Southern Namib Research Articles

Restoring Tire Tracks: Lessons from the Southern Namib Desert

Restoring Tire Tracks: Lessons from the Southern Namib Desert

Reconstitution of dune morphologies and palaeowind regimes in the Proto-Namib since the Miocene

Abstract In the Namib Desert, indurated fossil dunes are found between the active sands. Establishment of a biostratigraphic scale based on associated mammal faunas and ratite eggshells allows the study of palaeowind fluctuations that have controlled these palaeodunes since the Middle Miocene with a temporal resolution of between 1 and 3 Ma. In the southern Namib, the Proto-Namib aeolian systems consist of crescent dunes (barkhan type). In the eastern part of the Namib Desert, the aeolianite cross-bedding corresponds to longitudinal or pyramidal dunes. These aeolian structures are also controlled by local winds, resulting from high relief and the southern trade winds : this regime is identical to that which currently sweeps, through the Namib Desert, and is related to the presence of anticyclonic conditions in the Atlantic Ocean. A regime of seasonal winds from the northeastern sector is also recorded starting in the late Miocene.

Towards a density measure for assessments of arid plant populations – examples from the southern Namib Desert

AbstractPlant species in arid environments are often widely spaced making population assessments difficult. In order to minimise observer bias we tested two repeatable simple methods during red data assessments in the southern Namib Desert. Both the transect‐based and the quadrat‐based method resulted in over‐estimates of the true population size. However, because of case of application, the transect method warrants further development.

Determining landscape function and ecosystem dynamics: contribution to ecological restoration in the southern Namib desert.

The southern Namib is characterized by high biodiversity, changes in winter and summer rainfall, extreme climatic conditions and associated geomorphologic processes, a paucity of ecological knowledge, and multiple environmental threats posed by conflicting development options. With the long-term view to establish a research initiative focussing on restoration ecology in the southern Namib Desert, this article provides a review of current ecological knowledge and proposes potential research priorities. The aims of a research initiative would be two-fold. On the academic side, understanding processes operating at different scales will be critical to develop methods of ecological restoration suitable for southern Namib ecosystems. On the practical side, the development of appropriate methods will focus on facilitating ecological processes such as restoration of biologically active substrate, natural succession, and concentration of limited resources. The function of landscapes, plant-soil interface, soil-, vegetation- and plant population dynamics will likely provide some of the answers for ecological restoration.

Determining Landscape Function and Ecosystem Dynamics: Contribution to Ecological Restoration in the Southern Namib Desert

Determining Landscape Function and Ecosystem Dynamics: Contribution to Ecological Restoration in the Southern Namib Desert

Geophysical profile of the Roter Kamm impact crater, Namibia

Abstract— New gravity and magnetic data were obtained along ground profiles across the Roter Kamm impact crater in the southern Namib desert of Namibia. As the traverses of previous studies did not extend sufficiently beyond the crater rim, it had not been possible to adequately determine the regional background values. The gravity results of this study are similar to those obtained by Fudali in 1973, in that a negative, near‐symmetrical anomaly was obtained over the crater center. This anomaly conforms to the results expected for a sediment and impact breccia‐filled, simple bowl‐shaped crater. The magnetic results of this study, however, are different to those previously reported, which is most probably as a result of the longer profiles used in this new study. A slight positive magnetic anomaly was obtained over the crater interior. Short‐wavelength, high‐amplitude anomalies observed in the vicinity of the crater rim reflect magnetization contrasts that are probably related to brecciation and block rotation. Modelling of the positive magnetic anomaly indicates the possibility of a small magnetic body or lining at the crater floor‐breccia interface in the interior of the crater. Also presented is a 10 m contour digital elevation model of the crater and its environs.

Floristic affinities of an inselberg archipelago in the southern Namib desert—relic of the past, centre of endemism or nothing special?

The floral composition of an inselberg archipelago in the southern Namib desert was investigated with regard to species diversity, geographic range of the species, habitat niches, seed dispersal abilities and plant herbivore defence. Although no endemic species were found, the archipelago forms an extreme and often isolated outlier for a large number of species in their distribution area. The hypothesis is proposed that the isolated extreme habitat allowed survival of relic populations due to low immigration rate and low competition. The high proportion of species favouring long‐range dispersal might explain the lack of endemic species as well as the large contribution of species at their distributional boundary.

Water use efficiency of two succulents with contrasting CO2 fixation pathways

ABSTRACTTwo succulents with similar growth forms but different types of photosynthesis, Cotyledon orbiculata (crassulacean acid metabolism, CAM) and Othonna opima (C3 pathway), were investigated with respect to the modulation of water use efficiency (WUE) during the transition from the rainy season to subsequent drought. Environmental conditions were simulated in a controlled‐environment experiment on the basis of data collected in the habitat of the two species in the southern Namib desert. Experiments included one or more periods of hot bergwind, which frequently occurs in this region. When water was readily available, daily net CO2 fixation was similar in the two species. This result confirms that the daily CO2 fixation of CAM plants is as high as that of morphologically similar C3 plants adapted to the same habitat. As expected, both species reduced CO2 fixation and water loss through transpiration during simulated hot bergwind periods and their WUE values increased. However, after the second hot bergwind period, nearly identical WUEs were recorded: 41.0 and 40.0 mmol mol−1 for C. orbiculata and O. opima, respectively. Therefore the statement that a CAM plant is a better ‘water saver’ than a C3 plant does not necessarily hold for CAM and C3 plants with similar growth forms growing under the same environmental conditions.

Roter Kamm impact crater, Namibia: Geochemistry of basement rocks and breccias

The Roter Kamm crater in the southern Namib Desert has previously been identified as an impact structure on the basis of crater morphology and the presence of impact melt breccias which contain shock metamorphosed quartz and lithic clasts. To better define the variety of target rocks and breccias, we studied the petrography and chemical composition of a new suite of twenty-eight basement and breccia samples from the Roter Kamm crater. Based on chemical data for target lithologies and breccias we suggest that the crater was formed in a two-layer target region: an upper layer of Gariep metasediments (schist, marble, ± quartzite and sandstone) overlying the crystalline basement of the Namaqualand Metamorphic Complex. The basement was also heavily intruded by coarse-grained quartz veins and quartz- and quartz-feldspar pegmatites. The clast population in the melt breccias indicates that impact-induced melting involved mainly metasedimentary target rocks, with rarely detected contributions from pegmatite and granite/granodiorite. Three varieties of melt breccias can be defined: (1) “schistose,” (2) quartzitic melt breccias, (3) “true” impact melt breccias. These melt breccia types are chemically heterogeneous, and even the impact melt breccias may have been produced in situ and not from a coherent melt body. The shapes of the schistose melt breccias, previously thought to be ejected impact breccias, are most likely caused by erosion, and these breccias are now interpreted to be locally derived. The crater basement as exposed at the rim was structurally severely affected and, at least locally, considerable thermal energy was generated during formation of large volumes of cataclastic, mylonitic, and pseudotachylitic breccias. Analyses of mylonite and pseudotachylites from the crater rim, as well as their respective host rocks, show that these breccias were mainly formed from local material. Analyses of pseudotachylite-like breccias indicate that these possible friction melts are generated by preferential melting of hydrous ferromagnesian minerals and feldspar, similar to their tectonically produced counterparts. Although no significant fluid effects resulting from formation of mylonites or pseudotachylites are indicated, several breccias (compared to their host rocks) do show evidence of severe chemical alteration (chloritisation and sericitisation). The presence of large vesicles filled with hydrothermal mineral assemblages in some schistose breccias and other petrographic and chemical data support the hypothesis of an impact-induced hydrothermal event in the crater area.

Thermal constraints on prey-capture behavior of a burrowing spider in a hot environment

Seothyra henscheli (Eresidae) is a burrowing spider that lives in the dune sea of the southern Namib Desert, Namibia. Prey capture by these spiders involves a foray from a cool subterranean retreat to the undersurface of a capture web that can be lethally hot. Striking, disentangling and retrieving prey from the capture web typically involves several short trips to the capture web, alternating with retreats to the cool burrow. It has been suggested that this behavior limits the increase of body temperature a spider must experience while working at the hot capture web. We used biophysical models in conjunction with direct observations of prey-capture behavior and distributions of sand temperature to estimate body temperatures experienced by S. henscheli during prey capture. In the circumstances we observed, only the relatively long post-strike retreat from the capture web is important in keeping spiders' body temperatures from exceeding their lethal limits. After the post-strike retreat, shuttling appreciably limits the increase in body temperature of small individuals, but may have little effect on body temperature increase in larger spiders.

WATER EXCHANGE, TEMPERATURE TOLERANCE, OXYGEN CONSUMPTION AND ACTIVITY OF THE NAMIB DESERT SNAIL, TRIGONEPHRUS SP.

Water exchange, temperature tolerance and oxygen consumption of the snail, Trigonephrus sp., from the southern Namib desert of Namibia were examined and related to activity. At 25°C and 15% R.H. mean water loss and food and water uptake were 5.95 mg. day−1 and 630 mg.day−1, respectively. Body temperature tracked sand temperature. Snails tolerated sand temperatures as high as 45°C. Mean ± S.D. oxygen consumption rates were 32.0 ± 2.94 μlO2.g total body mass−1.h−1 at 15°C, when the snails were active, and 11.27 μlO2.g total body mass−1.h−1 at 25°C, when the snails were inactive. These values are 2-6 times lower than those recorded for the similarly sized mesic snail, Helix aspersa. Activity experiments indicated that low ambient temperatures and high humidities were favoured by the snails. This, together with the burying behaviour of these snails during high temperatures, suggests that they limit stress by restricting activity to physiologically-favourable periods, even though more-extreme conditions may be tolerated.

Convergent Evolution of the 'Homeria' Flower Type in Six New Species of Moraea (Iridaceae-Irideae) in Southern Africa

Convergent Evolution of the 'Homeria' Flower Type in Six New Species of Moraea (Iridaceae-Irideae) in Southern Africa

Respiration Beneath Desert Sand: Carbon Dioxide Diffusion and Respiratory Patterns in A Tenebrionid Beetle

ABSTRACT The massive dune sea of the southern Namib Desert is approximately 800 km in length and some 140 km wide. These dunes are among the highest in the world and, in spite of very sparse vegetation cover, they support a surprising variety of reptiles and arthropods (Koch, 1961 ; Louw & Seely, 1982). A feature of the arthropod fauna is the high species diversity of flightless tenebrionid beetles. These beetles do not construct burrows, but swim through the sand as if it were a fluid medium. For a description of the physical properties of dune sand see Lancaster (1981). Among the tenebrionids, Onymacris unguicularis is of special interest because it engages in fogbasking on the crests of the dunes when intermittent condensing fog occurs in the desert (Hamilton & Seely, 1976). This beetle occurs almost exclusively on the high slip-faces of the dunes, where it feeds on wind-blown plant detritus. To escape extremely high surface temperatures during midday and very low temperatures at night, O. unguicularis submerges beneath the sand to a depth of 5–10 cm for as much as 18 h per day (Seely, Mitchell & Louw, 1985). The present study assesses the respiratory consequences of such submergence by measuring the diffusion rate of carbon dioxide through a 5-crn layer of sand and the respiratory patterns, carbon dioxide production, and oxygen consumption of the beetles.

Availability of water controls Crassulacean acid metabolism in succulents of the Richtersveld (Namib desert, South Africa)

Features of Crassulacean acid metabolism (CAM) were studied in a variety of different succulents in response to climatic conditions between March 1977 and October 1983 in the southern Namib desert (Richtersveld). A screening in 1977 and 1978 revealed that nearly all investigated succulents performed a CAM, but overnight accumulation of malate declined gradually with decreasing soil water potential, tissue osmotic potential, and leaf water content. This was further substantiated by an extended period of insufficient rainfall in 1979 and 1980 which damaged the evergreen CAM succulents between 80 and 100%. In most of the species still living, neither CO2-gas exchange nor diurnal acid fluctuation, indicative of CAM, could be detected unless an abundant rainfall restored both CAM features. Plants persisted in a stage of latent life.Water supply is one necessary prerequisite for CAM in the Richtersveld. But even well-watered plants with CAM were sensitive to short-term water stress caused by high water-vapour partialpressure deficit (VPD) in the night, which reduced or prevented CO2 uptake and resulted in a linear relation between overnight accumulated malate and VPD. The results do not support the opinion that, for the Namib succulents, CAM is an adaptive mechanism to water stress since long-term and short-term water stress stopped nocturnal malate synthesis, but instead lead to the conclusion that nocuturnal CO2 fixation is only performed when the water status of the plant can be improved simultaneously.

Characteristics and occurrence of wind erosion features in the Namib Desert

AbstractSmall scale features resulting from wind erosion are widespread in the Namib Desert. They include selective erosion of lithological variations (etching), flutes and grooves, facetting of clasts and residual boulders, and smoothing and polishing of rock surfaces. Large scale features are tentatively identified and are restricted to the southern Namib, with its high energy, unidirectional wind regime.Wind erosion features are best developed on fine‐grained rocks of intermediate hardness. They are oriented towards or parallel to modern strong winds.The turbulent flow of wind armed only with dust particles is probably more important than wind driven sand in creating smooth polished rock surfaces, flutes, and grooves. Wind driven sand appears to be significant only in facetting rock masses.

Occurrence and changes of proline content in plants in the southern Namib Desert in relations to increasing and decreasing drought

Over a period of seven years (1977-1983) the proline content and its responses to climatic changes were investigated in plants - especially Mesembryanthemaceae - in the southern Namib Desert (South Africa). Among 95 species in 26 families, 61 had detectable amounts of proline. In several of these species the proline content increased considerably in years with insufficient rainfall but decreased when the rainfall was abundant again. When individuals of the same species were grown at different sites, water availability in the soil determined their proline content. Many of the investigated species showed a clear diurnal fluctuation in their proline content with a remarkable proline accumulation during times of highest evaporative demand. In general, the higher the proline content the more pronounced were the changes, indicating that in these species-predominantly annual plants - proline was most probably involved in drought tolerance. The observation that proline accumulation and degradation reacted sensitively to changing climatic conditions over many years confirmed the correlation of proline synthesis to increasing water stress as postulated by the results of laboratory experiments with Mesembryanthemaceae.

Water loss and malate fluctuations during the day for plants in the southern Namib desert.

The daily course of transpiration for 12 different plants growing in the southern Namib desert was investigated. Sclerophyllous species with C3 photosynthesis were the most effective water savers followed by succulents exhibiting CAM, while C3 pathway-succulents lose as much water as malacophyllous species. The different species showed either one or two peaked patterns of both transpiration and leaf conductance. With C3 plants the most common pattern is a single morning peak in leaf conductance followed by decreases in conductance over the remainder on the day. With CAM succulents leaf conductance is high in the morning, shows a pronounced midday depression and increases in the early afternoon when the malate pool is depleted, but malate consumption did not start earlier than 4 h after dawn. Seven of nine investigated C3 plants had rather high contents of malate, up to 180 μmol·g-1 dry matter. In these plants the malate content increased with increasing leaf conductance and disappeared when leaf conductance declined in the evening, indicating that malate was synthesized during photosynthesis.

CO2 exchange of CAM exhibiting succulents in the southern Namib desert in relation to microclimate and water stress

The responses of CO2 exchange and overnight malate accumulation of leaf and stem succulent CAM-plants to water stress and the particular climatic conditions of fog and föhn in the southern Namib desert have been investigated. In most of the investigated CAM plants a long term water stress gradually attenuated any uptake of external CO2 and led to CO2 release throughout day and night. No CAM-idling was observed. Rainfall or irrigation immediately restored daytime CO2 uptake while the recovery of the nocturnal CO2 uptake was delayed. Dawn peak of photosynthesis was only found in well watered plants but was markedly reduced by the short term water stress of a föhn-storm. Morning fog with its higher diffuse light intensity compared with clear days increased photosynthetic CO2 uptake considerably. Even in well watered plants nocturnal CO2 uptake and malate accumulation were strongly affected by föhn indicating that the water vapour pressure deficit during the night determines the degree of acidification.

CO2 exchange of CAM exhibiting suceulents in the southern Namib desert in relation to microclimate and water stress

The responses of CO2 exchange and overnight malate accumulation of leaf and stem succulent CAM-plants to water stress and the particular climatic conditiens of fog and föhn in the southern Namib desert have been investigated. In most of the investigated CAM plants a long term water stress gradually attenuated any uptake of external CO2 and led to CO2 release throughout day and night. No CAM-idling was observed. Rainfall or irrigation immediately restored daytime CO2 uptake while the recovery of the noctural CO2 uptake was delayed. Dawn peak of photosynthesis was only found in well watered plants but was markedly reduced by the short term water stress of a föhn-storm. Morning fog with its higher diffuse light intensity compared with clear days increased photosynthetic CO2 uptake considerably. Even in well watered plants noctural CO2 uptake and malate accumulation were strongly affected by föhn indicating that the water vapour pressure deficit during the night determines the degree of acidification.

Physical and biotic environments of the southern Namib dune ecosystem

The sand dune ecosystem of the Namib Desert of southwestern Africa contains a species-rich and highly endemic biota. As a first attempt to understand the evolution and ecology of this ecosystem, a number of abiotic and biotic characteristics are described and quantified, and the causal relationships between 18 of these variables are compared. Wind and dune morphology affect 16 of the 18 environmental parameters and are therefore considered major environmental factors in this ecosystem. Organic detritus cycles, fog moisture, soil temperature and sand grain size also have had important direct or indirect influences on the evolution of the dune biota. Four distinct biotic communities are defined and described for the north-central portion of the southern dune field; these are the interdune, dune base, plinth and slipface communities. Their distribution along the dune elevation gradient appears to be limited mainly by differences in sand particle size, soil moisture and sand stability. It is hypothesized that the subtropical, coastal Namib desert has an inordinately heterogeneous, and probably unique, physical environment that contains several adaptative zones not found in temperate continental dune ecosystems. Under such conditions, there has existed a high potential for extensive adaptive radiations within the numerous lineages of the psammophilic ancestral biota.