no stone cover.
2.4 Pupalia lappacea - Acacia mellifera bush-encroached lowlands
A total of eight relevés has been classified into this association. It is characterised by the
diagnostic species Boscia albitrunca, Gisekia africana, Phyllanthus pentandrus, Pupalia
lappacea, Senecio consanguineus, Citrullus lanatus, Acacia mellifera subsp. detinens,
Otoptera burchellii, Melolobium macrocalyx, Catophractes alexandri, Albizia anthelmintica and
Phaeoptilum spinosum. These are constantly associated with Pogonarthria fleckii, Mollugo
cerviana, Lycium bosciifolium, Schmidtia kalahariensis, Oxygonum species, Ocimum
americanum var. americanum, Nidorella resedifolia, Lycium eenii, Eragrostis porosa,
Commelina benghalensis, Acrotome species and Acacia erioloba. A total of 109 species has
been observed in this association, with, on average, 41 species per 1,000 m2.
This association occurs in various habitats, often on footslopes of the mountainous highland
of the Khomas Hochland, in patches on the Oanob plateau as well as in the undulating
landscape of the southern Khomas Hochland. Several large expanses of this vegetation type
are visible along the main road (B1) between Windhoek and Rehoboth near the Oamites
Mountain, as well as along the C26. These tall, moderately closed shrublands are dominated
by Acacia mellifera subsp. detinens and Catophractes alexandri, with Leucosphaera bainesii
conspicuous in the understorey. The grass layer is generally very weakly developed, with a
variety of herbaceous, often weedy, species (Figure 4h and 5f).
2.5 Schmidtia kalahariensis - Acacia erioloba woodlands of the Omeya valley
A total of 16 relevés has been classified into this association. It is characterised by the
diagnostic species Hypertelis bowkeriana, Nidorella resedifolia, Helichrysum
candolleanum, Ifloga glomerata, Geigeria ornativa, Eragrostis cylindriflora, Acacia erioloba,
Geigeria pectidea, Kyphocarpa angustifolia and Aptosimum albomarginatum. These are
constantly associated with Pogonarthria fleckii, Mollugo cerviana, Lycium bosciifolium,
Kyllinga species, Ursinia nana, Eragrostis lehmanniana, Commelina benghalensis, Chloris
virgata and Melinis repens subsp. grandiflora. A total of 111 species has been observed in
this association, with, on average, 40 species per 1,000 m2.
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These tall, semi-open woodlands are dominated by Acacia erioloba (Figure 4i and 5g). The
grass layer is dominated by the annual Schmidtia kalahariensis in association with Aristida
congesta subsp. congesta and Anthephora schinzii. The poisonous species Geigeria pectidea
and Elephantorrhiza elephantina are conspicious in the herbaceous layer. The association
has a park-like appearance (resulting in the development of a golf-estate within this
association), but because of the highly unpalatable, annual grass sward and high abundance
of poisonous species (Geigeria pectidea and Elephantorrhiza elephantina) it has low potential
for livestock farming.
3. Acacia hereroensis veld of the Khomas Hochland
Acacia hereroensis is the characteristic species of the Highland savannah sensu Giess (1998).
This third group of associations all contain this characteristic species. The other unifying
feature is the undulating to steep hilly and mountainous landscape, generally with shallow,
stony soils.
3.1 Ornithoglossum calcicola - Fingerhuthia africana mountain veld of the Oamites
mountain
A total of 12 relevés has been classified into this association. It is characterised by the
diagnostic species Ornithoglossum calcicola, Melhania damarana, Crotalaria kurtii,
Thesium xerophyticum, Monechma genistifolium subsp. genistifolium, Enneapogon
scoparius, Cleome suffruticosa, Stipagrostis hirtigluma, Zygophyllum pubescens,
Sarcostemma viminale, Euphorbia lignosa, Polygala pallida, Pelargonium otaviense,
Stipagrostis ciliata, Peliostomum leucorrhizum and Euclea undulata. These are constantly
associated with Searsia marlothii, Fingerhuthia africana, Eragrostis nindensis, Acacia
hereroensis, Enneapogon cenchroides, Talinum caffrum, Otoptera burchellii, Cenchrus
ciliaris and Acacia mellifera subsp. detinens. A total of 101 species has been observed in this
association, with, on average, 37 species per 1,000 m2.
This tall, semi-open shrubland is typically dominated by Acacia mellifera subsp. detinens,
Combretum apiculatum subsp. apiculatum and Acacia hereroensis in the shrub layer, and
Fingerhuthia africana, Stipagrostis hirtigluma, Monelytrum luederitzianum and Enneapogon
desvauxii in the grass layer (Figures 4j and 5h). The association occupies very steep slopes
(> 60%) with stony soils. Up to 40% of the soil surface is covered with rocks, with smaller
fragments also covering up to 40% in total.
3.2 Triraphis ramosissima - Manuleopsis dinteri veld of the rocky outcrops
A total of 13 relevés has been classified into this association. It is characterised by the
diagnostic species Pennisetum foermeranum, Cheilanthes marlothii, Bidens biternata,
Manuleopsis dinteri, Pellaea calomelanos, Combretum apiculatum subsp. apiculatum subsp.
apiculatum, Ozoroa crassinervia, Triraphis ramosissima, Eragrostis scopelophila, Kalanchoe
lanceolata and Phyllanthus pentandrus. These are constantly associated with Searsia
marlothii, Cenchrus ciliaris, Melinis repens subsp. grandiflora, Anthephora pubescens,
Acacia hereroensis, Eragrostis nindensis, Stipagrostis uniplumis var. uniplumis,
Enneapogon cenchroides, Tarchonanthus camphoratus, Talinum caffrum and Digitaria
eriantha. A total of 110 species has been observed in this association, with, on average, 45
species per 1,000 m2.
This association is found on the rocky outcrops scattered throughout the Khomas Hochland.
Although by definition a short, moderately closed bushland, the small sizes of these outcrops
(on average less than 0.25 ha) and patchy distribution in the landscape do not warrant this
description (Figures 4k and 5i). These rocky outcrops, due to the layered nature of the schists,
are well-fissured and provide habitats for smaller, shade-loving plants like ferns (Pellaea
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calomelanos, Cheilanthes species). Although not exclusively, most of these rocky outcrops
are south facing.
3.3 Brachiaria nigropedata - Acacia hereroensis veld of the central Khomas Hochland
A total of 99 relevés has been classified into this association. It is characterised by Ziziphus
mucronata and Brachiaria nigropedata, which are constantly associated with Searsia
marlothii, Anthephora pubescens, Acacia hereroensis, Eragrostis nindensis, Melinis
repens subsp. grandiflora, Stipagrostis uniplumis var. uniplumis, Monelytrum luederitzianum,
Aristida meridionalis, Schmidtia pappophoroides, Kyphocarpa angustifolia and Cenchrus
ciliaris. A total of 152 species has been observed in this association, with, on average, 39
species per 1,000 m2.
These semi-open to moderately closed tall shrublands, or occasionally, low bushlands are
dominated by Acacia hereroensis in the tree and shrub layer, and Eragrostis nindensis,
Aristida adscensionis, Brachiaria nigropedata, Monelytrum luederitzianum, Aristida
meridionalis and Stipagrostis uniplumis in the grass layer (Figure 4l). The altitude ranges from
1,760 to 2,100 m asl, whilst slopes are moderately steep to steep (Figure 5j). No affinity to a
particular aspect could be detected. Stone and rock cover is variable, but a high cover of
quartz pebbles (40 - 80%) (referred to as pebble mulch) is conspicuous.
3.4 Panicum lanipes - Pentzia incana veld of the Oanob Plateau
A total of 80 relevés has been classified into this association. It is characterised by Panicum
lanipes, Plinthus sericeus, Pentzia incana, Tragus berteronianus, Blepharis integrifolia,
Barleria rigida and Ipomoea bolusiana. These are constantly associated with Acacia mellifera
subsp. detinens, Eragrostis nindensis, Kyphocarpa angustifolia, Aristida adscensionis,
Microchloa caffra, Talinum caffrum, Schmidtia pappophoroides, Melinis repens subsp.
grandiflora, Hirpicium gazanioides, Hermannia modesta, Eriocephalus luederitzianus,
Chascanum pinnatifidum, Leucosphaera bainesii, Pogonarthria fleckii, Phaeoptilum
spinosum, Stipagrostis uniplumis var. uniplumis, Monelytrum luederitzianum, Searsia marlothii
and Fingerhuthia africana. A total of 206 species has been observed in this association, with,
on average, 53 species per 1,000 m2.
The Oanob plateau has a very distinct karroid structure (Figure 4 m), with the dwarf shrub
species Leucosphaera bainesii, Eriocephalus luederitzianus and Pentzia incana dominating.
Larger shrubs are generally fairly sparse, with a tendency for species from the adjacent
Brachiaria nigropedata - Acacia hereroensis association to encroach. In patches, Acacia
mellifera subsp. detinens forms dense bushlands within this landscape. The altitude of the
Oanob plateau ranges from 1,680 - 2,040 m asl., with an undulating to rolling relief (< 10°
slopes) (Figure 5k). The lithology is similar to the adjacent association, dominated by schists
of the Kuiseb, Auas and Chuos Formations. Rock outcrops are generally absent, with relatively
few large stones. The pebble mulch typical of the Khomas Hochland is, however, present (40
- 80% cover). The plateau is interspersed with numerous omiramba, as described for
association 2.3 (Figure 8).
3.5 Panicum arbusculum - Acacia mellifera veld of the southern Khomas Hochland
A total of nine relevés has been classified into this association. It is characterised by the
diagnostic species Kohautia cynanchica, Aizoon schellenbergii, Aptosimum
albomarginatum, Panicum arbusculum, Justicia guerkeana, Catophractes alexandri,
Ipomoea obscura var. obscura, Heliotropium ciliatum, Aptosimum lineare, Pentzia
monocephala, Hibiscus discophorus, Phyllanthus maderaspatensis, Phaeoptilum spinosum,
Ptycholobium biflorum subsp. angolensis, Ocimum americanum var. americanum, Aptosimum
spinescens, Eriospermum flagelliforme, Asparagus exuvialis, Melhania virescens,
Leucosphaera bainesii, Eriocephalus luederitzianus and Pogonarthria fleckii. These are
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constantly associated with Stipagrostis uniplumis var. uniplumis, Kyphocarpa
angustifolia, Fingerhuthia africana, Eragrostis nindensis, Acacia mellifera subsp.
detinens, Pogonarthria fleckii, Microchloa caffra, Melinis repens subsp. grandiflora,
and Hermannia modesta. A total of 118 species has been observed in this association, with,
on average, 60 species per 1,000 m2.
These moderately closed, high shrublands are dominated by Acacia mellifera subsp. detinens,
Catophractes alexandri and Phaeoptilum spinosum, with the occasional Rhigozum
trichotomum. The absence of Brachiaria nigropedata and the presence of Panicum
arbusculum, a grass species known as a climax grass in the rocky slopes of the Nama-Karoo
in southern Namibia, is conspicious (Figure 4n and 5l). The landscape is generally rolling (<
10°) and not as steep as the central Khomas Hochland. Stone cover, with the exception of
pebble mulch, is also lower than in similar landscapes within this cluster of vegetation
associations. The altitude ranges from 1,670 - 1,810 m asl.
a
b
c
d
e
f
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33
Figure 4: Box-and-whisker plots of the typical structure of the various associations and subassociations. a)
Eriocephalus dinteri - Danthoniopsis ramosa association; b) Danthoniopsis ramosa - Olea europaea association;
c) Acacia hereroensis - Tarchonanthus camphoratus - elephanthorrhiza suffruticosa subassociation; d) Acacia
hereroensis - Tarchonanthus camphoratus - monelytrum luederitzianum subassociation; e) Acacia karroo -
Cynodon dactylon association; f) Stipagrostis namaquensis association; g) Platycarphella carlinoides - Chloris
virgata association; h) Pupalia lappacea - Acacia mellifera association; i) Schmidtia kalahariensis - Acacia erioloba
association; j) Ornithoglossum calcicola - Fingerhuthia africana association; k) Triraphis ramosissima -
Manuleopsis dinteri association; l) Brachiaria nigropedata - Acacia hereroensis association; m) Panicum lanipes -
Pentzia incana association; n) Panicum arbusculum - Acacia mellifera association.
g
h
i
j
k
l
m
n
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a
b
c
d
e
f
g
h
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Figure 5: Typical views of the various associations and subassociations. a) Danthoniopsis ramose - Olea europaea
association; b) Acacia hereroensis - Tarchonanthus camphoratus - Monelytrum luederitzianum subassociation; c)
Acacia karroo - Cynodon dactylon association; d) Stipagrostis namaquensis association; e) Platycarphella
carlinoides - Chloris virgata association; f) Pupalia lappacea - Acacia mellifera association; g) Schmidtia
kalahariensis - Acacia erioloba association; h) Ornithoglossum calcicole - Fingerhuthia africana association; i)
Triraphis ramosissima - Manuleopsis dinteri association; j) Brachiaria nigropedata - Acacia hereroensis association;
k) Panicum lanipes - Pentzia incana association; l) Panicum arbusculum - Acacia mellifera association. Source:
Photo 5a: Dr A. Burke, all others by the author.
Environmental Gradients
The NMS produced an ordination in three dimensions, with the final stress for the best solution
being 17.248, and an instability of 0.000, after 76 itterations. The high mountain veld is
distinctly separated from the remaining clusters along Axis 1, whilst the lowlands and valley
habitats are clearly separated from the Khomas Hochland veld along Axis 3 (Figure 6a). The
overlap between association 2.3 (Pupalia lappacea - Acacia mellifera bush-encroached
lowlands) and the Khomas Hochland veld indicates the close relationship between these two
groupings, with association 2.3 most likely being an encroached / degraded form of the
Khomas Hochland veld.
Axis 1 presents an altitudinal and precipitation gradient (Figure 6b and c). Altitude is correlated
to Axis 1 with r = 0.802, whilst precipitation is correlated to this axis with r = 0.632. Axis 2 and
3 represent gradients related to primarily geology, which also manifest themselves in slope
and stoniness gradients (Figure 6b and 6c). In Axis 2 a clear split between relevés compiled
on the Kuiseb - and the Chuos Formations is evident (Figure 6b), whilst relevés compiled on
the Auas Formation form a link between these two groupings. Axis 3 depicts a gradient
between metamorphic rock types (mostly schists and quarzites, but also marbles) and more
recent fluvial sedimentary deposits (Figure 6c). This could also be interpreted as a gradient
from more skeletal soils with a high stone or rock content to deeper soils with a high gravel
content, typical of alluvial deposits.
i
k
l
j
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NMS all releves sorensen_reduced
Axis 1
Axis 3
Cluster
1.1
1.2
1.3
2.1
2.2
2.3
2.4
2.5
3.1
3.2
3.3
3.4
3.5
altitude
gravel
rock
slope
precipitation
NMS all releves sorensen_reduced
Axis 1
Axis 2
Stratigraphy
Recent alluvial deposits
Gamsberg Suite
Kuiseb Formation
Auas Formation
Chuos Formation
Hohewarte Complex
Kamtsas Formation
Kudis Subgroup
Billstein Formation
Duruchaus Formation
a
b
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Figure 6: Scatter plots of the NMS ordination: (a) with the classification results indicated as convex hulls; (b)
indicating the geological stratification as well as environmental factors as biplots; and (c) indicating the lithology of
the substrate as well as environmental factors as biplots.
Biodiversity indicators
The species richness per 1,000 m2, Shannon Index and Simpson's Index are depicted in
Figure 7.
altitude
small stones
medium stones
rock
slope
precipitation
NMS all releves sorensen_reduced
Axis 1
Axis 3
Lithology
Quartz gravel on schists
Quarzite
Schist
Metamorphic limestone
Conglomerate
Fluvial
Eolian
c
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Figure 7: Diversity indicators for the various associations described. a) Species richness; b) Shannon index and
c) Simpson's Index.
Vegetation map
It was not possible to map all individual vegetation associations separately, especially as a
number of these occur as small patches, embedded within larger associations (Figure 8). The
roughness of the terrain, coupled with numerous shade areas complicated mapping further.
For this reason, larger landscape units were mapped, incorporating the major (dominating)
vegetation associations, with included smaller associations (Table 2). This follows the SOTER
approach of landscape mapping (FAO 1995). A similar approach has been followed by,
amongst others, van Rooyen et al. (2008) and Hüttich et al. (2009).
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Table 2: Vegetation associations included in the various mapping units depicted in Figure 8, as well as their
measured area.
Mapping
unit
Major vegetation association
Included vegetation
association
Area within
conservancy
(ha)
High Auas
mountains
Eriocephalus dinteri - Danthoniopsis ramosa (1.1);
Danthoniopsis ramosa - Olea europaea (1.2);
Acacia hereroensis - Tarchonanthus camphoratus
(1.3)
10,840.8
Rivers
Cynodon dactylon - Acacia karroo (2.1)
Stipagrostis
namaquensis (2.2)
1,724.3
Omiramba
Platycarphella carlinoides - Chloris virgata (2.3)
4,298.8
Bush-
encroached
lowlands*
Pupalia lappacea - Acacia mellifera (2.4)
Panicum arbusculum -
Acacia mellifera (3.5)
12,831.2
Omeya
Camelthorn
savannah
Schmidtia kalahariensis - Acacia erioloba (2.5)
Pupalia lappacea -
Acacia mellifera (2.4)
7,818.0
Oamites
mountain
Ornithoglossum calcicola - Fingerhuthia africana
(3.1)
261.9
Central
Khomas
Hochland
Brachiaria nigropedata - Acacia hereroensis (3.3)
Triraphis ramosissima -
Manuleopsis dinteri (3.2);
Pupalia lappacea -
Acacia mellifera (2.4)
41,907.9
Oanob
plateau
Panicum lanipes - Pentzia incana (3.4)
Pupalia lappacea -
Acacia mellifera (2.4)
26,745.8
Southern
Khomas
Hochland
Panicum arbusculum - Acacia mellifera (3.5)
Pupalia lappacea -
Acacia mellifera (2.4)
18,038.8
* The full extent of this unit is unclear.
Figure 8: Map of the landscapes associated with dominant vegetation types listed in Table 1 within the Auas-
Oanob Conservancy and Auas Mountain range.
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DISCUSSION
Species Diversity
The low species richness of association 2.2 (Stipagrostis namaquensis sandbank vegetation),
coupled with low Shannon and Simpson's indices, is conspicuous (Figure 7). These are
indicative of the absolute dominance of the vegetation by this hard, reed-like grass (Figure
5d). The often- observed dominance by Acacia karroo in association 2.1 (riparian vegetation),
and Acacia erioloba and Schmidtia kalahariensis for the Omeya valley veld (association 2.5),
is also reflected in these Simpson's index ranges.
More diverse, but equally dominated by a single species, is association 2.4, the Pupalia
lappacea - Acacia mellifera lowlands, according to the Simpson's Index (Figure 7c). The
generally wide ranges in low Simpson index values for many associations are also
conspicuous, specifically the Khomas Hochland mountain veld (associations 3.1 to 3.5). This
is indicative of a tendency of these vegetation types to become bush-encroached (mostly by
Acacia mellifera), in this way showing a high degree of dominance and less diversity. This is
contrary to findings of De Klerk (2004), which indicate that the Khomas Hochland is largely
unaffected by bush encroachment.
Degradation trends
Bush encroachment is the biggest threat to the vegetation (and thus grazing resources) in
central and northern Namibia (Bester 1998, De Klerk 2004). This problem was observed in
the Brachiaria nigropedata - Acacia hereroensis, the Panicum lanipes - Pentzia incana and
the Panicum arbusculum - Acacia mellifera associations. A large number of plots from these
three associations are encroached by dense stands of Acacia mellifera and would fit, from a
structural point of view, into the Pupalia lappacea - Acacia mellifera association. For this
reason, the definition of this association is to be regarded as incomplete, and the mapping of
it questionable.
The omiramba, in their original state, formed extensive seasonal wetlands with extremely high
soil moisture, supporting dense perennial grassland. However, already in the early 1970s,
Volk and Leippert (1971) commented that the vegetation of the omiramba (their Themeda
triandra association) was badly degraded, with the only example of the original vegetation
remaining in an exclosure on the farm Voigtland. Within the study area, the Platycarphella
carlinoides - Chloris virgata association is also dominated by grasses, albeit a mixture of
perennial and annual grasses, with a strong component of pioneer grasses (e.g. Aristida
congesta, Tragus racemosus, Eragrostis pilgeriana, Pogonarthria fleckii, Anthephora schinzii,
Chloris virgata, Melinis repens subsp. grandiflora and Eragrostis porosa) (Müller 2007).
Climax grasses like Eragrostis superba, Digitaria eriantha, Cymbopogon pospischilii and
Themeda triandra only occur occasionally in protected niches. The soils are highly erodible,
forming extensive gullies (Volk & Leippert 1971, Shamathe et al. 2008). In this way, the rivers
are extending steadily uphill into this association. A similar situation has been described for
western Australian, where erosion is leading to extensive landscape desiccation (Pringle &
Tinley 2003). This means that erosion of the omiramba will not only lead to further degradation
of this ecosystem, but will be likely to also have a detrimental effect on the adjacent upstream
landscapes by drying these up.
Species conservation
The Auas Mountain range has been identified as a botanically important area in need of
protection, because it supports rare, endemic plant species as well as relicts of Grassland and
Succulent Karoo vegetation within high altitude habitats (Hofmeyr 2004). Within the three
vegetation associations on this mountain range 217 species occur, of which 15 (or 6.9%) are
endemic to Namibia. The Khomas Hochland vegetation contains 23 species endemic to
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Namibia, which is proportionally less (5.8% of 399 species), but also nine exotic species (or
2.3%). Two of these, Opuntia stricta and Prosopis glandulosa, have been classified as the
most aggressive invaders in Namibia ( Brown et al. 1985, Bethune et al. 2004,). Neither has
reached problematic proportions within the conservancy yet, but the danger of spreading is
real.
In conclusion, the Auas-Oanob Conservancy contains several unique landscapes, which can
be characterised by high species diversity, a high number of endemic species, high-altitude
mountainous habitats, but also arid wetland habitats in the form of the omiramba. Current land
use is no direct threat to the species richness, but, through various forms of land degradation
like erosion, bush encroachment and/or the threat of alien invasive species, will be likely to
cause a change of habitat, which in turn, will threaten the plant species diversity and
consequently also the habitat of wild and domesticated animal species. The presented
description, as well as the original relevé data with associated photographs, will serve as a
valuable resource in monitoring changes in the vegetation within this conservancy.
ACKNOWLEDGEMENTS
Thanks are due to the farmers and members of the Auas-Oanob Conservancy for their friendly support during the
fieldwork. The assistance of the staff of the National Herbarium of Namibia for the identification of plant specimens
is gratefully acknowledged. Data and photos from the Auas Mountain range were generously contributed by Dr
Antje Burke and Mr. Martin Wittneben. This project was funded by the Government of Namibia through the recurrent
budget of the Directorate Agriculture Research and Training, Subdivision National Botanical Research Institute.
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