Southern Namib Research Articles

Occurrence and formation of clast-free circular depressions in the southern Namib Desert, Tsau ǁKhaeb (Sperrgebiet) National Park, Namibia

Circular depressions are concave, shallow depressions found on planar landscape surfaces in the southern Namib Desert. They occur on gravelly substrates with nearly level to very slightly inclined surfaces. The depressions range from 6 to 10 m in diameter with centers typically depressed 10–20 cm below the level of the surrounding terrain. Locations of individual circular depressions were mapped at one site using ground-based measurements and at three additional sites using Google Earth imagery. At all sites, circular depressions are highly overdispersed with densities ranging from approximately 10–20/ha and corresponding nearest neighbor distances of 17–24 m. Large fragments of weathered calcrete and stones occur on soil surfaces surrounding circular depressions, but not within the depressions. Circular depressions at one site contained active burrow systems of Brants’ whistling rat ( Paratomys brantsii). Bioturbation by these rodents contributes to the non-cohesive nature of the sandy substrate, which promotes aeolian deflation and formation of the depressions. Excavations of the burrow systems by the honey badger ( Mellivora capensis) in search of rodent prey can transfer large stones and calcrete fragments from the subsurface to the surface and subsequently move those materials about the surface. Even if such sequential, horizontal displacements are in random directions, such movements can eventually yield a central, clast-free area surrounded by a peripheral zone where the clasts accumulate once they have been displaced beyond the margin of the area to which the predator is drawn in search of rodent prey. A conceptual model consisting of a two-dimensional random walk of large clasts about the surface until they are displaced from the focal “arena” of rodent occupation provides a novel explanation for origin of a spatially organized pattern that is initiated through the random displacement of those materials. Comparable microtopographic patterning associated with bioturbation in other arid and semi-arid environments may have similar origins.

Vegetation change on mine sites in the most arid part of the Succulent Karoo Biome and their implications for mine closure

Restoring biodiversity is an important component of mine site rehabilitation. It needs to be evaluated regularly to assess success or failure of restoration methods and compliance with mine closure completion criteria. Yet vegetation dynamics in the driest part of the Succulent Karoo Biome are poorly understood. This study contributes to the question whether succession theory applies in this ecosystem by investigating 40 sites in a completely altered mining landscape and accompanying reference sites in the southern Namib within a 17-year time interval. Plant cover and species composition of the same site were compared between a field survey in 2005 and 2022. Multivariate analyses indicated that the importance of investigated environmental variables shifted between the survey periods, affecting the secondary vegetation on altered landforms as well as the undisturbed reference sites. This complicated the interpretation of perceived changes between the 2005 and 2022 surveys, making it difficult to detect successional trends. A drying of the general area is indicated which reduced species richness and cover overall and may have precipitated a change in habitat from stabilised to shifting dunes with concomitant changes to more sand-tolerant species in the southern part of the study area. These changes in both natural and secondary vegetation pose challenges when deciding on appropriate measures for restoration monitoring and setting completion criteria for mine closure.

Geophysical and geological exploration of the Aurus clay pan as an archive of the long-term climatic and environmental history of the Namib Desert

The Namib Desert is one of the oldest deserts on Earth. Whilst the history of the desert during the last glacial-interglacial cycle is relatively well understood, information on the older Pleistocene and Neogene history is very scarce, because longer-term records such as those in lakes and caves have not been formed due to the aridity. Here, we investigate the potential of the sedimentary record in the Aurus clay pan in the Sperrgebiet Domain in the southern Namib Desert, Namibia, for longer-term paleoclimate research. For this purpose, we conducted a multi-disciplinary geophysical site survey along with initial sedimentological investigations. We used a combination of transient electromagnetic (TEM), seismic and magnetic data, that allows us to derive information with respect to tectonic features as well as the deeper sedimentary architecture of the Aurus clay pan. TEM and seismics were applied along two perpendicular transects. The data obtained reveal a three- to four-layered subsurface structure with a maximum sedimentary thickness between 31 and 65 m overlaying resistive bedrock. The aeromagnetic data show a fault zone cutting the southwestern corner of the clay pan that correlates with deep conductive anomalies visible in the TEM data. Radiocarbon ages of plant remains from the sediment surface evidence modern deflation, probably due to a particularly dry climate. The composition of a 4-m-long sediment core, which probably reflects the depositional history from 3400 to 23,000 cal. yr BP, however, suggests that most of the Holocene was characterized by a relatively high humidity, whereas the latest Pleistocene again was much dryer. According to an extrapolation of the sedimentation rates in the upper 4 m to the base of the sedimentary infill the record in Aurus clay pan penetrates in the order of 340,000 cal. yr BP. This would be longer than most other paleoclimate records in the Namib, but much less then recently recovered in the Atacama Desert in Chile. A comparison of the geophysical and geological data from Aurus with comparable data and two drill cores from clay pans in the Atacama furthermore shows that the physical parameter in all cases allow to decipher the sediment thickness and general architecture, but alone provides limited information on the sediment composition.

Not one but several: Morphological diversity within the sand termite

Termite species are generally identified based on morphological differences of soldiers. This approach is here applied for the sand termite species Psammotermes allocerus. By using high quality resolution images, measurements of 26 traits of head, labrum and mandibles, the CART method, as well as geometric-morphometric shape analyses of mandibles of large soldiers (> 7mm in total body lengths) a discrimination between soldiers of three geographical regions is possible. Soldiers collected from the Succulent Karoo, Southern Namib and Northern Namib differ significantly from each other. This study is a first attempt to show strong morphological variation and differentiation within the sand termite Psammotermes allocerus from the arid regions. Further research in additional geographical regions and the comparison of different development stages of soldiers are needed for a complete assessment of the morphological diversity of species.

Phylogeny of sand termites

The sand termite Psammotermes allocerus is widely distributed in in southern Africa. To investigate the genetic diversity of this species 113 samples were collected in the arid regions of South Africa, Namibia and Angola in fairy circles and also in landscapes without fairy circles. The mitochondrial markers COI and COII were selected to perform Bayesian Inference analysis. The phylogenetic tree is composed of eight well-supported genetic groups within the species complex. In a synopsis with also studied morphological features the eight groups Northern Namib, Western Kalahari Basin, Nama, Southwestern Kalahari, East Gariep, Southern Namib, Tsau IIKhaeb and Succulent Karoo should be considered as distinct species. The species name P. allocerus should be used for termite collections of the Succulent Karoo due to their basal position in the phylogeny.

Cryptic subterranean diversity: regional phylogeography of the sand termite Psammotermes allocerus Silvestri, 1908 in the wider Namib region

Psammotermes allocerus Silvestri, 1908 is the only described species representing the genus Psammotermes Desneux, 1902 in Southern Africa. The large geographical range of this subterranean termite covers both summer and winter rainfall regimes. Deadwood is the preferred food when available, but in more arid habitats, both live and dead grasses form the major dietary component. Along the Namib Desert margins, the species’ localised herbivory creates circular bare patches known as fairy circles. For a regional phylogeographic study of this species, we sampled 65 sand termite populations within drier parts of Namibia, South Africa, and Angola. Based on combined molecular and ecological data, we found considerable genetic diversification within P. allocerus. Analyses of two mitochondrial markers (COI, COII), including a Bayesian inference tree, haplotype analysis and genetic distances suggest a delineation into seven highly differentiated genetic groups. The ‘Succulent Karoo’ group is additionally characterised by unique features of the royal chamber, nest and tunnel system. In conclusion, our data suggest that P. allocerus should be not regarded as one species but as a species complex. Termites of each analysed group ‘Northern Namib’, ‘Western Kalahari Basin’, ‘Nama’, ‘Southwestern Kalahari’, ‘East Gariep’, ‘Southern Namib’ and ‘Succulent Karoo’ should be considered as distinct species. The species name P. allocerus should be used for termites of the ‘Succulent Karoo’.

Impact of extreme rainfalls in the year 2006 to the vegetation cover on different geomorphological units of the central Kaukausib catchment, southern Namib Desert (Namibia)

Impact of extreme rainfalls in the year 2006 to the vegetation cover on different geomorphological units of the central Kaukausib catchment, southern Namib Desert (Namibia)

Field surveys and revegetation experiments show that simulation of topographical habitat features could improve the chances of successful restoration for the threatened succulent Juttadinteria albata

Species with highly restricted ranges are disproportionately at risk of extinction, particularly where habitat loss occurs as a result of mining. Postmining restoration of rare species populations is considered as an appropriate response to counter such threats, but requires a careful, evidence‐based, and information‐driven approach. The economically important diamond mining at Sendelingsdrif in the southern Namib Desert occurs in the highly diverse Succulent Karoo Biome and threatens a significant part of the population of the narrow endemic succulent plant species Juttadinteria albata. To decrease the inherent risks of restoring such a rare species, we studied the habitat features of premining J. albata populations and experimentally tested whether some features could assist future reintroductions in postmining substrates. Plots where J. albata occurred were mostly south‐ to west‐facing and had among others higher rock cover and steeper slopes than plots where J. albata plants were absent. A revegetation experiment, with J. albata cuttings that were established on postmining substrate mounds, revealed that plants on steeper slopes, facing south to west, grew faster than plants on other slopes and aspects. Slope and aspect are therefore important habitat properties to recreate when restoring J. albata populations. These, and other preferred habitat properties such as higher levels of organic C, should now be tested in larger‐scale field trials. Validation of habitat requirements of J. albata through the revegetation experiment has decreased the risks at least partially and provides additional empirical evidence of the importance of establishing reference conditions to enhance ecological restoration.

Factors influencing colonisation processes in two contrasting mine sites in the Namib Desert

I investigated vegetation recovery in a uranium mine in the central Namib and a diamond mine in the southern Namib to better understand natural recovery processes in arid areas. Environmental variables influencing colonisation processes were investigated using multivariate analyses to examine the relative importance of environmental variables on vegetation recovery at the two mines. Disturbance level was the most important variable influencing colonisation processes in the central Namib whereas distance to seed source was the most important in the southern Namib. At both mines, the proximity of a species pool to the disturbed areas (reference species pool) was more important than the pool of available species in the broader ecological community in which the mines were located (ecological species pool). The study indicates that merely relying on spontaneous, natural recovery is insufficient in the Namib if (1) pre-mining conditions are the restoration goal and (2) presently undisturbed or well recovered areas in the mine cannot be preserved as a suitable seed sources in the southern Namib. Appropriate completion criteria need to be developed that consider local conditions in order to ensure that restoration of mines in arid areas is realistic.

50,000 years of vegetation and climate change in the southern Namib Desert, Pella, South Africa

This paper presents the first continuous pollen record from the southern Namib Desert spanning the last 50,000years. Obtained from rock hyrax middens found near the town of Pella, South Africa, these data are used to reconstruct vegetation change and quantitative estimates of temperature and aridity. Results indicate that the last glacial period was characterised by increased water availability at the site relative to the Holocene. Changes in temperature and potential evapotranspiration appear to have played a significant role in determining the hydrologic balance. The record can be considered in two sections: 1) the last glacial period, when low temperatures favoured the development of more mesic Nama-Karoo vegetation at the site, with periods of increased humidity concurrent with increased coastal upwelling, both responding to lower global/regional temperatures; and 2) the Holocene, during which time high temperatures and potential evapotranspiration resulted in increased aridity and an expansion of the Desert Biome. During this latter period, increases in upwelling intensity created drier conditions at the site.Considered in the context of discussions of forcing mechanisms of regional climate change and environmental dynamics, the results from Pella stand in clear contrast with many inferences of terrestrial environmental change derived from regional marine records. Observations of a strong precessional signal and interpretations of increased humidity during phases of high local summer insolation in the marine records are not consistent with the data from Pella. Similarly, while high percentages of Restionaceae pollen has been observed in marine sediments during the last glacial period, they do not exceed 1% of the assemblage from Pella, indicating that no significant expansion of the Fynbos Biome has occurred during the last 50,000years. These findings pose interesting questions regarding the nature of environmental change in southwestern Africa, and the significance of the diverse records that have been obtained from the region.

Natural recovery of dwarf shrubs following topsoil and vegetation clearing on gravel and sand plains in the southern Namib Desert

Natural recovery of vegetation in arid areas is expected to take long time spans. Relatively rapid turn-over times have, however, been reported in Succulent Karoo vegetation in southern Africa, suggesting a certain resilience. This study investigated natural recovery of coastal Succulent Karoo shrubland five, six and seven years after geological sampling in the southern Namib Desert. The geological sampling resulted in a grid-pattern of cleared and topsoil stripped 5 × 5 m areas in three study sites in slightly different environmental settings. Using plant species richness, vegetation cover, plant species composition and an adaptation of dominance–diversity curves as measures, the initiation of natural recovery was demonstrated at all three study sites. Calcrete and gravel plain vegetation showed the best recovery, followed by sand plain vegetation and gravel plain vegetation in a deflation valley (wind corridor). An adaptation of dominance–diversity curves proved useful in illustrating successional patterns in these sparsely vegetated, arid dwarf shrublands and provided a tool to extract information for species-level management intervention. An exceptional rainy season, a large edge effect due to the localised impact of the geological sampling, intact subsurface structure and natural sand deposition are hypothesised to have favoured the initiation of natural recovery in this arid area.

The application of COSI‐Corr to determine dune system dynamics in the southern Namib Desert using ASTER data

ABSTRACTThe Coregistration of Optically Sensed Images and Correlation (COSI‐Corr) algorithm was used to estimate dune migration and sand flux rates from a series of remotely‐sensed optical image pairs. Several areas of barchan and transverse dunes transport sand along definite pathways, feeding the southern part of the Namib Sand Sea from beaches and deflationary basins in the Sperrgebiet. We give a detailed description of the pre‐ and post‐processing routines used for our COSI‐Corr analysis, and we evaluate the sub‐pixel dune migration results at the dune and dune‐field scales. The best set of parameters for the application of the algorithm was systematically derived, resulting in methodological refinements of the cross‐correlation windows. The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor aboard the Terra satellite provided the remote sensing imagery. Few comparisons have been made between dune migration measurements using COSI‐Corr and other types of change detection analysis, and few dune fields have served as test cases for COSI‐Corr. We suggest that analysis and interpretation of COSI‐Corr output from ASTER data is more valuable if combined with other image analysis techniques, such as manual digitization of dunes in a geographic information system (GIS), processing of write memory function insertion (WMFI) imagery and multispectral image (MSI) analysis of composition. The dune migrations estimated from COSI‐Corr results were validated using GIS and WMFI, and later MSI analysis added an important regional, contextual framework of sand transport pathways. The synthesis of these works lends more confidence to understanding the Namib dune system's dynamics and provides a basis for future comparisons to other dune fields. Copyright © 2013 John Wiley & Sons, Ltd.

The effect of altitude on arid inselbergs along a bioclimatic gradient

Twenty-four inselbergs in four distinct landscapes along a biogeographic gradient from mesic winter rainfall in the southern Namib to arid summer rainfall in the northern Namib were analysed. Declining similarity of vegetation between matrix (surrounding plains) and inselbergs with altitude were exhibited in most inselbergs, irrespective of biogeographic region, rock types and height above surroundings. Some inselbergs showed the least congruence with matrix vegetation on upper slopes, others with the top of inselbergs. This is probably related to the nature of the individual inselbergs, e.g. with steep cliffs in the upper reaches supporting plant species adapted to more mesic conditions. Most trends of declining similarity between matrix and inselberg habitats were maintained across the investigated bioclimatic regions. Even low inselbergs clearly supported vegetation very distinct from the surrounding area. Their contribution to species richness of a landscape is therefore unquestionable and adequate protection of such landscapes is needed where human impacts are severe.

Phylogeny of the Dorotheantheae (Aizoaceae), a tribe of succulent annuals

AbstractDorotheantheae (Aizoaceae) is a small tribe of succulent annual herbs with flat leaves belonging to the generaAethephyllum, Cleretum and Dorotheanthus, which are endemic to the winter–rainfall region of South Africa. Several species are well–known for their conspicuous and striking flowers but others possess very inconspicuous flowers. Analysis of molecular data from combined plastid and nuclear sequences shows that Dorotheanthus is monophyletic while Cleretum is diphyletic, since the two species of Cleretum with lyrate leaves are most closely related to the vegetatively similar, monotypic Aethephyl–lum. In the absence of clear, diagnostic characteristics for separating Cleretum and Dorotheanthus, a revised classification for the Dorotheantheae is proposed in which the tribe consists of a single genus, Cleretum. Within Cleretum, we recognize 14 species, by raising/re–instating two subspecies to species rank and including two that are still undescribed. The major clades occupy distinct climatic and edaphic niches. Repeated shifting from large to small flowers may indicate changes in the breeding system. Since succulence in annuals is rare in the winter–rainfall region of southern Africa, we compare patterns of distribution between Dorotheantheae and the other leaf–succulent annuals in the region, namely in Mesembryanthemum (Aizoaceae) and in Crassula (Crassulaceae). We find that similar to the non–succulent annuals of the Poaceae and the Brassicaceae in this region, the annual species of Mesembryanthemum show a preference for arid areas. In contrast, in Dorotheantheae and in Crassula the hot–spots of diversity lie mainly in the wetter parts of the winter–rainfall area, with a decrease in the number of annual species northwards to the arid, winter–rainfall areas of the Richtersveld and the southern Namib.

A new Drimia sp. from the Namib Desert in Namibia

Summary A new miniature species in Hyacinthaceae, Drimia occultans G. Will. is described, from the arid southern Namib Desert. A diagnostic line drawing, photographs of the plant and flower and a locality map are included. The new species appears to be endemic to the schistose rock outcrop close to the Lower Orange River, about 12 km NE of the river mouth.

A new species of Polemanniopsis (Apiaceae) from Namibia

A new species of woody Apiaceae, Polemanniopsis namibensis B-E. Van Wyk, A. Burke & C. Mannheimer, is described. The species is allied to Polemanniopsis marlothii but differs in the much smaller habit of (up to 0.6 m vs 1.5–4 m in Polemanniopsis marlothii), the white or greyish colour of the twigs (brown in Polemanniopsis marlothii), the 3-foliolate leaves (predominantly 5-foliate in Polemanniopsis marlothii), the smaller inflorescences with persistent involucral bracts (caducous in Polemanniopsis marlothii) and anatomical details of the leaves. The species is endemic to Namibia and is known only from a localised area in the southern Namib desert near Lüderitz, where it is relatively common.

The use of new elevation data (SRTM/ASTER) for the detection and morphometric quantification of Pleistocene megadunes (draa) in the eastern Sahara and the southern Namib

New freely available digital elevation data derived from the Shuttle Radar Topography Mission (SRTM-3) and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) allow, for the first time, the use of area-wide high-resolution elevation models. They are useful for various investigations, especially in regions that are devoid of detailed topographic maps. In this contribution, the geomorphological, sedimentological, geochronological, and archaeological results of the Cologne Collaborative Research Centre ACACIA are presented, showing that the southern Great Sand Sea of Egypt (eastern Sahara) is composed of a complex dune pattern of Pleistocene megadunes (draa) and Holocene dunes. A conceptual model is presented suggesting draa, formed by a strong unimodal trade wind regime that acted, most probably, as helical roll-vortices. The draa differ conspicuously in their dimensions and in their evolution from the smaller longitudinal dunes, which arise from the recent bimodal wind regime. First results of comparative investigations in the southern Namib Erg (Namibia) yielded similar conditions and a synchronous landscape development. Based on these results, the second part of the paper evaluates the elevation data for both regions with respect to their availability for the determination of draa and dunes and the morphometric quantification of the draa. Due to their different resolution (SRTM-3: 90 m, ASTER: 30 m) and their different sensor technology (SRTM-3: radar, ASTER: stereo data), both models have assets and drawbacks but can complement each other. With the calculation of geomorphometric parameters (elevation, slope), the ASTER digital elevation model allows the location of the draa of the Great Sand Sea of Egypt, the quantification of their spatial distribution, size, volume, and consequently the Holocene dune movement. In conclusion, the resolution of the ASTER elevation model is applicable for quantitative analyses of well-defined morphological elements with altitudinal variations > 40–50 m. The volume estimation presented provides a tool for a realistic assessment of the sand mass balance of sand seas. This is not possible with the SRTM-3 model and generally impossible with both models if the altitudinal variation is < 20–30 m (e.g. the draa in the southern Namib Erg). As most of the world's ergs contain draa and dunes of respectable heights, the demonstrated methodology could be useful for further investigations concerning dune movement, desertification and climate change.

Extreme rainfall in the Namib desert during late summer 2006 and influences of regional ocean variability

AbstractThe desert country of Namibia (southwestern Africa) and adjoining areas received anomalously heavy rains in austral summer 2006. In particular, the hyper‐arid southern Namib Desert experienced an extreme rainfall event in mid‐April 2006 which led to the coastal station of Luderitz receiving 102 mm, or about six times its annual average rainfall. Based on extreme value theory, this event appears to have a return period of 145 years—the rainfall received at Luderitz during 16–23 April 2006 was about twice that of the annual rainfall recorded at this station in the previous wettest year (1956).It is argued that this event occurred as a result of the unusual interaction of an anomalously westward‐located tropical‐extratropical cloudband, and an anomalously northward‐located cut‐off low. In turn, the location and behaviour of these synoptic features may have been aided by regional ocean conditions, a strong warm event in the tropical southeast Atlantic and a positive dipole‐like sea surface temperature event in the south Indian Ocean, together with La Niña conditions. It is argued that the combination of these regional ocean patterns with the La Niña event led to the very wet conditions over Namibia and neighbouring countries during austral summer 2005–2006. Copyright © 2007 Royal Meteorological Society

Neogene climate change and emergence of C 4 grasses in the Namib, southwestern Africa, as reflected in ratite 13C and 18O

Stable light isotopes in ratite eggshells have been shown to be reliable indicators of shifts in climate and environmental conditions in the past. Here, we show that δ 18O and δ 13C values in fossil and modern ratite eggshells collected in the aeolianite deposits of the southern and central Namib Desert track regional distinctions and global climate shifts throughout the Neogene. δ 18O values, although variable, are consistently higher in the central compared to the southern Namib throughout the record. δ 18O trends during the Miocene differ for the two regions, but track each other post-Miocene. Throughout the Miocene, δ 13C values for ratite eggshells from both the central and southern Namib regions are indistinguishable showing that the flora remained C 3 throughout. The overall negative (− 3‰) shift in mean values for Miocene biostratigraphic zones is consistent with the response of C 3 photosynthesis to pCO 2 shifts from 180 to 320 ppmv as estimated from marine alkenone studies and/or evolution of the δ 13C of the atmospheric CO 2. Evidence for C 4 plants occurs post-Miocene, with the development of the southern, winter rainfall and central/northern, summer rainfall zonation apparent today. These data provide independent corroboration that the expansion of C 4-dominated ecosystems after ∼ 7 Ma cannot be attributed to a reduction of pCO 2 below a 500 ppmv threshold, as earlier proposed. Proliferation of C 4 plants in the Namib after ∼ 5 Ma and, elsewhere, may be related rather to energy budgets and rainfall seasonality shifts resulting from large-scale atmospheric and oceanic circulation reorganisation.

Status of succulent shrubs in the southern Namib Desert, Succulent Karoo Biome

AbstractThe ratio of dead to alive succulent shrubs as an indicator of turnover was investigated to test whether the accepted notion of rapid turnover rates and cyclic succession in the Succulent Karoo Biome are applicable in the southern Namib. Based on counts of dead and alive plants, twelve species in two habitat types were investigated. These short‐term data which could not incorporate recruitment rates or changes over time, generated, however, some hypotheses. (i) Rapid turnover rates are likely not supported by all succulent plants at the Succulent Karoo Biome's northern boundary. With the exception of one species, mortality rates in succulent shrubs across a range of plant functional types were lower than reported in other parts of the biome. (ii) Sand plains appeared to support some transient species, indicating that there may be differences in vegetation dynamics between habitats. (iii) Species of different plant functional types showed no differences in ratio of dead to alive plants related to habitat. These species may have broad ecological tolerance limits and are perhaps less affected by changes in their environment. (iv) Shrubby Mesembryanthemaceae have high turnover rates and hence a short live span in the investigated area, but are longer lived than elsewhere in the Succulent Karoo Biome. As these hypotheses have implications for management and conservation of succulent species in this global biodiversity hotspot, a detailed evaluation of vegetation turnover, balancing mortality versus recruitment, should be investigated over a longer time span.