Weathering and soil properties on old granitic catenas along climo-topographic gradients in Kruger National Park

Catenas, the geomorphologically and hydrologically linked soils on hillslopes between crests and toeslopes, are arguably the best heuristic tool to underpin pedological work for understanding soil landscape evolution. Under this pretext, I investigated the expression of soil forming factors in shaping weathering and soil development in Kruger National Park’s diverse granitic landscapes. The quantitative investigations in four data chapters are preceded by narrative on how the soil landscape can most compellingly be viewed in pedological research. This synthetic perspective provides an interdisciplinary framework derived from the fusing long-standing pedological, geological and geomorphological thinking. Briefly, the framework articulates (1) the origin and fate of rock derived soil constituents; (2) how those constituents, primarily clay minerals and soil’s dissolved load, interact and react to the soil forming factors viz. climate, rock type and topography and (3) the distribution of soil properties along gradients of soil forming factors. My field sites were located in Kruger National Park, on the eastern seaboard of South Africa, nestled between the Great Escarpment to the west and the Lebombo Mountains to the east along the Mozambican border. Kruger affords an opportune test-case to study the interaction of climate, topography and geology on soil landscape patterns in a model ecosystem that acts as a natural laboratory. Kruger functions as a model ecosystem for earth scientists because physiographic gradients are well demarcated and can be studied iteratively by focusing on the variation in one gradient while others are kept constant. For example, the north-south axis of the park separates granite from basalt; and rainfall increases from north to south. Therefore the park has four well-bounded blocks within which the interaction of rainfall and geology can be investigated. Also, the influence of rainfall on soil properties can be isolated by fixing geology and varying climate, as illustrated in the first data chapter of the thesis. There, I used new and established tools to set boundaries on granitic weathering and soil development. In addition to constraining geology along the 400 to 700 mm mean annual rainfall gradient, the topographic dial on weathering was also set to zero by restricting sampling to crest soils. In this way, the sole impact of increasing rainfall on weathering and soil development was examined in exclusion of added complications imposed by geology and topography. Results suggested by geochemical mass balance, cosmogenic 10Be concentrations as an erosion proxy, soil exchange properties and clay amount and mineralogy all demonstrated indiscriminant loss of rock-derived constituents as rainfall increases from north to south. This means that granitic crests in Kruger are losing material under all climatic regimes, from dry in the north to wet in the south. The next question then was: what is the destiny of the material lost from crests? Does the material remain on the catenas or is it lost to the rivers for transport towards the ocean? How are the patterns different on catenas in distinct topographic contexts? These questions were the subjects of subsequent chapters in the thesis. Therein, analyses for crests were extended to capture patterns between those crests and toeslopes across three relief classes in each of three climate zones for a nested matrix of nine catenas. Material removal from crest soils was evident across all nine catenas, but was nuanced by depletion and accumulation zones whose extents were context-dependent within the topo-vii climatic matrix. Depletion zones invariably occurred at or near crests whereas accumulation zones were near toeslopes in all catenas across climate and relief. Depletions were most expressed by base cation and silicon losses while zones of accumulation were of heightened concentrations of aluminium and titanium. In terms of concrete soil properties, the depleted soils were clay-poor with low exchangeable base cation concentrations whereas soils at distal ends of catenas were high in both clay and exchangeable base cations. These general patterns were modified by the amount of rainfall and hillslope relief. Clay-rich zones were narrow in both high and low rainfall areas but widest at intermediate rainfall. Similarly with relief, it appeared that intermediate steepness facilitated the retention of material on catenas. Hence, coupled rainfall-topography interactions provide a thrust on material transfers on catenas resulting in a spectrum of catena types across gradients in soil forming factors. Hence, the catena is far from the static entity it is commonly imagined to be, but is dynamic if quantified across gradients of variation in soil forming factors

Using mulching to reduce soil surface temperature to facilitate grass production

Ecosystems in semi-arid and arid Southern Africa experience high temperatures which translate to extremely hot soil surface temperatures. High soil surface temperatures lead to a decrease in seed germination and consequently less plant cover in these areas. To facilitate maintenance of optimum plant cover, soil surface temperature should be moderated with appropriate mitigation techniques. Temperature variations in low (kg.0.5 m−3) and high density (1 kg m−3) brush packing treatments were compared to bare soil. We also measured the grass productivity (g.m−2) against the effect of temperature in the three treatments. iButtons® were used to log soil surface temperature every hour for seven months. Daily and nightly temperatures of the hottest months were compared amongst the three treatments. Mid-day temperatures, corresponding to peak heat stress were also compared between the three treatments. There was a significant difference (p < 0.01) in soil surface temperature between the three treatments. The high density treatment was the most buffered against temperature variation, when compared to the bare soil. Grass production was generally higher in the high density treatment. Productivity can be increased by mulching the soil with brush packing as this will improve soil surface conditions such as moderating abrupt changes in temperatures to assist plant growth

Does the landscape functionality approach provide insight into rangeland conditions in the Tanqua Karoo region, South Africa?

The harsh environmental conditions coupled with a long history of overgrazing have altered the ecology of the arid Tanqua Karoo rangelands in South Africa, which necessitates rehabilitation. However, a suitable method for monitoring rangeland function over time is required for sustainable management. In this study, vegetation characteristics and landscape function indices were used to rate and compare rangeland conditions in 43 sites distributed among three vegetation types: Tanqua Karoo, Tanqua Wash Riviere, and Tanqua Escarpment Shrubland, which occupy different landscapes in the Tankwa Karoo National Park. The results showed low values of vegetation volume (mean of 10.1 m3 per 100 m−2) and low vegetated patches (mean of 29% patches vs 71% fetches). The overall landscape function indices (soil stability, water infiltration, and nutrient recycling) were low and amounted to 55%, 28%, and 17%, respectively. Amongst the various examined landscapes, the escarpment had the highest values of most of the measured landscape functionality parameters, and the open plains had the lowest values. This revealed high heterogeneity of soil properties and vegetation characteristics amongst the different vegetation types, mainly influenced by altitudinal gradients. The higher-lying landscapes on the escarpment are relatively more functional and more susceptible to improvement when compared to the lower-lying landscapes on the plains. The landscape functionality approach (LFA) approach demonstrated that some of the examined vegetation types had insignificant improvement in landscape functionality likely not to improve in the near term due to existing low patchiness, higher fetch space and low LFA indices coupled with the low annual rainfall of the region. The landscape functionality approach has provided a suitable benchmark for assessing and monitoring the diverse vegetation types in this arid part of the world

Evidence for aeolian origins of heuweltjies from buried gravel layers

Although&nbsp;heuweltjies&nbsp;(19–32 m diameter) dominate the surface of much of the southwestern Cape of South Africa, their origins, distribution and age remain controversial. Current hypotheses are that the&nbsp;heuweltjies&nbsp;are (1) constructed by the excavation and mounding habits of burrowing animals; (2) the result of erosion by water of areas between patches protected from fluvial action by denser vegetation or (3) the product of localised aeolian sediment accumulation beneath denser vegetation associated with termitaria. At a site where quartz-containing gravels occur on the soil surface in areas between&nbsp;heuweltjies, these gravels were found to extend as a relatively intact layer of uniform concentration from the inter-mound area into the mound at the same plane as the surrounding soil surface. This buried layer suggests that&nbsp;heuweltjies&nbsp;were either built-up by deposition on a previous soil surface layer or eroded from sediment accumulated above the buried gravel layer. Mounds contain a relatively large proportion of silt consistent with sediment deposition. Mound sediment elemental composition was strongly correlated with that of local shale, indicating a local source of sediment. Pedogenesis was considerably more advanced off- than on-mound. There was no evidence of extensive regional aeolian sediment mantling over the vast area in which the&nbsp;heuweltjies&nbsp;occur. These findings and observations support the aeolian deposition hypothesis of&nbsp;heuweltjie&nbsp;origins combined with a degree of erosion, rather than a termite bioturbation hypothesis or a predominantly erosion-based hypothesis

Inselbergs persist as islands of diversity in a heavily grazed rangeland mosaic at the nexus of three arid biomes

Inselbergs are regarded as ‘islands of diversity’ due to the high number of plant species present and level of endemism. They also act as natural sources of fodder for livestock and thus risk becoming homogenised in a heavily grazed rangeland. The aim of this study was to compare plant diversity on inselbergs and matrices inheavily grazed sections of three arid biomes of South Africa’s west. The inselbergs are scattered in a matrix of rangeland vegetation in the Desert, Nama-Karoo and Succulent Karoo biomes where pastoral herding is the main land use. Plant composition, cover and growth forms were quantified in plots and along transects in 21 inselberg and matrix sites. Inselbergs had more diverse plant and growth forms and greater cover than adjacent matrices. Defoliated plants on inselbergs are able to recover due to more mesic conditions and because inselbergs are largely grazed during drier periods, species are able to flower and set seed during winter. Inselbergs are also less accessible to livestock due to their unique geomorphology. Therefore, inselbergs are able to persist as islands of diversity and warrant inclusion into conservation and rangeland farming management plans since climate change will result in biome boundary shifts.Keywords: arid biomes, communal rangelands, inselberg

The composition of the soil seedbank and its role in ecosystem dynamics and rehabilitation potential in the arid Tankwa Karoo Region, South Africa

Very few soil seedbank studies have been conducted in South Africa, especially in arid rangelands. Insight into the soil seedbank could therefore improve assessment of rangeland dynamics and enhance rehabilitation efforts. This study aimed to characterise the soil seedbanks in various vegetation types of the Tankwa Karoo National Park, an arid environment in South Africa. At 43 sites soil was sampled twice at the end of spring and the end of autumn, and the composition and structure of aboveground vegetation were described at the peak growth period in spring. Seeds were isolated from the soil samples by means of flotation in a salt solution, seedbank species composition was obtained by seedling germination, and the vegetation was investigated using a line-point survey method. Overall seed density was 8 034 seeds m−2 of which 55% was viable, and the common life-forms were therophytes and chamaephytes. Species richness and diversity were lower in the seedbank compared with those of the vegetation, and the two vegetation spheres were 25% similar. While seedbank composition suggested poor veld condition, there was enough seed density and viability for future regeneration and rehabilitation initiatives. However, this may be impeded by the absence of many perennial species in the soil seedbank.Keywords: arid environment, interpatches, life-form spectra, patches, rangeland condition, species composition, Succulent Karo

Vegetation structure and composition at different elevational intervals in the arid Tankwa Karoo National Park, South Africa

Study area: The study was conducted in the Tankwa Karoo National Park, one of the driest areas in South Africa. Historic overgrazing has resulted in the poor and often degraded state of vegetation in large parts of the Tanqua Karoo region. Aim: This study assessed the spatial variation of vegetation structure and composition in the three main vegetation types namely: Tanqua Karoo (TK), Tanqua Wash Riviere (TWR) and Tanqua Escarpment Shrubland (TES), along an elevational gradient. Method: Using the point intercept survey method, vegetation cover, plant height, species diversity, life forms, proportions of perennials and annuals were examined in 43 sites, widely distributed in these vegetation types. Results: There were 150 vascular plant species belonging to 83 genera and 29 families recorded across all sites. The most diverse vegetation type was TES with 96 species belonging to 61 genera and 26 families. The vegetation structure was comprised mainly of shrubs and dwarf shrubs with a high proportion of leaf and stem succulent species. The mean perennial vegetation cover throughout the study area was 28 and annuals covered 22%, but this cover varied significantly between the vegetation types. The most dominant life forms were chamaephytes, which comprised 64% of all species, with cryptophytes (18%), therophytes (16%) and nanophanerophytes (2%) less abundant. Surveyed sites in the TES showed a clear association with each other but there was an overlap in the species composition and environmental conditions between some TWR and TK sites. This study highlighted the important role of elevation and topography as drivers of vegetation characteristics. Conclusion: The findings from this study can be used as a vegetation baseline to identify and prioritise degraded areas for active restoration in order to limit further degradation. Considering climate change, elevational studies may provide additional insight into species dynamics across landscapes. Taxonomic reference: Plants of Southern Africa Checklist (South African National Biodiversity Institute 2016). Abbreviations: ANOVA = analysis of variance; GPS = Global Positioning System; LFA = Landscape Function Assessment; NDVI = Normalized difference vegetation index; PCA = principal component analysis; PCo-A = principal co-ordinate analysis; TES = Tanqua Escarpment Shrubland; TK = Tanqua Karoo; TKNP = Tankwa Karoo National Park; TWR = Tanqua Wash Riviere

Shaping post-orogenic landscapes by climate and chemical weathering

The spacing of ridges and valleys reflects the competition between diffusive transport on hill slopes and advective transport in channels, although the underlying lithologic, tectonic, and climatic controls have not been untangled. Here, we measure geochemical and geomorphic properties of catchments in Kruger National Park, where granitic lithology and erosion rates are invariant, enabling us to evaluate how varying rainfall (MAP=470, 550, and 730mm) impacts ridge-valley spacing. Catchment-average erosion rates, based on 10Be concentrations in river sands, are low (3 to 6 m My-1) and vary minimally across the three sites. Our lidar-based, slope-area trends reveal that hill slopes in the dry site are gentle (3%) and short (area of hill-valley transition is 75m2). The terrain has low relief and is highly dissected by small channels. With increasing rainfall, hill slopes lengthen (transition area increases from 220 to 700m2) and increase in gradient (6 to 8%), resulting in fewer, higher relief catchments. The chemical depletion fraction of ridge-crest regoliths increases with rainfall from 0.3 to 0.7, indicating progressive increase in chemical relative to physical erosion. In the dry site, catenas are relatively undifferentiated, but intermediate and wet site catenas have leached sandy crests and upper side slopes complemented by clay accumulation zones further down slope. The texture of this granitic landscape appears to be set by climate-driven feedbacks among chemical weathering, regolith fabric differentiation and hydrological routing that enhances the vigor of diffusive transport relative to advective transport