Conservation implications: This paper describes six plant associations of the Nama-Karoo biome in arid southern Namibia. The information presented forms a baseline description, which can be used for future monitoring of the vegetation under altered land use.

Prof. O.H. Volk was one of only a few vegetation scientists active in Namibia before the 1980s. During 1956, he conducted extensive vegetation surveys on selected farms in the former South West Africa. Unfortunately, only one of these data sets had been analysed and formally published (Volk & Leippert 1971); the others that could be retrieved were all in the form of raw data sets with brief comments on the vegetation, generally held in farm archives. Nonetheless, these data sets have proved to be of immense value in reconstructing vegetation changes over the past five decades of land use, especially with regard to the effects of land use and/or global climate change. One of these data sets, surveyed in April 1956, was from the farm Haribes.

This study was initiated to provide data on the current vegetation, for later comparison with Volk’s historic data. The aim of this paper, however, is to classify and describe the present vegetation of the farm Haribes as an attempt towards filling the immense gap in vegetation data in Namibia (Strohbach & Jürgens 2010). In a subsequent paper, we intend to compare the present vegetation with that documented in 1956.

FIGURE 1: Location of the farm Haribes in the Hardap region of southern Namibia. Also indicated on this map are the long-term average annual rainfall isohyets (in grey), with their values (in mm) indicated to the right.

The geomorphology can be described as a plateau broken up by many dry washes and some bigger ephemeral rivers that are responsible for the drainage of the area [a dissected plain according to the terminology of the Digital Soils and Terrain Database (SOTER) (FAO 1995; Oldeman & Van Engelen 1993]. The Riet River drains into a big pan surrounding the farmhouse complex. In and around the pan several well-developed dune hummocks have formed over time. The farm lies between 1120 m and 1320 m above sea level. The geological strata belong to the Fish River subgroup of the Nama sediments. The eastern part of the farm is situated on the Gross Aub Formation dominated by red shale and sandstone, whilst the larger, western part of the farm forms part of the older Nababis Formation with red sandstone and shales (Geological Survey 1980). The soils in the region (at a scale of 1:1 000 000) are classified as eutric leptosols, which means that the soils are fertile but generally very shallow (ICC, MAWRD & AECI 2000; Mendelsohn et al. 2002).

The farm falls within the Nama-Karoo biome (Irish 1994) and its vegetation is classified as Dwarf shrub savanna (Giess 1971, 1998). This can broadly be described as open, short-to-low shrublands dominated by C. alexandri, R. trichotomum, Boscia foetida and several dwarf shrubs. The grass layer, which includes species such as Stipagrostis uniplumis, Eragrostis nindensis, Enneapogon cenchroides and Schmidtia kalahariensis, is relatively sparse. Trees and larger shrubs, including species such as A. nebrownii, Acacia karroo, Euclea pseudebenus and Ziziphus mucronata, are largely confined to dry washes and rivers.

To compare the present composition with that documented by Volk in 1956, we made exhaustive attempts to relocate his original sampling sites according to route descriptions, odometer distance readings (in miles) and old farm maps. Although this information greatly eased the task of relocation, the exact position of those plots could not be determined. Additional vegetation types were sampled using a stratified sampling approach to achieve a good average for the main vegetation types. For the purpose of stratification, a satellite map of the farm (based on Landsat 7 ETM scene path 177 row 077, 26 March 2001) was used as base map. Relatively homogenous areas were delineated as derived from segmentations calculated by Definiens 5 (Definiens 2006). Occasional azonal vegetation types (especially small vleis and related features) were also sampled.

Surveying followed the Braun-Blanquet concept as applied during the Vegetation Survey of Namibia project (Strohbach 2001). For each 20 m × 50 m plot, all species were recorded, as well as their typical growth form and average height. Abundance was estimated according to the modified Braun-Blanquet scale (Mueller-Dombois & Ellenberg 1974). In addition, the position of the plot and environmental parameters such as altitude, aspect, slope, topography, stone cover, erosion and disturbances were noted. Owing to time constraints, no soil samples were taken. Instead, available soil data (ICC et al. 2000; Mendelsohn et al. 2002) were interpolated for the various vegetation associations described. Unknown plant species were collected and identified at the National Herbarium of Namibia. The relevé data were captured in the TurboVeg database (Hennekens & Schaminée 2001).

The data were classified with JUICE (Tichý 2002), using the Modified TWINSPAN classification procedure (Roleček et al. 2009). The pseudospecies cut level was set to presence/absence, and total inertia was selected as heterogeneity measure. The classification process concluded after 14 divisions. At this stage, two communities (hummock dunes and the ranteveld) were already subdivided owing to high internal heterogeneity and had to be recombined manually (Figure 2). The classification result was ordered as a table using the phi coefficient fidelity measure (Chytrý et al. 2002) as sorting criterion.

FIGURE 2: Dendrogram based on the Modified TWINSPAN classification of the vegetation types in the study area.

Mapping was performed using a Landsat 7 ETM satellite image (path 177, row 077; 26 March 2001). In addition, a digital terrain model derived from Shuttle Radar Topography Mission (SRTM) data (available from http://srtm.csi.cgiar.org/) (Jarvis et al. 2008) was used to elucidate the topography. 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 IDRISI Andes software package (Eastman 2006).

The classification of these relevés revealed eight communities, of which three were divided into subcommunities. The phytosociological table is presented in an online appendix 1, whereas online appendix 2 is a synoptic table. Figure 2 depicts a dendrogram of the classification result. The ordination diagrams of the DCA are shown in Figure 3a (all data except for two outlier relevés) and Figure 3b (also excluding the relevés representing pans and associated hummock dunes). In Figure 3c, the lithology (i.e. the rock types which gave rise to the soils) has been imposed onto the DCA ordination diagram of Figure 3b to illustrate the habitat relationships between vegetation communities.

FIGURE 3: Detrended Correspondence Analysis ordination diagrams compiled from (a) all data, excluding two outlying wetland relevés and (b) data excluding wetland, hummock dune and pan relevés; (c) indicates the lithology of each plot, with a biplot indicating the stoniness of the substrate.

The Lycio cinereum – Salsoletum is typical of the large Haribes pan on the farm. The soils originate from weathered sandstones and some calcretes consisting of fine material which is very prone to capping and surface sealing (Figure 4c). The cation levels are probably elevated to form a halophitic environment, but not sufficiently so to render the pan a salt pan (M. Coetzee, pers.comm., March 2010).

Two subassociations can be distinguished, as described below.

Along the outer parts of the pan and on the floodplains to the pan the vegetation is better developed, forming a tall, moderately closed shrubland (Figure 5). Few perennial herbaceous species occur here; this layer is dominated by ephemerals. The species density is on average 14 species per 1000 m² and in total 39 species have been observed here.

The Salsolo – Tetragonietum schenckii is found on hummock dunes probably originating from a deflation process during the formation of the Haribes pan, similar to that described for the Koës pan and other depressions in the Kalahari (Bullard & Nash 2000; Lancaster 1986; Shaw 1988; Shaw & Thomas 1997). The dunes vary in length, generally between 30 m and 50 m. The slopes of the dunes are often quite steep. The occurence of Salsola spp. on these dunes indicate a high base status, which, in turn, supports the hypothesis that the dunes are the result of the deflation process of the pan.

There are very few of these small pans that differ greatly from the big pans on the farm. Some are relatively small depressions, mostly on top of the plateau area, but there are also small pans below the plateau, especially near the old road along the tar road between Mariental and Maltahöhe. These depressions allow water to accumulate, resulting in microhabitats with a wetland character.

The Anthephoro pubescentis – Ziziphodetum mucronatae represents the riverine woodlands (sensu Giess 1998). There are two types of drainage system on the farm, with dry washes, which contain water only for very short periods during rain storms, being most common. Geomorphologically they can be classified as very young river systems. In many cases the slope is relatively steep and the run-off very fast. They occur mostly on the plateau or at the edge of the plateau. Towards the centre of the farm, more mature rivers, like the Riet, Holz and Lewer, are responsible for drainage. There is a big dam near the farmhouse, where the water of these rivers is captured. These two different habitats support two different vegetation subassociations (5.1 and 5.2).

Although the vegetation of the riverbed and the riverbank differ, they were sampled as one for the purposes of this study and are thus included in the same plant association.

The Ehretrio albae – Ziziphodetosum mucronatae occurs along the dry washes. Their slopes vary from steep to almost level (15°–20° and < 3°, respectively). Erosion is a common occurrence on the steeper slopes. Most of these dry washes start on top of the plateau and generally run eastward or southward to join the Riet, Holz and Lewer rivers. They are all typically ephemeral and are active only during and shortly after thunderstorms. Varied soil character is observed: although most soils are very shallow, occasionally with big rocks, gravel and small stones also represent a common soil cover. At the base of the plateau some of the washes have deeper soils, which are also higher in nutrient content.

The Heteropogo contorti – Ziziphodetosum mucronatae occurs along the more mature rivers (Riet, Holz and Lewer). Their slopes are normally less than 3° and water remains standing in river pools for long periods. The soils in the riverbeds are normally rough gravels, which can be very stony with big rocks. The nutrient level in these gravels is low, but water content is usually relatively high. The soils of the river banks are sandy loams with a relatively high nutrient content, whilst the moisture content varies depending on the slope of the river.

Zygophylo pubescentis – Leucosphaeretum bainesii is associated with shallow calcrete soils, either along the northern Haribes pan edge, as part of the flood plain system of larger rivers or on dolerite ridges. The soils are shallow (20 cm – 30 cm deep), whitish in colour and have a high clay content. The soils are expected to be alkaline with a pH of more than 7 (M. Coetzee, pers. comm., March 2010). Stone cover in the form of calcrete gravel and pebbles is often found mixed with medium to large dolerite blocks and can reach up to 80%. Steep terraces, with slopes of between 5° and 10°, are formed along the pan edge.

The Monsonio umbellatae – Boscietum foetidae typically occurs on the torras on the farm. Torras are defined as flat plains, with little or no internal relief, covered with reddish brown gravel on the surface (often more than 80%). The soils are loamy, with a crust, and with very low water infiltration capacity. This low water infiltration capacity results in patchy, near-banded vegetation, with run-off from the open patches feeding the vegetated patches (run-on). Similar vegetation patterns have been described as brousse tigree in the Sahel (Boaler & Hodge 1962; Bromley et al. 1997; Couteron et al. 2000; Thiéry, D’Herbès & Valentin 1995) and banded mulga in arid Australia (Dunkerley 1997a, 1997b).

The Schmidtio kalahariensis – Boscietosum foetidae occurs on flat plains and footslopes as well as on ranteveld proper and forms a transition between the torras and the ranteveld. These plains are underlain by sandstones of the Nababis formation. Fragments of these form the stone cover on the soil surface, ranging from gravel to medium-sized stones and covering up to 80% of the soil surface. The low vegetative cover indicates a tendency to degrade to a similar vegetation as the Monsonio umbellatae – Boscietum foetidae on the present torras (Figure 4j).

The Acacio senegal – Catophractetosum alexandri forms the ranteveld and occurs mainly in areas on top of the plateau. The plateau is dominated by very stony outcrops and small escarps (rante) of Nababis sandstone. The soils are leptosols, red in colour and very shallow, of moderately coarse to medium texture, with many coarse fragments and well drained. The surface layer is typically 10 cm – 20 cm thick. These soils have a moderate hydraulic conductivity and present a moderate infiltration rate with a low water holding capacity (ICC et al. 2000). The landscape is relatively flat on top of the plateau, but forms occasional low hills and an escarpment area to the west of the farm, with steep-sloped ravines in between.

FIGURE 4: Typical vegetation at different sampling sites on the farm Haribes.

FIGURE 5: Box-and-whisker plots of the various structural components for each of the described associations or subassociations.

Differentiating between the Monsonio umbellatae – Boscietum foetidae and the Schmidtio kalahariensis – Boscietosum foetidae proved extremely difficult owing to their similar structures and especially similar low vegetative cover. For this reason, a landscape map was produced (Figure 6) to differentiate between plains and torras, and ranteveld. Table 1 lists the mapped landscapes, their spatial extent and their included vegetation associations or subassociations.

Giess (1998) superficially described the Dwarf shrub savanna, as follows:

Characteristic for the greatest part is Rhigozum trichotomum (…). Parkinsonia africana, Acacia nebrownii, Boscia foetida subsp. foetida, B[oscia] abitrunca var. albitrunca and Catophractes alexandri as well as Karoo bushes such as Pentzia spp., Eriocephalus spp., and others are typical for this vegetation type. (Giess 1998)

This description is illustrated with three plates. Yet the Acacio senegal – Catophractetum alexandri has a too tall structure (2 m – 3 m) to be regarded as true dwarf shrub savanna. Within the broader Nama-Karoo (of which Giess’ dwarf shrub savanna forms the northern extent), the Acacio senegal – Catophractetum alexandri can structurally be compared to the Acacia mellifera open shrubland of the Blouputs karroid thornveld as described for the Augrabies Falls National Park (Bezuidenhout 1996; Mucina & Rutherford 2006). However, the shrub component here is dominated by A. mellifera subsp. detinens, a species rarely found in the Acacio senegal – Catophractetum alexandri. In this respect, the Acacio senegal – Catophractetum alexandri is to be regarded as a xeric outlier of the highland savanna (sensu Giess 1998). The rocky habitat with shallow soils, and often steep slopes, is similar in both vegetation types.

The Salsolo – Tetragonietum schenckii resembles the Saline desert and dwarf shrub savanna fringe (sensu Giess 1998) described for the Etosha pan. Here the structure of the hummock dune vegetation can specifically be compared to the Okondeka duneveld (Le Roux 1980). However, the composition varies widely, with T. schenkii not occurring in northern Namibia. For the Lycio cinereum – Salsoletum no comparable vegetation type is known. It is likely that these, in a future synopsis of syntaxa, will form a class on their own.

The very close relationship between the Monsonio umbellatae – Boscietum foetidae and the two subassociations of the Acacio senegal – Catophractetum alexandri is well illustrated in the ordination diagram in Figure 3b. This leads to the assumption that these three units form degradation stages of one another: Acacio senegal – Catophractetosum alexandri degrading to Schmidtio kalahariensis – Boscietosum foetidae, and in very extreme cases (possibly also owing to soil degradation processes such as erosion and capping) to Monsonio umbellatae – Boscietum foetidae. This hypothesis needs to be tested and described in a paper in which dynamic trends are considered.

With reference to biodiversity patterns, species density is comparable to other descriptions within the Nama-Karoo biome in southern Namibia. The Nico observatories (Nico North and Nico South) of the BIOTA Southern Africa project are the nearest to Haribes and are also situated on relatively stony soil, albeit of origin from the Dwyka formation (Geological Survey 1980). The Nico observatories display an average of 21 and 20 species per 1000 m², respectively (Jürgens et al. 2010), compared with the 17–19 species per 1000 m² for the Zygophylo pubescentis – Leucosphaeretum bainesii, Monsonio umbellatae – Boscietum foetidae and Acacio senegal – Catophractetum alexandri. The slightly lower values observed during the current study can be related to the survey being conducted during a single, comparatively dry year, whilst the Nico observatories had been resurveyed over a period of seven years (but unfortunately not in 2005). However, the overall number of species observed at Haribes (218) is higher than the total number of species observed at both Nico North (166) and Nico South (204). This can, in turn, be attributed to the fact that the area sampled at Haribes is far bigger and more diverse than the 1 km² sample areas of the BIOTA observatories. Comparing the Zygophylo pubescentis – Leucosphaeretum bainesii to the Narias and Duruchaus observatories, which also have shallow soils on calcretes (thus a comparable edaphic habitat), a far lower species density was observed (19 vs 36–37 per 1000 m²). The climatic environment at the Narais and Duruchaus observatories is, however, far more moist, with a mean annual rainfall of et al. 2010).

Within the Nama-Karoo biome in Namibia, livestock farming depends mainly on browsing of dwarf shrub species (Espach et al. 2006), with grazing playing a secondary role. This does not hold true for Haribes. A comparison of the shrub, dwarf shrub, grass and total vegetation cover of the various associations (Figure 5) shows that farming on Haribes is more dependent on grazing than browsing. The pans and surrounding dunes are mostly covered by shrubs dominated by Salsola spp. and T. schenckii, which are both known to be poisonous (Mannheimer, Marais & Schubert 2008). This makes roughly 13% of the farm unsuitable for grazing. The Zygophylo pubescentis – Leucosphaeretum bainsii on the calcrete ridges provide some dwarf shrub browse, but is very limited in extent. The Monsonio umbellatae – Boscietum foetidae (torras) and the Acacio senegal – Catophractetum alexandri (ranteveld) provide the main grazing resources on the farm. However, as pointed out in the discussion, these seem to form a degradation gradient from each other; thus, extensive overgrazing will lead to desert-like torra plains.

FIGURE 6: Map of the major landscapes on the farm Haribes.

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